Brian Greene and Adam Riess: The Accelerating Universe

[Music] [Music] [Music] you [Music] hey everyone welcome to another one of our live sessions here at world science festival this of course is some hybrid of your daily equation that many of you are familiar with also with world science you the education platform in which you can explore all sorts of scientific subjects with world leading experts and we're doing something a little bit different for today's live session normally as you know i just kind of sit here for an hour or two hours or sometimes three hours and more and you know i enjoy just fielding your questions sometimes throwing questions back at you today we'll do some of that but i'm also going to be joined today by a guest nobel laureate adam reese is here who i'll bring in in just a few minutes and we're gonna discuss you know the things that led to the breakthrough that he and his colleagues achieved back in 1998 and has rippled through our understanding of physics ever since so that's going to be an exciting conversation and look as as we just have a little preamble here if you have particular questions in cosmology having to do with issues of einstein the cosmological constant the expansion of space the accelerated expansion of space supernova explosions as standard candles allowing us to determine cosmic distances any questions in that arena i'm going to give those priority when adam is here so ask those and you're more likely to get your question addressed if it's in that domain but again as always feel free to bring up any questions at all spanning the gamut i'm happy to field and we'll happy to address any questions that come along so before we get into the conversation just a couple things i always like it when you guys chime in on where you are tuning in from around the world i like to see all the countries scroll by it's kind of fun to see where you all are from so do that and i'll take a look at that as well but i thought i'd spend a a minute or two just on some sciency news type things of late some of you have asked me in fact a number of you have sent me emails or in twitter asked questions about the xenon experiment that are searching for dark matter has it found dark matter and the answer is i don't have an answer i don't think anyone does at the moment or maybe some people do and i'm in the dark on this question of dark matter but there's some anomalous data that has come to light oh i'm really sharp today on my metaphors and anti-metaphors so this data has come to light about dark matter that there is an excess of certain kinds of processes apparently happening that don't agree with the most pristine and economical standard model of particle physics which would suggest possibly additional physics and that additional physics might perhaps involve the dark matter so it's really too early to tell and look if you use history as your guide most of the almost breakthroughs in physics turn out to not be breakthroughs at all and that's the nature of the beast every so often you have some spectacular insights such as the one we're going to talk about with adam reese but most of the time you think you have some great breakthrough and after you study it more fully and after other teams of researchers investigate it you find that the result is actually nothing unusual nothing unexpected at all so of course that is a very conservative perspective my guess is that that is the case but who knows and if the experiment has detected dark matter that will be one of the great achievements of our age so stay tuned on that another fun little piece of physics news that i was i don't know how i came upon it but you guys know this this weird effect that sometimes hot water appears to freeze more quickly than cooler water i mean it seems completely counterintuitive because for hot water to get to a freezing temperature it has to pass through every temperature in between you would think and therefore you would think that hot water is going to take longer to freeze because it has to get to the lower temperature that the other sample of water already began with but remarkably some people think that's not the case and there's now some data supporting that and i don't understand it fully maybe adam does but the physics appears to be the case that if your initial sample is not fully in thermal equilibrium name there's not one unique temperature that applies the entire sample then different parts of the sample can cool at different rates and somehow can choreograph their cooling in some unexpected way that finds shortcuts to freezing so it's kind of a interesting piece of you know who would have thought that there's still any question about you know freezing of water right you know so those of you who think that physics is done we're not even done with understanding how water freezes so there is a there is a long way to go anyway so that's just a little bit of sciency new stuff that's going on but as i mentioned we're going to begin in this first hour with a conversation with one of the faculty members who is given a course in world science you a course on cosmology and the expansion of space and that faculty member is adam reese who i'm going to bring in now he's the bloomberg distinguished professor and thomas j barber professor of physics and astronomy at the krieger school of arts and sciences at johns hopkins university and he is the co-recipient of the 2011 nobel prize in physics so adam i think you are with us can you hear me i am great thank you for being with us this this afternoon and i don't know if you um are aware we started this little series when the pandemic hit just to give people something in the afternoons on well we were doing it every day for a while but the live sessions were just every friday and we sort of have formed this little community of like-minded interested seekers from countries from around the world who just love to tune in and chat about physics and science and sometimes even more philosophical things so the folks who are watching will be asking some questions in real time and i'll try to pick some of those out as you and i are talking about things but um basically i just want to have a little conversation about cosmology and i thought i'd begin though by just uh you know it's hard not to think about the things that we are all facing at the moment how has the pandemic been treating you are you home are you in baltimore now or yeah i'm in baltimore i'm at home you know the one of the beautiful things about working um with space and observatories and in particular the hubble space telescope which is what i mostly work with is you can do it from anywhere you can do it from a starbucks you can do it from you know your home under uh under a lockdown uh in fact when all the observatories were shutting down during the initial outbreak you know somebody has to go to the mountaintop and put you know liquid nitrogen in the doer hubble just keeps going and so uh it was probably the least affected uh of all that's sort of amazing and and and i i should know this i apologize but do you have kids at home do i do um i have a nine year old and a 16 year old so you know going from teaching at johns hopkins university back to uh third grade uh math is uh you know take some time take some adjustment and take some study you have to remember that you let go of so so i'm just wondering at johns hopkins what is the disposition of the administration are they putting any pressure on you guys to go back into the classroom are they leaving it totally up to you yeah so you know up until a few weeks ago there was the optimism that uh we were gonna come back in kind of a hybrid mode but just yesterday uh they uh retrenched that position and so everything's gonna be online now yeah yeah yeah i was thinking how you know ahead of our time we were with the world science festival when i uh or uh school when i taught that uh class you know because everything's online now exactly exactly you know it's funny i was talking to a colleague a friend of mine i don't know if you know malik parrik at arizona state and he was saying how everybody he spoke to at the beginning of the pandemic thought okay we're now all going to be isaac newton right newton retreated during the black death invent calculus and then calculus figure out the universal law of gravity we're going to come back to the world with some sparkling new insights in fact i feel quite bad about it because the truth is i haven't done anything that good so exactly you know we were all given our newton moment and uh maybe some of us will come through with it but anyway let's just turn now i thought we jack suggested if it's okay with you just sort of stroll through the history of cosmological thought going back to einstein really and then leading up to the work that you and your colleagues so famously gave to the world so obviously it was einstein who gave us the general theory of relativity back in 1915 he sits down the equations and i guess you could really say he was the first person to use general relativity in a cosmological setting but he kind of came to a conclusion that was at odds with what we think about the world today uh so i mean he did not like the notion of an expanding universe even those equations seem to suggest that many people say it was just a philosophical prejudice is is that what it was yeah you know i'm gonna so i'm gonna throw in here that right from the beginning um there was the tension between the theory you know gravity it's beautiful general relativity you could solve it if you knew what circumstance you were solving it in and the observational component which is where are we you know what is are we in a gravitational bound structure well we are the milky way and then what's beyond that so um when einstein was first thinking about the universe as a whole astronomers were still sorting out uh whether our galaxy the milky way was everything uh or not um and so at the time they told einstein who asked you know are things expanding contracting moving and you know the response from astronomers at the time was really to measure the velocity of stars around us and say you know nothing's really moving very much i mean there's some random motions but you know for the most part there's no evidence of anything expanding or contracting again thinking perhaps the milky way was everything and so einstein struggled with how to get in general relativity a static universe so it was more than philosophy from so what you needed you needed the right boundary conditions and the initial ones from astronomers were i would say not correct and you know i always think einstein gets a bad rap for making this blunder about the cosmological constant when he was really given i think you know bad data yeah so you mentioned the cosmology constant of course einstein introduced that because his equations did not allow for a static universe so he fiddles with the equations introduces this new term that does allow for a static universe i mean so just to give a sense how how does this new term do that um how does this new term do that well um you know the the tendency for matter in the universe is to be attractive of course and pull things together and so any way you arrange the universe with matter it's going to do that some degree of that so how do you how do you hold back that tendency and you know einstein made this amazing discovery that really the gravity of empty space itself could be repulsive you know what he called the cosmodrome constant and today we would call more generally vacuum energy the fact that if you try to expand the universe and every time you do that you're going to put more vacuum energy in the universe you're going to have to do some work in order to create that energy and the way to think about that really is it's like a universe inside a cylinder if you want to expand that cylinder and create more energy in that cylinder you're gonna have to do work therefore it's gonna be hard for you to open that cylinder pull that piston and that's what we call negative pressure it's the opposite of what you think of as positive pressure when you do work to compress it and the weird thing is uh unlike newton's theory of gravity in einstein's theory of gravity negative pressure has the opposite sign of gravity and so it is a direct consequence really of that property having uh new energy anytime you create more space so did einstein and and it's interesting to talk to you about the stuff because look i i lecture on this stuff all the time and sometimes i say things that i'm not completely sure that i'm hitting the historical progression with with full accuracy and i guess nobody really knows what was going on in einstein's mind but i've never found in einstein's writings that he refers to repulsive gravity we all know that the cosmological constant as you note does gives rise to this outward push as opposed to the usual inward pull of matter did einstein ever think of it that way um so i've done a fair bit of reading and i would say i've never seen him refer to it that way but i have seen him later on in fact there's a famous i think almost paper that they found in the archives in israel where he refers for the first time to the cosmodrome constant as possibly due to energy in space and so um you know that connection between this mathematical term and energy in space but you know in terms of thinking of it as a repulsive gravity it's really a term that shows up in the friedman equation that describes how space expands but i think it i think it's reasonable for people in a basic way to think of it as gravity going in the other direction right right and then and then i guess it was would be a friedman who you mentioned alexander friedman george lemaitre as well in the early years of the 1920s and so forth who alerted or at least they thought they were alerting einstein to this idea that the bare general theory of relativity did not allow for static universe it's basically expanding or contracting but einstein tried to shut them both down i mean he felt really strongly i guess the quote for lemaitre was your calculations are correct but your physics is abominable or something like that you know right now now there's always been this very long history of physicists using more than mathematics but a kind of idea of elegance um in order to bring intuition to a problem and i think he thought he had strong intuition but you know again your intuition is only as good as your observations of the world around you and he you know was limited in that way yeah i mean one suspects that most intuition is built up from experience and observation so like you're saying if your observations somehow have missed something vital about the world it stands to reason that your intuition will be missing something vital too but you know if einstein said to me that that my physics was abominable you know i have a feeling that i would have like dissolved yeah right but but you know friedman who einstein actually said he said your calculations are wrong to friedman freeman just stayed at it and ultimately convinced einstein that the math was right and the interpretation perhaps was wrong uh lemaitre stayed at it too so i look at these two figures as kind of remarkable to stand up to einstein but i guess it was in 1929 when the observations changed edwin hubble comes into the story and i guess and einstein famously visits edwin hubble at the um california observatories and comes away gives his famous quote to the media who were sort of following him at the time oh okay you know i'm convinced the universe is expanding or this interpretation and i think you know i made a big mistake by introducing this cosmological term which he wrote to lemaitre as well saying you know probably not right yeah there's also an interesting quote there i think on that trip that you mentioned elsa went with him and as the astronomers were showing einstein around the observatory elsa einstein got separated so she started to talk to some of the folks there and she said oh what do you what do you guys do and they said oh we like try to figure out the nature of reality the nature of the universe with this big machine big telescope she goes oh my husband he does that on the back of an envelope right that's right that's right so so in 1929 we learned that the distant galaxies are all rushing away as you say einstein kind of smacks himself in the forehead realizes that he could have predicted this year before but he didn't but at that time i would think that everybody came to the conclusion that okay space is expanding but it's got to be slowing down right in its expansion right was there anybody who was thinking otherwise um you know uh of course the consequence of this is that it all started at some point in time and of course uh there was another school of thought sure it's expanding but i don't like this start this origin of the universe and so fred hoyle and uh bondi and gold developed the steady state idea of it sort of just continuously expanding forming new matter in between space but not that it ever had to start kind of like watching a river flow you know it's always flowing but it doesn't you know have to start at a spot uh and uh you know it was really not until the 1960s that uh the heat left over from the big bang was first discovered and of course that's a direct consequence of the um idea of starting at a singularity and then expanding not this steady state continuously expanding but forming new material to keep it always looking fresh so the notion of singularity is of course a vital one and and the point that you're making in terms of it hard to square that with say steady state cosmology just raise a question that people ask all the time and perhaps somebody will ask it here too we often think about the big bang is like a little point that then grows into the cosmos as we know it but if space is infinite at the moment then that picture is a little a little bit wrong right i mean can you expand i mean you know it's a it's a tough concept to get intuition about but you know our best understanding now is that it's always infinite even you know going back up till that point in time it's just bigger um you know my dad used to love the joke you know what's bigger than infinity you know infinity plus one in a way that's sort of what we're saying you know the universe is infinite and getting bigger right or at the very least that the distances between finite distances between objects is right it's always important to point out there is a horizon which is as far as we can see and you know the rest is you know reasonable extrapolation past that so we have a a question from purisha mishra who asked did the fabric of space exist before the big bang thoughts on uh well i mean we don't know i would say that um you know our theory of what happens uh in the universe really is a theory that starts you know 10 to the minus 34 seconds or something after this singularity and describes very elegantly what happened after that point it really i i don't know maybe you have thoughts brian but um anything is going to be much more theoretical about what preceded that yeah absolutely i mean the whole notion of before the big bang raises all sorts of conundra even linguistic ones is there is does the concept of before apply to this arena or are we extrapolating the notion of before that applies to more ordinary events in the universe to an arena where it just doesn't apply or was the big bang an interesting event but not the actual creation event of the universe maybe it was just an interesting event that took place in a pre-existing universe in which case the the spatial fabric might have existed so yeah these are these are pretty tough questions but back to our story right so so so you know by the 60s 16 big bang is in steady state is out and uh and at that time just about everybody is on board with the idea that the expansion is real yeah but because everything pulls on everything else it should be slowing down over time and and so when did you start to think i mean as a kid were you thinking about cosmology and i mean i was fascinated by the stars i think i suffered the same problem einstein did which is as a kid i knew about the stars i didn't really know about anything beyond that i mean that i don't know you know when you're a kid you know what you can actually see with your eyes and i guess maybe i wasn't well read enough but i was pretty fascinated by just uh the amazing distances and time look back times two stars you know my dad would tell me you know a star you look at may be gone now you know it's millions of light years away so it may have it may have died millions of years ago and isn't there those were to me cosmic ideas even though they were still within our our galaxy yeah hugely so i mean i just had that conversation with my kids it turns out i mean because normally we're in new york city where a starry night means there's two stars in the sky but we're actually up in upstate new york there's a helicopter yeah right exactly like the helicopter but now we go out it's like starry night all the time so it gives opportunity and that is such a crazy idea that you're looking at something that may not be there any longer and just it's remnant light that it's shed thousands or hundreds of thousands or millions or billions of years ago still in transit but of course this turns out to be the very tool or trick astronomers like me use to answer this question because if we look out we're looking back in time when we measure a fact about the universe as we infer it from a distant object we are inferring that fact about the universe as it was in the past we're inferring the past state of the universe and so we can see the universe expanding at a different rate in the past and we can measure how that rate has changed over time and answer this question you know is the expansion slowing down and if so is it enough to stop the expansion which just turns the physics problem around from was there enough matter in the universe to cause that it's it's you know very analogous to escape velocity for a rocket so your goal so if we jump even ahead right now to the work that you did i mean you went you're an undergraduate at mit is that right and then uh graduate student at at harvard and your your advisor was kirsh bill christian bill cress right yeah yeah so press was my undergraduate bill president undergraduate advisor uh yeah years earlier uh but uh so so you set out to figure out if the rate of cosmic slowdown was such that you could say something about the ultimate fate of the cosmos was that the thinking that was driving you yes i mean you know to be very honest right you're a graduate student your postdoc your goal is to survive get your phd you know to write a paper to you know get a job but you know the bigger glorious uh idea of the projects is okay astronomers had been measuring for years that there was not enough matter to stop the expansion in the the terms that we use omega omega matter that represents the fraction of the matter necessary to stop the expansion and so uh theorists told us it's probably going to be one that's almost a prediction of uh our best theory of the big bang inflation um and yet when astronomers would go out and try to count or or quantify take a census of the universe around us they would end up with only about 30 percent of that total and so in desperation i would say uh cosmologists said look further out maybe you know there isn't uh that much matter right around us but it's further out there it's sort of like maybe we live in the boonies but you know you haven't seen the cities um look further out and so how do you look further out um and see the matter especially when much of it might be dark you know you may not even see it is to actually measure how much the expansion is slowing down to essentially go directly at the question and uh in the late 1990s it first became possible to use a kind of exploding star called a supernova type 1a to measure these distances in red shifts and those are the two things you really need to measure you need to measure distance and redshift that tells you how fast the universe is expanding and then you need to do that at a bunch of different distances or redshifts to get these slices in time and measure how much the expansion was slowing and so my doctoral thesis was on improving the precision of those kinds of measurements just at the time when two teams of astronomers formed to find more distant versions of those explosions and make this measurement and you were leading you you were leading up one of those teams with brian schmidt is that right brian schmidt was leading it and i was leading the analysis for our first big set of data and at the time this would have been 1997 you know we thought that the expansion would be slowing down uh it's captured in a quantity um we try to measure called the deceleration parameter it's the second derivative of the scale factor of the universe and the sign tells you what that is and the number itself tells you relates very easily to how much mass is in the universe and so come late 1997 we had assembled our first large set of distant supernovae to compare to nearby ones and this was something that i was working on and you know wrote a little computer program to tell me okay you know how much deceleration is there and what is the implied mass of the universe and i kept getting a negative number for the mass which really meant that the deceleration was acceleration and i hadn't even noticed what was going on yet and uh after staring at that and thinking about that and you know going back to you know the great textbooks on this uh realized you know matter can't do that it needs to be something with the other sign and enter again einstein's cosmos constant and interestingly we didn't mention this in the beginning when einstein realized that this was a possibility and then it wasn't necessary in this idea of physicists have liking simplicity and elegance it became oh well if it isn't dominant if isn't necessary then it's zero or probably zero or we don't understand but let's assume it's zero and i would say you know it only shown the light more brightly on this problem that we had never really understood that physics and what that value should be and so we were simply saying the only physics we've ever heard of that could cause what we're seeing acceleration is if einstein's cosmological constant exists and it has a particular value now are you jumping over a lot of hand wringing in there because i mean look every time that i've been fortunate enough to get an unexpected result which is only which is rare like in a physicist's career yeah i think i made a mistake yeah and i heard your beginning your prologue here you're talking about dark matter yes non-experiment and it is true that you know 98 of the time everything you hear about or experience as a scientist if it's exciting it's probably wrong but now you i almost want to jump in and say you know we can't become too cynical about that otherwise you exclude the possibility of discovering and i guess i was fortunate that i was young enough to not uh have enough of that experience so yeah did you have like you know literally or poetically you know sleepless nights because too in the world absolutely claim right the expansion is speeding up instead of slowing down going against decades of expectation if you were wrong yes no one would listen to you again right i mean that that is true so brian and i really sort of took the lead in checking the numbers the calculations the measurements and you know we kept exchanging these kind of amusing messages about if this is wrong what are you gonna do next you know and he liked wine so he was gonna do a winery and i thought you know with my math skills maybe i could do something on wall street because we were both thinking we're pretty young guys and you know this will be the thing we're most famous for is getting this wrong sure you you definitely sweat that a great deal um but i think the key is to find a middle ground between uh being wrong or being wrong for an interesting reason and so what that really means is you know you got to get the calculations right you got to get the measurements right because it's not interesting to say oh i had a negative sign in my code sorry everybody but if it only looks like the universe is accelerating if it's i'll call it an apparent acceleration and then somebody smart person comes along and says you know why it looks like this because of some subtle effect that nobody's written a paper about or even something surprising in the universe there is some kind of intergalactic gray dust that nobody has ever seen before then you go oh okay i mean you know this is as far as i can go is is trying to get the measurements right um and so you know you you find peace at that level if you can you can get to that point now there was another team that was absolutely undertaking led by saul from mutter right close contact was it was it just yeah um you know they were doing the same experiment at the same time and of course you know competition drives uh a kind of um bringing together of the timing of things if somebody's a header behind they sort of scramble to not be falling too far behind i mean we're competing for telescope time and so it so happened then that we really sort of converged at this point in time within months of each other um not in direct communication but you know nevertheless you know doing the same experiment and you know in early 1998 we started both showing up at conferences and going we're seeing this and they were saying we're seeing this and you know it became very exciting uh for the community in particular because it sort of gets you past these issues of you know are you sure you don't have a negative sign wrong or you know is there some background that you you know in your experiment is being ignored so for the most part it you know accelerated no pun intended the discovery itself so in 1998 you you both write the papers yeah and look sometimes papers of that sort generate a lot of backlash uh a lot of uh people who just don't believe it right is that the case here was it pretty much like wow and acceptance right so i would say there was a little bit of a limbo for a couple of years i mean it certainly caught people's attention uh you also have to understand anytime somebody makes a claim of anything right we have so much established knowledge and measurements that you know first it has to fit it has to fit other things too and so right away people started pointing out you know this actually makes certain sense we were missing i was about 0.7 of the universe in this critical okay we thought it was in matter but maybe it's an energy it still works for inflation um you know there was a crisis an age crisis where some of the oldest things in the universe looked um to be older than the universe itself this fixed that so first people check it against things they know and say is this already ruled out is this going to make something worse and the quick answer was actually it makes some things better so we survived i would say those first couple of years simply because of that and then of course by the early 2000s observations of the cosmic microwave background the anisotropies were actually used to independently measure the energy density of the universe and came to the same conclusions yeah in case you're just joining us first i want to apologize for my dog that's barking in the background i can't i can't turn her off i wish i could at the moment but we're talking to nobel prize winner adam reese from johns hopkins university about this wondrous discovery that the expansion of space is speeding up not slowing down that it's accelerating so so so so people began to warm to this idea as you're saying but as a theorist right i have to say that this this sent us into a bit of a tail spin at at first because for many reasons actually it made it at first it seemed like it was difficult to square with certain of the beyond the standard model ideas that we've been developing for a very long time but then people found ways to do that but the number the actual amount of this cosmological constant or call it dark energy this source of anti-gravity that's pushing things outward instead of pulling things inward sure it is say 70 percent of the mass energy budget of the universe but when you put that into other units the units that perhaps are more familiar to the folks who study particle physics the number is utterly bizarre right i mean in those units it's 10 to the minus 120 10 to the minus 122 or so you know a decimal point with a string of zeros and then like a one at the end of it so so to us that's just an incredible challenge to ever explain that number is that something that how do you look at that do you look at that as a as a problem of the way the theorists are are modeling how the universe works or is this a real fundamental issue in how humans will ever understand the deep structure of reality right you know it's funny i uh always thought of this as a pre-existing problem that had been swept under the rug and that by making this observation we only lifted up the rug again um we it didn't make the challenge any worse that sword was cast in the stone before and i go back to stephen weinberg who had written a paper a number of years earlier saying geez if the cosmogenical constant uh had the natural value none of us would be here you know would have caused space to accelerate with such great you know gusto that gravitational structures never would have formed and so that can't be close to right i i have to imagine that you know there's some kind of anathropic argument or you know maybe there'll be deep physics that sets the value but um you know he in some ways almost predicted the value that we have because that it's got to be compatible with life yeah yeah yeah i want to so so there were theorists who are on the books at least as suggesting this possibility but i want to go back to the actual approach that you took to get to this discovery you have this wonderful little visual that i'm going to bring up now and you know say a few words if you like and then i have a couple questions on it so yeah i think i'll get out of the way yeah there we go that's space it's expanding and there's supernovae going off in those galaxies and uh you know if we watch some galaxies we're gonna find one and we're gonna measure that light and that light is getting red-shifted it's getting stretched by the expansion of space um with that graphic also you can imagine that light is heading out in all directions and so the further away we are the smaller slice of that light we get so it also is fainter and so as observers we would like to measure the property that you see there which is that grid size that's what we call the scale factor the size of that grid how that grid changes over time and because there is no grid we have found in space uh instead of measuring the grid size we measure that wavelength of light that you see there we measure how stretched it is um and instead of measuring how far away the other objects are uh well sorry we do measure how far away they are instead of measuring time how far back in time or how long it's taken the light uh to reach us um and so this is how we we map the expansion rate of the universe so there's so many moving parts in a way to to get to the result yes that that like you said you and and and brian schmidt and and the other team led by saul promoter just i assume you know possibility by possibility of just ruling out any more prosaic explanation but i still see papers yes i'm just wondering what you i still see papers even recently coming out which like it's not correct i challenge them so so is it obviously i'm assuming there's nothing to them but but what what what are the criticisms that at least people posit and what were they doing wrong so so you should know first of all that you know we have now about i would say five or seven depending on how you count it five to seven independent routes so the route we used was to look at exploding stars and see the acceleration but completely different ways of mapping or measuring the universe measuring other features other ways of measuring distance other physics of the universe so we have you know the five to seven ways makes us very confident um and so uh the most common phenomenon when somebody says oh this is wrong is they're starting out their argument by throwing out five or six of the seven ways and they only address one of them and of course this is the challenge in sciences you know cosmology is hard we do need multiple ways we weren't confident or sure that the universe was accelerating until we had these multiple techniques so i think it's you know it's always unreasonable to discard data um if you don't have some really strong rationale and then of course as you said it's very challenging to explain the cosmological constant and so there's always the the willingness i would say to say okay let's say we throw out six flavors of data that show this and what if we uh imagine some like i said some gray dust in the universe that makes distant objects look dimmer what if we analyze the data in a different way one can often play games with sort of statistics and then you know they may reach a conclusion in in a recent case where you know the universe isn't so different in terms of its expansion history as a universe without any matter or without any cosmodrome constant and you know maybe that's only you know three standard deviations away from the best fit and somehow through a series of uh translating errors i would almost say to media it becomes maybe the universe is not accelerating okay okay yeah yeah but i mean the result stands is the bottom line and it's one that we have to deal with now key to the observations that you're describing of course is light traveling from say the supernova through expanding space to our telescopes and one of the folks who's watching us right now who describes their name as egg leaves i'm not exactly sure i presume that's not their real name asks the question but what if the speed of light has not always been the same is that something that could affect the results it it could it i would probably say it would you know it's an assumption we make right from the beginning and we do make a number of assumptions i should point out in cosmology we almost always start with a few assumptions like we don't live in a special place um we don't live in a special time and so these assumptions which are often wrapped up into a fancy name the cosmological principle um sort of allow us to do physics they allow us to make measurements and interpret them because if you said every domain could have completely different physics in in the universe you know you would only be able to look out at the stars and the galaxies and say isn't that a pretty show you wouldn't be able to actually try to turn it into hard measurements now having said that we start out with that on zots that you know physics is not different in other places in the universe and then we analyze the data in that context and it fits these model lines beautifully i mean it's not like our measurement is one object you know we have supernova after supernova at different redshifts all fitting the predicted now accelerating line and so we say you know it seems so so it it doesn't look like the speed of light is changing uh over time everything seems to agree very well if we found uh anomalous results and i guess i would even say outside the cosmological constant you know we'd have to struggle again and say you know could there be something else could the speed of light change with time right and and you know every result every observational result at some level is is theory dependent because you've got to place your observations within some theoretical structure there's been i don't know what the right word is kerfuffle argument so you mentioned inflationary cosmology and and many do look at it inflationary cosmology as the best approach that we have to understanding physics from just after the big bang or depending on how you define the big bang maybe including the big bang through the accelerated expansion that that you guys found but there are some who think that we're jumping the gun and committing to this inflationary theory which uses the very same physics that you're talking about right you know repulsive gravity of a much higher magnitude in the earliest fraction of a second giving rise to the outward swelling then things do slow down for a while and then they pick it up again as the dark energy that you and your colleagues discovered causes the expansion space to accelerate what's your view of inflation is it i mean do you do you question it or is it you know it's it's an i think it's a powerful framework you know it relies on physics that we already know in terms of general relativity and the idea of vacuum energy it solves an observational problem we didn't understand any other way how you know you can solve the the flatness problem how space can be so flat how you can solve the horizon problem how uh opposite ends of the sky uh seem to be in equilibrium even though they've never been in contact um and uh it may not be a unique answer it may not be the only way to address that but you know it is powerful and um and it's it's it's a useful model but you know the expression you know uh you know not all models are right but some are useful uh you know it i think it fits that right and you know another thing that comes out of the inflationary paradigm and it's quite relevant to the dark energy that we've been talking about is one explanation for the value of the dark energy could be that we live in a multiverse that there are many distinct universes each has say a different value of the dark energy you have enough of these universes in fact i got a little visual i kind of make use of our technology oh yeah there they are each of these guys here is its own universe each with a different value of the cosmological constant then the fact that we have this weird value would just be uh you know an artifact of where we happen to be and as you noted it's life right imagine you you know you encountered some you know crude physicists on another planet some distance from their sun trying to derive from first principles the separation of their planet from their sun and and you'd step in and say hey i have news for you you know there are many planets and many stars and you know this is just a random environmental parameter but it had to work out for you had to be not too close not too far and uh so you know will are we in that same situation or not yeah i mean so copernicus right spent a long time trying to calculate 93 million miles earth's on distance and as you're saying now we'd be like hey copernicus you're a real smart guy but there isn't a first principles calculation of a number that's a historical artifact that is compatible with us being here but beyond that probably has no fundamental significance so let me ask you this question then because i think this is more up your alley um i'm comfortable with an anthropic argument like that when we talk about you know something like uh why we're at such a great distance from our sun you know because i'm able to see other planets around other stars and i get oh there's a distribution there were lots of options when we talk about the multiverse you know we really talk about options that i'm not sure or distributions i'm not sure that you know we can claim either exist you know that we know they exist it seems like there are um prerequisites to making an anthropic argument and i wonder have we checked those boxes yet yeah i i think that's the big question and it's an argument that well i mean i've been having a an ongoing conversation with paul steinhardt for years now and and paul as you know and i'll just remind the folks that are watching us he's one of the architects of the inflationary theory with alan guth and andre linde and andreas albrecht and so on so you know he has skin in the game for the inflationary theory and yet for the very reasons that you're saying adam the fact that okay if this theory gives rise to many universes then if you're going to be able to say anything concrete about its predictions you need to know the demographics of those universes you need to know their probabilistic description of the kinds of qualities that these universes have and and so paul whenever i talk to him about this he says absolutely you need that and that he says in his view is such a new quality of the theory that doesn't come out of the mathematics that was developed into the 1980s and 1990s he says that's no longer the inflationary theory that's that's a wishful theory that doesn't yet exist right and so whereas many folks like me who sort of view this distribution of universes as an add-on that we need to tack on to the inflationary theory he says don't think about it that way it's too big a thing to tack on it is the theory at some deep level right and that's why he has been developing alternate ideas that don't require emotion and you know i would almost say it in a more simplistic way you know i would say to my theoretical brethren and and sisters um you know don't be satisfied with our cosmological constant as um uh a consequence of a multiverse anthropologically set because you know we haven't yet demonstrated those distributions and those demographics and we could miss something important just by waving our hands and saying well that's how it comes to be i mean you know in the same way almost that people would have waved their hands you know in the 1950s and 60s and said i guess the cosmological constant is zero because we don't see any evidence of it you know it's similar sort of of story you know you don't actually have all the information yet to make connect rigorously connect those dots and uh you know we could lose out on this clue by doing that prematurely yeah yeah without a doubt and and you know some people view the cosmological constant as this uh very painful development in physics some view it as the greatest opportunity that that theorists have ever had and i should know the answer to this question but i but i don't actually so let me ask you in your paper do you mention cosmological constant or dark energy or do you just stick to the observation the title of our paper is observational evidence for an accelerating universe and a cosmological conflict constantly so that's interesting to me because when i think back to the papers on the microwave background radiation the observational paper of penzias and wilson famously excess radio tempe excess uh antenna temperature excess tenon there's no cosmology no expanding universe and so forth because i guess their view was you know we don't want to commit to anything like that the observations of the observations they are unassailable but the interpretation might change over time did you worry about including say cosmological constant so bold faced and i forgot that it was right in the title did you worry about that because nothing will obscure the validity of your observations but cosmological constant may be an idea that goes away right and i think the answer is you know in a way we had the benefit of our ignorance as observers you know even you know at that point in time we did not we were not aware that one could generalize the idea of a cosmological constant to dark energy it doesn't have to have a specific equation of state uh as though that energy is static it could be the result of a field and uh that field could be changing and so you know there is a broader range of possibilities and i think at the time that was the only physics you know we were aware of and we wanted to desperately fit the data you know we had the data and we wanted to fit lines through it and that was you know the only piece of physics we knew and so you know i think we put it in and amazingly to this day it is still the best fit to the data even as we open up the possibility the range to dark energy with all kinds of different what we call equations of state um the best fit is still within you know a few percent of minus one the famous you know value for the cosmological constant so i hope you don't mind me asking feel free to move on if you don't want to go here at all but so you're lying in bed you're asleep the phone rings it's early morning and it's the nobel prize committee what in the world was that like it's bananas i mean it's you know as as a scientist right i mean we're we're not wired for or expecting any kind of experience like that we're not celebrities you know i'm uh you know grunt through the data figure out what's going on making measurements and so it's it's a complete shock i mean to your system um i mean it's exciting but it's uh and it's you know i mean you're certainly gratified that the work is being recognized and that your colleagues are being recognized with you and so there's that tremendous excitement but it's like a bolt of lightning so i think it's the case that both you and einstein got the nobel prize at the same age i think you're both 42 if i'm not mistaken i was 41 actually 41. even you peter i know einstein so he's einstein you know what one of the first things you have to realize about the nobel prize is it's not for being the smartest person you know it is you know for making a discovery which you know people can make by being lucky or you know maybe a little dogged um so yeah did it affect your um motivation i mean in some sense you sort of hit the highest accolade now we all know that it's not about the accolades per se but look we're all human when you reach the pinnacle in that way did it affect your motivation in any way you know i uh i knew at that time that i really liked doing science and i really you know it you know hours are lost you know the clock spins by you know i'm not i'm you know my job is not a job and so i didn't really want to change that and you know being 41 it was way too early to think about stepping away from the thing that i really enjoyed the most and so um i decided i was going to keep doing research pretty much the same way and and i have uh you know i continue to use the hubble space telescope you know what i've been working on more recently is very precise measurements of what we call the hubble constant the rate at which the universe expands and also interestingly in the last few years we've seen that those values measured in the we call the late universe around us now do not seem to agree with the predicted values based on the causing microwave background in conjunction with our standard model now of the universe which includes the cosmological constant um we are still off by you know about nine percent which may not sound like a lot but in the last decade we've improved the precision of the measurements by about an order of magnitude and so that nine percent has become plus or minus two percent and so you know it's again looking like something isn't quite fitting and you know maybe there'll be another wrinkle yeah so people have have kicked around measurements and discrepancies in the in the holocaust for a very long time right do you think we're finally settling down is that yeah these this right this period is much more interesting in my mind because you know decades ago uh people might debate the value of the hubble constant when they were measuring it in exactly the same way so there wasn't really a possibility that it was interesting physics uh it was really you know the challenges of systematic uncertainties but in the last 10 years uh i think we've been able to attack systematics in a much more methodical way um and we have much more precise observatories like the the planck observatory the hubble space telescope the european space agency gaia mission which measures parallaxes that is used to calibrate uh ultimately these distance ladders and um now what we're seeing are discrepancies not based on who is measuring it or what technique but what epic in the universe they are measuring it from are they measuring it from the beginning or from the end and that smells to many people like something in the story that we used to connect the beginning to the end yeah right so so what what value is it settling down say today too yeah so it so it turns out if you measure it starting at the beginning of the universe you predict the value should be about 67 plus or minus 0.5 and if you measure quickly tell people the units and what that's doing that's uh in units of kilometers per second per megaparsec which is a it's actually units of inverse time or you could think of it as it's in units of a rate of expansion meaning how much time before the universe doubles again so if i turned it into units of doubling again it would be about uh 10 billion years before it doubles again in the present universe but we use these more units that are closer to the actual measurement so the the people who measure the early universe predict it should be 67 plus or minus a half and people who measure in the late universe get something more like the low 70s like 73 is a pretty typical number and the uncertainties are coming down to about one and a half we're hoping to reach one percent later this year and uh so the tension we call it has risen to about five sigma five standard deviations um and appears to be fairly robust because uh we look at different ways of measuring in the late universe or different ways in measuring the early universe and they agree with each other but don't agree across this time divide right and and so that again is a fruitful place no doubt for uh someone to uh sort out that that interesting story so adam it's been great talking to you i have two final questions before i let you get back to real work uh one is as i mentioned at the outset this uh series emerged during the pandemic we called it your daily equation and what i would do is i would sort of pick an equation each day explain it in everyday language and try to illustrate it and so on so in that spirit do you have a favorite equation yes it's i mean it would have to be the friedman equation which is the uh equation it's a solution to einstein's general relativity as applied to the universe on the left-hand side of the equation is the kinematic description of the universe it's how the scale factor a that movie that you saw it's that uh it's a differential equation it just simply says this is how the universe expands on the right hand side are the sources or uh of uh the exchanging expansion of the universe the matter density the dark energy any physics that causes it's almost like the f equals m a really for the universe where you know we try to uh measure the a and learn about the f and the m yeah so for those of you uh who have been following your daily equation there is an episode i can't tell you which number it is where i describe the fremont equation so you can actually go back and see the mathematical details but what i will say is that friedman is the same freedman that adam and i were talking about at the beginning of the conversation who einstein didn't believe when he put forward these possibilities of a non-static universe so he gets his due right he's like this equation is named after him and it's one that's useful and used in understanding the cosmological history final question comes from one of our viewers who calls him or herself or their self i should say quantum maths and says i'm about to enter high school and i'm really interested in physics and mathematics especially theoretical physics what would you suggest for me to pursue this interest right uh that's a great question um you know i always think that at that stage um if you're interested in that you know you want to take all the physics and mathematics that's available in your school um and rather than quote try to get engaged in you know the very advanced research which can be very off-putting because it can be very difficult is to pick up popular books i'll say you know books by brian greene and other people that tell you about the current state of play where the questions are because those things are inspiring um without sort of ruining it for you in a way before you have really the tools to actually get involved in that work so you develop the tools but keep your inspirations in your forefront by you know reading the sort of late person or interested person uh books and you know documentaries and online courses uh again meant more for lay people yeah no i fully agree and i'm not saying that so someone goes out and buys my book so thank you for mentioning it but you know you got to learn the basics but you kind of hunger for what's happening at the frontier and there's just so much out there today you know when i was a kid there just wasn't all that much and it's an exciting thing that there is so much online material now for kids to keep their their excitement going including world science you so i'll give it a plug you know adam gave a wonderful master class in world science you where you can go and and hear about the things that we spoke about today you know in greater detail directly from from adam and there are all sorts of other classes at world science hue from string theory some in philosophy of consciousness those very broads of things i i recommend to the young and the mature to go check that out it's an interesting thing to do so adam again thank you so much for joining us today thank you for the world science you master class and um and good luck in nailing down the the hubble parameters so we we want to know how quickly space is expanding all right good luck and thanks a lot all right thank you thank you so for everybody else i'm gonna carry on with our more traditional way of doing these live sessions by just turning right to some of your questions and uh let's see so i saw one that streamed by that was on uh tachyons uh where is oh yeah steve bermeo uh oh but maybe he's talking about the jello photo kizmelakas question i can't know but uh having to do with tachyons and whether tachyons can be so there it is right can we use taco to time travel to the past is what is asked there and as steve bermeo begins to answer tachyons are indeed hypothetical particles that's correct completely correct steve and uh the question is could you use them for backward time travel well again time travel to the past is a very thorny issue in physics we full well understand how to time travel to the future that's not mysterious it's a technological hurdle to achieve but as we've discussed in these sessions before there are many ways to do it you can travel out into space near the speed of light you come back near the speed of light your clock relative to a clock on planet earth will have ticked off far less time so when you step out of your ship you may be a year older but everybody around you could be five years older or a billion years older all depending on how close to the speed of light and how long your journey was so that is time travel to the future the past is much much more difficult and we don't know if you can do it most i think physicists would suspect that we can't but here's the thing with tachyons the weird thing about tack outs these hypothetical faster than light particles if i have a say a tachyon gun and i say fire it from one hand to the other from my right hand to my left hand and a tachyon in my frame of reference my perspective is traveling from the tachyon gun and hitting my hand if you're traveling in a spaceship in a particular direction of particular speed your interpretation of the scenario will be completely different from your perspective you will conclude that the tachyon hit my hand before i pulled the trigger that the tachium will hit my hand before i pulled the trigger so it isn't actually time travel to the past but it's an inversion of causality it's something that normally we would think about as happening in the future the tachyon hitting my hand you will say that that took place prior to the event that normally we would say happened earlier namely my pulling the trigger tachyon hits my hand before i pull the trigger from your perspective so that's the notion of the weirdness of time that emerges if tachyons are real that cause and effect can be interchanged now again as steve pointed out we do not think that tachyons are real but as a hypothetical exercise in understanding the properties of relativity it's an interesting exercise but who knows whether these particles are real many think that they are not road runner relevant to the conversation that i just had with adam isn't the cmb everywhere and therefore everything is relative to it yeah in a sense the answer to that is yes so when we talk about the cosmic microwave background radiation we're talking about radiation that means heat light is entering our telescopes or if you allow me to be more poetic it's entering our eyes we can't literally see it with our eyes but just imagine that we could and that might lead you to think that there's something special about where i am because the heat is now hitting my eyeball right here from the big bang but regardless of where i am in the cosmos the idea is that the heat would be there because this heat fills all of space so imagine that the early universe it is a ball filled with photons filled with heat and as it expands the photons are traveling this way and that but they continue to fill the space so regardless of where your eyeball is there is a photon from the big bang that is there ready to enter your eye so there's nothing special about where i am there's nothing special about the cosmic microwave background radiation that i my eyeball or my telescope receives there is something quite special though about a particular frame of reference with respect to which the heat left over from the big bang will appear uniform it'll appear homogeneous because if you are moving relative to that cosmological frame of reference then the photons will be redshifted they'll be blue shifted depending upon how you are moving and you will not have that homogeneity and indeed planet earth is moving relative to that cosmological frame of reference so when adam was referring to the cosmic microwave background radiation and its properties what we often do is when we analyze the heat left over from the big bang we subtract out the motion of planet earth namely we do some data analysis on the measurements of the microwave background radiation to figure out what it would have looked like if we were not looking at it from a planet that is orbiting a star that itself is orbiting the center of the milky way galaxy so we do this processing of the data to figure out what it would look like from that uniform cosmological frame of reference so in that sense everything is relative to the cosmo cosmic microwave background radiation as it sets the frame of reference relative which relative to which that radiation appears uniform roadrunner asks why does our frame of reference matter well our frame of reference matters because our goal our goal is physicists and and adam's goal we just heard from him is not to merely come to some understanding of what the universe looks like from planet earth that's interesting but however much we love our planet there's something else that we physicists love even more and that is the true nature of reality so we don't want to be biased by the observations that happen to be available to us and the features that they happen to have as we walk around earth we want to figure out qualities of the universe that would be true anywhere and any when and that is what forces us to understand what reference frame we are actually in relative to say the microwave background radiation subtract out the bias which that particular motion through space results in and thereby conclude what the microwave background radiation looks like to any observer anywhere who is not moving relative to this cosmological reference frame and that allows us to pinpoint qualities of reality as opposed to qualities of how reality happens to look from planet earth that's the difference and that's why it matters okay so what else do we have here anything else on cosmology or quantum mechanics um so um srinivas krishna asked did the big bang start one universe or the entire multiverse and and you know i can't help but bring up my fun little multiverse video i'm gonna have more i don't have very many interesting visuals today but look if each one of those spheres represents a universe okay that means that each one of those spheres has its own big bang if you will now how did it all get started from the get-go nobody knows but when we talk about big bang in the context of what you're seeing behind me in the context of a multiverse we're really talking about multi-big bangs each universe has its own big bang giving rise to its own expanding portion of space but the different universes have distinct big bangs and therefore the big bang did not start off the whole thing rather the big bang is the name that we give to the process that yields our expanding universe that is the difference between them and that's the language that we typically use so let's see so um shuvik mandal asks hello sir which latest research and physics do you think could bring a revolution in science like the revolutions associated with relativity and quantum mechanics and i i don't know i don't know obviously if i knew the answer to that question i would quickly leave this live discussion and get to work on that particular idea and yeah that's the big question that we all ask like we live these finite lives with a limited amount of time and when you start on a research project you could be embarking on something that takes months to years and when you don't have that many years left you want to choose carefully you know when i was a young graduate student i i really in some sense didn't care what i worked on i mean the area i wanted to work in cosmology and quantum mechanics but if there was an interesting sounding problem i'd be like yeah okay i'm in let's go and i'd work on it because at a young stage you have all these years of research ahead of you and you just want to get in the game and i think that's the way young researchers should approach the research projects that are available to them but as you get older you start to ask the question that shuvik asks which is which of those research projects really has the potential to be revolutionary right if you're already in the game then the next stage is hopefully to work on something that will be utterly revolutionary and and obviously no one knows the answer to these questions you only have post-facto analyses and you're like oh okay that was the idea that led to the revolution i mean look at adam reese when he started to work on the measurements of cosmological expansion he and his colleagues were set on determining the rate of slowdown of the expansion of space the rate of slowdown of the expansion of space and that would have been an interesting result to find but i don't think had it turned out that it really was slowing down i don't think it would have been revolutionary i i don't think i could be wrong but i should have asked him but uh but i suspect that that result would not have been awarded the nobel prize unexpectedly working on a project that was important but perhaps not revolutionary a revolution ensued because the result was not what anyone expected rather than slowing down and speeding up so it's just to say that it's very hard to to answer that question but my gut feeling shivok mandal is that the next revolution will come in fully understanding what space is right we know a lot about space we know a lot about time we know that space is expanding we know that time has these interesting qualities some of which i addressed in the first question about time travel but we still are at a loss to fully articulate what space is and what time is are they made of ingredients finer gradient ingredients is that the way we should think about the nature of the fabric of space and time is fabric the right word is space actually stitched together by threads like a piece of fabric is and if so what are those threads are those threads the threads of quantum entanglement are those threads the strings of string theory i don't know but i suspect that answering those questions is where the next revolution in physics is likely to come from that that is my guess all right let me head back to the question so viraj kapani is a new book coming from me well i'm there are new books not not from me i'm not at the moment working on a new book you know my last book thankfully is not that old yet it came out in february and the paperback has not even been released yet trying to figure out when to release the paperback in this strange pandemic world part of me wants to hold the paperback in reserve for when the world is a little bit more normal and you can go out and talk to people about your ideas and have events and things of that sort but but we'll see but that that's really uh the next book if you will it's the paperback of the old book i'm also working starting to work on some things that may not be interested interesting i should say to this audience in fact i'd be interested if you have any thoughts on it but i'm you know i wrote a fiction book for kids a very short book years ago icarus at the edge of time it was called about a boy going to a black hole and einstein's general theory of relativity does kick in and dictates how the narrative unfolds so there is an intersection with the kinds of ideas that we typically talk about here but i was thinking of flushing that out into a very short more of a parable i'm thinking of fleshing it down to a full-fledged full-fledged book that you know for kids sort of but also for adults that are just interested in in odysseys that have a scientific through line so that may be the next thing that i work on very different from the kinds of things that i've done before i'm just wondering anybody would anybody read that so if you would put in the comments give me some encouragement if that's a direction that that i might go um so at ram three letch asks mr green i'm extremely confused about the prominence of the intuition that smaller particles make up larger structures in physics do you think leaving it could offer any hope and i i'm not sure what the confusion is age ram adrian iii letch because to me it's so intuitive but maybe it's just because i have been living within the reductionist perspective all of my professional life and even before i became a professional physicist the notion that things are made of smaller things is so central to the way we have organized our understanding of reality and it has been so powerful right if you were to ask and i think it was richard feynman who was asked this question he was asked if we were to leave one sentence summary of the most important thing that science has ever discovered let's say we were given one it's sort of like a twitter for the future right we were given like 280 characters that would be delivered to the future or maybe delivered to a civilization that replaces us after we destroy ourselves and want some record of the most important thing that we have ever discovered what would that be and richard feynman said that though that matter is made of atoms that the world is made of atoms so the atomic hypothesis that all of the structure all the stuff in the world around us is nothing but a collection of atoms that we can see in the periodic table arranged in different configurations the fact that all the richness of the world right from from tables and chairs and trees and grass and clouds and mountains and dogs and people and and ponds and water all that stuff that all that stuff is nothing but atoms and a small collection of atoms brought together into different configurations he said that is the most important thing that we've discovered and and we have of course carried on with that very reductionist perspective that the atomic hypothesis articulates because we now know that each atom is nothing but a collection of smaller particles protons and neutrons and electrons arranged in certain patterns and we also know that neutrons and protons are also made of smaller particles quarks arranged in particular combinations and particular patterns and so that idea that reality is constructed from finer and finer ingredients brought together in ever more complex and interesting patterns when articulated not just in the english language but through the power of mathematics has allowed us to understand so much about the world so at dram three lech when you say do you think leaving that perspective could offer any hope i don't i don't think so and by that i don't mean that there shouldn't be other ideas because the reductionist perspective needs to be married together with synthesis not only do you want to know the fundamental ingredients you need to truly understand the rules by which they come together and hopefully understand even complex configurations of those particles that come together in a living system and a brain things that we've not yet been able to figure out so if you marry the reductionist perspective with this perspective of synthesis then to me that is the most powerful framework for making progress are there other ideas that might supersede these of course there might be but if you ask me i am quite confident that for a very long time if not forever that this will be the way that we think about reality all right um rob vandenberg asks if you were a betting man what would you put money on time is a partial or not i'm not exactly sure about the second half of the sentence i don't understand the first half but perhaps what you mean that time is emergent as opposed to fundamental maybe that is what that language means and let me know rob if i'm misinterpreting your question and if i was a betting man i'm not a betting man i'm very conservative in those regards i don't understand why people bet i don't understand people going to casinos where if you stay long enough you will lose why is that fun if you stay long enough you will lose the odds are skewed in the favor of the house why is it fun to play a game that's rigged anyway that's a separate question so let me take the betting person out of the story and just sort of adjust the second half i do suspect the time is emergent i do suspect that time ultimately will be viewed as not a fundamental quantity or concept within our understanding of natural law but rather will be viewed as something that emerges for something finer and what that finer thing is at the moment we don't know but if that is a correct interpretation of your question rob then if i were a betting man i would bet on time being emergent so lisa asks me what my favorite equation is i asked adam what his favorite equation is and i guess my favorite equation would be einstein's field equations from general relativity why do i feel that way well i have a certain nostalgia in some way for those equations because they're the very equations that i so desperately wanted to understand as a kid which drove me to study physics and ultimately understand those equations or at least as well as i can you know those equations r mu minus a half g minu times r right equals 8 pi g newton's constant over the speed of light raised to the fourth power times t muni and it's a geometry expression on the left hand side it's a matter energy expression on the right hand side and that summarizes einstein's brilliant breakthrough that gravity which emerges from matter and energy is nothing but a geometrical property of space and time and to me to think about the geometry the shape of space and time as being vital to the force of gravity that is just such a beautiful idea and the equation is such a simple reflection of that idea that there's nothing there's nothing quite like it bluesmokerh1 asks what is love i don't know what is love well at one level [Music] love is a human emotion and all human emotions emerge from my perspective and from that of anybody who thinks about themselves as a physicalist or a materialist emerges from the motion of particles inside of a human body inside of a human brain and so love is simply a particular pattern of those quantum mechanical particle motions now framing it that way sounds so mechanistic and so lacking in the feeling of the poets that you could easily turn to me and accuse me of reducing the most wondrous qualities of the world to the most base description and i don't accept that characterization because i am not saying that we should leave a description of love in the reductionist language i'm saying that's useful to recognize that human emotion emerges from the very same particle motion that allows a piece of swiss cheese or blue cheese or the moon or a bottle of water or anything to emerge everything physical in the universe every physical process in the universe is nothing but the quantum mechanical motion of particles period and a story and it's useful to recognize that human emotion doesn't stand outside that paradigm having said that human emotion can be experienced and it can be described can be communicated at a whole variety of higher levels of coherence and so we can describe human emotion in the language of the poet in the language of human self reflection and i am not saying that that's a secondary language i'm not saying that's a secondary description i'm saying that the deepest understanding of something like human loves comes from an amalgam of the description at the level of human self-reflection with the description given by the great poets that have walked the face of the earth together with the description that emerges from a quantum mechanical understanding of matter when you blend it all together you have the richest understanding of reality and in a sense that really is the a dominant theme in my latest book which is trying to understand the emergence of self-reflective conscious beings and to understand the kinds of behaviors they undertake within the rubric of a universe understood in terms of its fundamental ingredients and its fundamental laws so blue smoker i don't know if that's the kind of answer that you were looking for but that's my response to your very simple but deep question about what is love and look i'll just follow it up with a question from master e.t what is consciousness consciousness i would describe in exactly the same language no difference whatsoever consciousness i believe i cannot prove nobody can consciousness i believe emerges from the physical processes inside of a brain or my consciousness emerges from the physical processes inside of my brain the hard problem of consciousness that philosophers struggle with that psychologists struggle with that thinkers struggle with is how can that be how can it be that the inner world of self-reflective experience how could it emerge from the motion of electrons and quarks and protons and neutrons which themselves seem to evidence no inner world seem to have no ability to engage in self-reflection that seems to be an emergent property that only comes on the scene when you have enough particles in the correct arrangement but how could it be that particles that don't have inner worlds can somehow yield inner worlds i don't know the answer nobody does but i suspect that one day we will not be as puzzled by that mystery as we are today especially when we finally have sentient computers artificial intelligence that really convinces us that they the system the computer whatever convinces us that it is having the same kind of inner worlds that we have how will it do that i i don't know but i mean if you i may have said it before but if if i walk along central park in new york in the far future if i'm still here and on a bench there is a computer maybe it's even in the form of a humanoid and that computer is like doubled over in angst and i have a conversation with the computer say what's wrong computer and the computer says i'm so i'm so concerned with the nature of life i'm so concerned with how it is that i'm having this inner world of self-reflection how is it possible the computer says to me at that point i'll i think i'm likely to be convinced unless someone's maliciously just programmed the computer to do just that barring that possibility i will be convinced that that computer really is having an inner world and at that point i will really be convinced that inner worlds simply emerge when particles undertake certain processes and that will be that at least from my perspective so that that will be my notion of of how it we can talk about what consciousness is now i'm being told by some of you that my um my video is out of focus i forget how to fix the focus there's some advanced setting on this camera that i don't know so what i'm going to do for half a second is i'm going to shut my video for a split second and hopefully it'll come back and look better i don't know hopefully that helped otherwise i'm not sure how to i forget how to how to do it for this camera so maybe someone can tell me if it's really continues to be out of focus all right so other questions um what do we got here um uh pranav krishnasami any updates on the general relativity course in world science u i i i answer that question by first hanging my head in shame i've been promising that course for like six years oh my goodness five years or something like that well here's the update you know i'm not i'm not able to go back uh to to normal life nobody is and so we're gonna have the opportunity i think over the next few months to in our seclusion create some new things that otherwise i wouldn't have had time to do without traveling and my teaching will be all online so i am hopeful that i'll produce a new world science eu course i mean a full course not a master class but a full course and i was thinking of not doing general relativity and again you guys chime in because you guys are are part of that audience what i am thinking is that i would do a course on quantum mechanics i mean a full course on quantum mechanics you know soup to nuts as they say starting with the basic idea of classical physics the motivation for quantum mechanics really get into schrodinger's equation spend some time on the interpretational issues give you some of the mathematical technology for those of you who are interested to to analyze systems quantum mechanically and then end the course with some philosophical speculations about what quantum mechanics is really telling us about the nature of the world if that is a course that some of you would like please let me know if some of you would far prefer the general relativity course let me know there too but that that is the i'm i'm really hopeful that i'll be able to produce that by by january so so don't hold me to it but uh that is what i'm thinking now uh having said that let me ask you guys one other question and you know there's like this time delay in the stream so i won't see your your answer in in in real time but i'll see it 20 seconds later and so if i don't catch what you say i'll look back at it later but i want to continue doing these uh these these your daily equation live sessions we probably should change the name but i'm so used to it now but um number one it'd be really good feedback if you enjoyed the conversation with adam reese for those of you who were here for the first hour if you weren't here you know don't don't say anything but if you were hurt let me know because i think that's something that we're planning to do for subsequent of sessions we'll bring in more of the faculty from world science you number one and question number two is timing because it turns out that i do have to go back to teaching at columbia columbia university is where i i suspect maybe some of you know maybe you don't know i'm professor of physics and mathematics at columbia and the term is starting up and one of the courses that i'm teaching meets on fridays from 2 p.m to 5 p.m which as you will no doubt note conflicts with this time period so if i have to change i do have to change the time what should i change the time too my thought is what if we still keep it on friday but just me earlier like 11 to 1 11 to 2 depending on how much energy i have eleven to two be tight i'd have to go right from three hours with you guys to three hours of my class that may be hard but just friday mornings work what do you think about that or should i just do a different day like thursdays from from two to five or something like that so i'm more than happy for you guys to chime in with a suggestion on what time we should use as we go forward in the future but let me leave that at that and go back to some of the physics questions that you guys have been asking so i'm going to scroll back to see what i've got here and monjeb's tv asks according to string theory when particles collide some of the energy goes to another dimension and energy may also come from another dimension to ours that is all true could it be that some of that energy is dark energy that is a very good question and let me just note a few things you don't even have to who asked this question monjeb's tv mon jabs you don't even have to have the exchange of energy between the various dimensions to have the potential for the extra dimensions to be intimately involved and perhaps even explain the dark energy that i was talking about with adam it's a highly speculative idea but you have to bear in mind the extra dimensions of string theory they're not located at some particular spot in our dimensions they're everywhere they're located everywhere in our dimensions i mean what's a good example of that i have this um this little piece of paper here just happened to have it but i'm gonna roll this piece of paper up it's an instruction manual for something i don't remember what it was but here that piece of paper is now a tube right and if i was to make this tube ever tighter in its circular girth its circular dimension it'll become so skinny that you don't see it right so you would think that all i had in my hand was a line you wouldn't see the thickness if i made it sufficiently small but of course there is a circular part to the paper it's just harder to see but my point is this if i were to ask you where is the circular part where was the extra curled up dimension for the piece of paper you wouldn't point to one location along this linear dimension you'd point to every location there's a circle here there's another circle right next to it there's a circle here there's a circle here and right down the line circle after circle after circle after circle so the extra dimensions in this curled up piece of paper exist everywhere along the linear dimension that we can easily see and that's the model for how we think about extra dimensions and string theory some dimensions are big and obvious to see like the linear dimension of the curled up piece of paper and those dimensions in space we call left right back fourth and up down those are the three dimensions of everyday experience string theory says there are extra dimensions where are they well like with the circle in the curled up piece of paper they're everywhere the extra dimensions are here and here and here and here and here so they're all over the place they permeate space that's very interesting for many reasons but it's very interesting for dark energy this energy that suffuses space that we don't see why is it interesting well imagine that those extra dimensions of space each harbor some energy where would that energy be well that energy then would be everywhere throughout our three dimensions of space because the extra dimensions exist everywhere throughout our three dimensions of space that begins to sound like dark energy dark energy is energy that fills space that we don't see if there's energy in the extra dimensions which we don't see and those extra dimensions are everywhere in space and energy itself will fill space so that's an interesting idea and it's one that some people have developed i wrote a paper a few years ago on this idea with a physicist named jan 11. some of you may know jen 11 she also has written some wonderful books most recently black hole blues about gravitational waves and anyway so we wrote this paper some years ago suggesting that the dark energy might be bound up with a quantum mechanical source of energy called the casimir force or the casimir energy that naturally would give energy to these curled up portions of space and an answer that's a long answer to mon jeb's question that we suggest it might be the dark energy there are there are challenges with making this idea fully work but we did write out some of the mathematics of this and at least in principle i consider it an idea that that it's a dark horse candidate but you're talking about dark energy so perhaps a dark horse kennedy is the right thing but it is at least a possibility that the dark energy that we believe fills our space may indeed originate in the energy that suffuses the extra dimensions that a theory like string theory posits to be out there okay so that's a great question and allowed for a lot of physics but if there is a follow-up on that man jebs feel free to ask it jordan where jordan wire where asks hey brian how is string theory doing these days just fine i will say the last i checked there were a lot of problems that needed to be solved well that's a healthy state of any vibrant cutting-edge field of research once you've run out of problems the the theory is done have any of these been figured out in say the last 10-ish years thanks and the answer that is yes so there are you know tremendous amount of unsolved issues in string theory that's why it still keeps people generations of physicists busy but many questions that i would say physicists thought would be beyond our ability to address in say a 10-year time frame say like 20 years ago that they posited or imagined would not be addressable or not would not be addressed 10 years later which would be 10 years ago to match with jordan's time frame had been addressed for instance we even have non-perturbative formulations of the theory for a very long time all we had were approximate formulations of the theory and now in certain environments certain space-time environments we have exact of the theory there are certain descriptions of black holes that i don't think anybody thought we'd fully understand within string theory and there are certain aspects of those descriptions now we do understand now those have raised even more questions and that's sort of the nature of research you solve question number one and the very solution that you put forward now allows you to ask questions you couldn't even ask you couldn't even imagine before so so yes the theory is alive it's well it's making progress the big question though that we've not really been able to make much headway on is whether this theory can describe actual physics observations experiments that we can do today or in the not too distant future and that's the only way that we're really going to know whether the theory is an interesting piece of mathematics that might on paper put together quantum mechanics in the general theory of relativity but beyond that it's not actually relevant to the physics of our world it could be the case that that's what string theory is to go beyond that we really need the theory to make a prediction or explain some observation that happens out there in the real world and except for the most prosaic of things string theory has not distinguished itself in that domain as yet and that's that's where we hope things will change okay good question uh jordan also the previous one by mon jobs is a good question as well so i'm going to scroll you know there's so many comments here that it's always hard to stop on one that is [Music] relevant to the kinds of things that we're talking about today but i'm again happy to go elsewhere so a random question rigid asks is is warp drive possible and there are warp drive ideas that emerge in the general theory of relativity it's one thing for something to be possible that is not incompatible with the laws that we understand it's quite another to claim that we will realize that possibility so i would say that there are warp drive like scenarios that people imagine in the general theater relativity but we're nowhere nowhere near realizing them so don't think that anyone's going to be on a starship asking for warp 9 anytime soon okay anas asks can you do a daily equation on your paper dark energy and stabilization of extra dimension so that is the paper that i was just referring to an answer to mon job's question punjab tv dark energy and the stabilization of extra dimensions can i do a daily equation i could um it's really a matter of finding the time with so much to do but i will keep that in mind and maybe one weekend you know the kids are out maybe uh playing tennis or looking for frogs that's unlikely my kids don't really look for frogs much these days but if i have a free half hour i will try to sit down and do that yeah the daily equation on that paper would be kind of fun sandeep asked can you please confirm that a conscious entity is not needed to be an observer in a quantum mechanical experiment for example a detector one of the slits can make the observation and sandeep i do agree with you when we talk about quantum mechanics and the role of the observer to me observer is a stand-in for any physical system conscious or not that's able to interact with the system that's being studied and interact with it in such a way that the interaction itself disambiguates the quantum mechanical possibilities what i mean by that well in any physical system described by quantum mechanics which is any physical system because quantum mechanics rules across the board you're in a situation that the quantum laws predict the probabilities for one or another outcome but as far as we understand things today the quantum laws don't select one or the other outcome all of those outcomes that have non-zero probability are in principle possible but yet when an observer makes an observation only one of those outcomes is actually found and so i would define an observer as any physical system that can interact with a system that you're interested in that you're studying can interact with it sufficiently to ensure that one of those outcomes is selected and i do not believe that you need consciousness to play that role can i prove that i can't why can't i because i can't become aware of that singular result that one outcome from the many that quantum mechanics allowed i can't become aware of it without my consciousness being part of the story so the very fact that consciousness cannot be aware of something without consciousness means i can't be aware of my physical device having disambiguated the possibilities without actually looking at the meter and seeing one outcome and then you could say to me it was not the case that one result had been selected by that non-conscious piece of equipment it was only when you looked with your consciousness that that piece of equipment was realizing one outcome from one state of the observed system so i can't prove it but cindy my strong belief is that consciousness is not special to the quantum formalism others may disagree with that and i'd be happy to hear from anyone who does in the comments section so quantum mechanic asked brian we know nature is quantum mechanical so why is so much effort going into quantifying a classical theory like general relativity rather than begin with quantum mechanics and search for space-time gravity as an emergent phenomenon so look quantum mechanics you're you're right but you're describing a state of affairs that's very sophisticated when we study most physical systems we make use of a paradigm that was laid down you know roughly 100 years ago is we understand the system as best we can with classical physics and then there's a process by which we sew quantum mechanics into the formalism we quantize the classical system and you're right we shouldn't do that if we really were fully enlightened quantum mechanical beings we would start with quantum mechanics from the start it's a funny sentence we would start with quantum mechanics from the start we would begin our mathematical analysis with the quantum formalism as opposed to begin with the classical formalism and then overlay quantum mechanics on top of it but for many physical systems we just don't know how to do that we still use classical physics as our way in to our partial understanding of the system be it baseballs be it atoms be it a light itself right even in the study of light we begin with maxwell's equations maxwell's equations are classical equations they're partial differential equations that don't know about quantum physics that describe how electric and magnetic fields influence each other and in that way give rise to propagating wave disturbances that we call light in order to make that quantum mechanical we take maxwell's classical equations and we run them through an algorithm that algorithm is called the quantization algorithm and using that algorithm we can yield quantum mechanical versions of maxwell's equations and when we use those equations to describe things like photons and electrons and how they interact the quantized maxwell equations work fantastically well wow that is a beautiful trajectory from ignorance to understanding but in a more enlightened way of going about things we would simply write down the quantum mechanical description of electricity and magnetism from the get-go no one really knows how to do that without relying even in some hidden way on our classical understanding and so when it comes to the general theory of relativity we similarly would imagine we start with einstein's equations and try to quantize them now that's an approach that is taken by one particular school of thought loop quantum gravity if you will is an approach that takes more or less that perspective that einstein's classical equations are the starting point and we inject quantum mechanics to that that is not the approach that we take in string theory so in string theory what we do is we introduce a holy secondary idea which is that matter is made up of vibrating filaments called strings these linear filaments of energy they vibrate in different ways and that's a system that we can describe using classical physics anyone who's taken basic classical physics one of the systems that you study is the vibrating string and we know how to write down the equation for that indeed we know how to write the equation for that including relativity so the relativistic version of the vibrating string and then in essence and string theory we quantize that classical equation so we're not quantizing the general theory of relativity in string theory the way the loop quantum gravity people do we're quantizing the motion of a filament and then the remarkable thing is when we study the equations that emerge from quantizing the vibrations of a string lo and behold within that system we unexpectedly bump into einstein's equations in the general theory of relativity but now we are in a quantum mechanical description so we get a quantum description of gravity by quantizing the classical vibrations of a string who would have thought that that's like crazy talk but it's true so in answer to your question and i forget who asked this question i don't know sorry whoever asked this question it's a good one but in some sense the loop quantum gravity people are following the more prosaic trajectory quantizing einstein's equation and look then they do all sorts of miraculous and wonderful things that that are not obvious and are deeply creative so i don't want in any way take away from what they do but the string theorists approach to quantum gravity is far more indirect it is not simply taking einstein's equations and quantizing them it's doing this other intermediate step of introducing the motion of a vibrating filament recognizing that within the quantum mechanical description of that vibrating filament we have einstein's general theory of relativity and then saying aha string theory gives us a quantum theory of gravity that's the approach that we take which is quite quite different all alrighty so anything else you guys would like to talk about i'm going to scroll through here and um anas about salham in capital letters tells me that i found a way to unite gravity with electromagnetism in four-dimensional space-time and and in us look you may have discovered the final theory i'm not taking it away from you i have no idea what you found i can't pass judgment on it and i don't mean this in a mean way but i don't even want you to send me your ideas i'll tell you why i get so many unified theories sent to me through ordinary mail at columbia through my email which is not that hard to find that i could make a full-time job of reading other people's proposed papers on unification rather but i'd rather you do annas is take your paper and send it to one of the journals journals are set up for the refereeing process and if the referee thinks that your idea has promised we'll send it out to be judged by a member of the community maybe that'll be me i don't know although i have to tell you journal editors don't send me papers as frequently as they used to because i am really slow in reviewing them because i got so many other things going on but that's a footnote but on us my point is number one if you really think you have the deep theory send it to a journal go through the review process but the point i want to make is that simply uniting gravity with electromagnetism is not that hard if you're talking about classical ravaging classical electromagnetism in fact we have an algorithm for doing that the simplest way to do it is you simply look at the approach to classical physics that's called the least action principle you write down an action you write down the einstein hilbert action for gravity and you add to it the action for electromagnetism which anyone who studied electromagnetism knows that that action is f mu nu contracted with f mu nu where f mu nu is the derivative of the four vector potential a so f mu is d mu a new minus d new a mu and the contraction of f mu nu with f mu nu that is you contract them with the metric tensor on the four-dimensional space-time manifold that you're studying and you add that to the einstein hilbert action and there you have a unification of gravity and electromagnetism in four-dimensional space time and a story we know how to do that the more difficult thing is writing down a quantum mechanical union of general relativity and say electromagnetism that works in an arbitrarily curved space-time environment the quantum mechanical story is less obvious you can try taking that action and putting it in the quantum mechanical version that makes use of the action which is known as the feynman sum over histories so now if you want the transition amplitude between quantum state 1 and quantum state 2 you look at the transition probability that you get by integrating over all field configurations that join the initial configuration with the final configuration weighted by e to the i action over h bar sorry for talking in technical language but i think many of you know what i'm talking about here that however doesn't work for gravity or at least it's hard to make it work for gravity and if you use a standard approach so if you can solve that problem and us then i think you really have something and that's worthy of sending to a journal but a classical union of gravity and electromagnetism is something we know how to do okay let's see what else we got here scrolling down the questions maybe i should scroll up the questions there are a bunch here donatello asks couldn't you tell you're at the edge of the universe if all the galaxies you see are to one side but not the other and yeah you could in principle i mean if if our universe had a wall out there and actually there are some real theories that suggest that there is a a version of string theory where one of the 11 dimensions is compactified on a line interval and if that's the case and you could move along that extra dimension that line interval is finite in size and you'd hit a boundary and that boundary would be the edge of the universe so these ideas are not as as ridiculous as they might sound and yeah if there is a boundary to the universe and you happen to be right next to the boundary then what you see toward the interior would be different from what you see as you look to the boundary and yeah you know that you're there possible but that's not what we see is the point what we see when we look out into the universe is what appears to be appropriately interpreting the data a universe that's homogeneous and isotropic these are ideas that that adam briefly mentioned i don't know if he used that language but he yeah he did he used exactly that language now that i think about it in our earlier discussion from 3 to 4 pm and if those of you have come late go check out we had an interesting conversation with nobel laureate adam reese between three and four associated with world science u but he mentioned that the universe is homogeneous and isotropic by which we mean that if you look out in any direction it seems to be more or less on average like looking out in any other direction and moreover if i was to transport my location to any other place in the universe the view from that location on average would be the same as the view that i have from here homogeneity and isotropy but if the universe was not homogeneous it was not homogeneous and isotropic sorry starting to get dry mouth as we enter into our third hour here so i need to uh wet my whistle as they say if the universe were not homogeneous and isotropic then our theories of cosmology would be quite different and in the most dramatic case that donatella is suggesting if we were to look out in a direction and see a brick wall that would certainly affect the way we mathematically model reality we simply don't see that not that it's impossible but our most precise experiments have not revealed that road runner has a very cryptic equation as a question roadrunner says er equals epr and that is perhaps one of the deepest suggestions to emerge in the last few years er equals epr and it's an especially timely moment to discuss that equation er equals epr because er refers to a paper that einstein wrote with his colleague rosen in 1935 which is 85 years ago and in that paper they introduced what's known as the einstein rosen bridge which in more sci-fi language that we physicists also use is known as a wormhole a tunnel through space epr which is the right hand side of roadrunner's equation refers to another paper that einstein wrote in 1935 this time with two colleagues one of whom was also rosen but the p was podolski boris podolski and i do not think that einstein thought that these papers had anything to do with each other because the first paper as i mentioned had to do with wormholes and the second paper epr has to do with quantum entanglement nobody thought that there was a relationship between these ideas at all but in recent years led by juan maldasena at the institute for advanced study led by lenny gross lenny gross lenny suskind lenny gross brain faltering moment there my good friend leonard suskind at stanford there is a suggestion on the table that those two 1935 papers may be deeply connected and that's what this er equals epr is all about roughly speaking the idea is that quantum entanglement two particles that are quantum entangled there may be in some sense a secret wormhole that is connecting them so the epr of quantum entanglement may secretly be the er the einstein rosen bridge of that other 1935 paper so we're actually going to have a conversation at this year's world science festival on exactly that subject i'm not sure when that program will be out obviously we're doing it fully digitally but we're going to have a conversation that will involve i believe i believe lenny suskind will be in that program as will be some of the folks who are experts in quantum entanglement i don't think zeilinger has said whether he'll be in the program yet i hope he will be if you know anton zeilinger and you're maybe in his research group i mean i know him but if you're in his research group maybe hit him from another angle and say hey it'd be fun to do that program so i'd love for him to be part of it if he has the time and we will discuss er equals epr so in the next few months look for a program from the world science festival that will explain this idea in accessible detail but in more detail than i have time or even expertise to discuss here right now okay other other questions that we might oh here goes one that not quite scientific but uh those are always kind of fun i don't know who said it but somebody just asked what was the last movie that i saw and sorry i can't it went by too fast and i don't know who asked that but uh i'll answer the question what's the last movie i saw the last movie i saw was selma i'm pretty certain that's the last movie i saw which very timely movie you know about the uh the civil rights activism that was led by people like john lewis who just died us congressperson and uh it was a wonderful movie before that i think this movies that i saw were the uh were seven of the eight harry potter films that my son had me watch they were great to see i'd never seen any of them before and um yeah whatever expect his patronus or whatever it is he'll correct my uh my harry potter lingo no doubt patronum or something like that yeah so that's uh that is the uh the last movie that i saw um okay um shivam asks uh please listen to me brian okay i'm listening why is action such a big deal in theoretical physics why is it so beautiful well there actually is a daily equation that a full episode that i did on the least action principle so in more detail i would suggest if you want to see the equations which i won't do here right now you can go back to that iteration of your daily equation but why isn't such a big deal here's why it's such a big deal in physics and in our observation of the world we encounter so many distinct phenomena right you got whatever you got shooting stars you got the motion of planets you've got the motion of baseballs hit by a bat you've got the motion of electric and magnetic ripples through an electromagnetic field i mean you've got a wealth of different physical phenomena and so naively you would think you have a wealth of different mathematical descriptions of that wealth of different physical phenomena but amazingly at least speaking classically for a moment amazingly the equations that describe all of those phenomena we have found all emerge from one basic principle and that is the principle of least action which is roughly speaking the idea that associated with any physical system is a quantity called the action and every physical system if they differ sufficiently they have a different formula for their action but if you minimize the formula for the action of a physical system the equations that result from minimizing that action are the classical equations that govern that system so here you've got a wealth of physical phenomenon that can be described mathematically by a single algorithm write down the action that's relevant for that system which if you care about it is the kinetic energy minus the potential energy so you write down the kinetic energy minus the potential energy that's relevant for that physical system you minimize it and the equations that ensue are the equations wow right what a beautiful unity at the core of classical physics that all of classical physics can be reduced to finding the minimization or the extremization of this quantity called an action when you go to quantum physics richard feynman showed that that unity carries over in a far more complicated way but as i mentioned before you now take that action you exponentiate it you put an i in front of the action so you're looking at e to the i action you divide through by planck's constant h bar and then to find the quantum mechanical properties of that system you do the path integral the sum over all histories that that system can engage in weighted by e to the i action over h bar that's a little bit more complicated i don't expect you to understand that if you haven't studied the path into approach to quantum mechanics but my point is even quantum mechanically all physical phenomenon slot into the same rubric path integral where the dominant term the essential term in that sum involves the action so that is the answer to why the action is so beautiful and how it is so ubiquitous in our understanding of physical law uh shavok mandal says i read your book until the end of time and there you describe boltzmann brains but i didn't understand that can you explain it to me okay i'm happy to do that chevoke so in the far far future as i explain in until the end of time i use the metaphor of you know walking up the empire state building and letting each floor represent the logarithm say of the time scale so floor 50 would represent the time scale 10 to the 50 and for a hundred would represent the time scale 10 to 100 years since the big bang that's the units that i use but the numbers are so big that it doesn't matter what unit you use and what i argue for is that in the far future the universe will have decayed it will have disintegrated even black holes will have evaporated and all that will remain are particles that are wafting through the void that's the fate that awaits all matter all complex matter and you could at that point say that that's it that's the end of time right once everything is just disintegrated into particles wafting through the void the particles continue to meander this way and that but but that's it and for a very very very long time that would be a correct assessment that nothing happens and therefore you call it the end of time but if you wait exponential of exponential times like 10 to the 10 to the 68 years not 10 to the 68 years not the 68th floor of the empire state building in our logarithmic visualization but rather a 10 raised to the 10 raised to the 68th power over those time scales something else can happen and what can happen is even the random motion of particles in the void every so often can cause them to collide and stick together and yield interesting agglomerations interesting conglomerates of particles and among the interesting conglomerates of particles that on rare occasion can spontaneously form from particles wafting through the void the configuration that can emerge is that of a human brain a human brain is just a constellation of particles as i mentioned before and so if you wait long enough on the order of 10 to the 10 to the 68 years then just the random motion of particles through the void can cause them to overlap to coalesce into a structure whose physical configuration is that of a human brain and that's what's known as a boltzmann brain a boltzmann brain is a brain that forms out there in the void of space from the random motions of particles obviously it is a rare occurrence and that's why i mentioned you'd have to wait at least 10 to the 10 to the 68 years to have a fighting chance for this to happen but if a brain did form out there in the void the suspicion the hypothesis the speculation is that that brain would think i mean literally if that brain happened to have the particulate arrangement that my brain is enjoying right now then that brain out there in the void would think it's me that boltzmann brain would have the thoughts that i'm having right now because that brain by hypothesis has the same particulate arrangement as my brain and nothing but the particulate arrangement determines the inner world that i'm now experiencing so that brain in the void would think it's having a live session in a little room in upstate new york sitting in front of a camera and having a conversation called your daily equation live that brain in the void that boltzmann brain would think it's having that experience now that brain you wouldn't expect to last very long because space is inhospitable to brains that are just floating there but you know you can imagine even more extreme circumstances more extreme particle configurations in which not only does a brain form but maybe a body and maybe a planet and maybe a solar system and therefore you could imagine that the random motion of particles could spontaneously cause the whole thing that i'm familiar with to emerge in the void and that sets up an interesting conundrum because then how do i know that i am not now that brain i don't i could be a boltzmann brain in the void that's a possibility i think it's unlikely but it's something that we have to admit the possibility of and in my book and in you know who asked this question somebody asked this question now shabook mandala said so when you read about this in until the end of time my point there in those late chapters not to convince you that we are boltzmann brains although we could be but rather to note that physicists pretty much use boltzmann brains as a diagnostic tool if our theories really allow for boltzmann brains we question our theories or if our theories make boltzmann brains likely we question our theories we are more interested in theories that don't set up this skepticism that what we see here is real and boltzmann brain certainly do set up that skepticism because that brain floating in the void even though it thinks me me thinks it is me and thinks that it's having my experience and has all of my memories none of those memories are real because they just were imprinted on that brain by virtue of the highly choreographed motion of particles reproducing my brain out there in the void and that really sets up a kind of skeptical nightmare where we if we buy into this view can't trust anything we can't even trust the very observations that support the general theory of relativity and quantum mechanics which are themselves vital to concluding that there could be boltzmann brains at all so everything that you care about winds up having its legs cut out from under it if you buy into this boltzmann brain perspective and that's why many of us hope that our theories will one day steer clear of this possibility show why this possibility can't be true but that i hope that clarifies what boltzmann brains are and why it is that people talk about them so chris mortlock asks apologies if this is a somewhat dumb question don't worry about it man we're all just here hanging out not judging i'm no science expert but i read that atoms are mostly empty space but what sort of empty is this is it a vacuum of some sort or something else that's a great question chris great question in fact you know my my daughter who's with us up here as my son is too decided to take an online chemistry class this past week i was really surprised pleasantly surprised that she she actually took an online camera and had to pay for this chemistry class this is free right she gotta pay 60 for five hour long chemistry lessons but it was worth it she learned all about the various atomic models that have been developed over time and she actually made a version of the rutherford model which is the solar system like model where you've got the electrons going around the nucleus and as chris notes between the electrons and the nucleus it is mostly empty space like my daughter she made it out of you know coat hangers put the electrons on the coat hangers and you got the nucleus hanging from you know the top of the coat hanger but it's empty space between them and i told her if you made that model to scale and your proton is the size of a ping ping-pong ball then your electrons i haven't worked it out but your electrons would be like you know like miles away that's how much empty space there is if the proton's like a ping-pong ball i think i once worked this out that if the proton was the size of a ping pong ball that my daughter used but rather the size of a p then the electrons would be sort of in the bleachers in yankee stadium if the p was on the pitcher's mound so anyway i don't know if those details are correct but the point is there's a huge amount of empty space between the electrons and the nucleus and chris mortlock asks what kind of empty is that and the idea is that it's not the vacuum in the sense of the a region of space that's completely devoid of any matter because protons have positive charge electrons have negative charge which means that they are surrounded by an electromagnetic field so you've got electromagnetic field that's you know permeating around the particles and it spills off into space overall the atom is neutral because the positive of the nucleus cancels against the negative of the electrons but there's a lot of interesting electromagnetic phenomenon that's happening within the empty space of the atom so on average we would say that that space has zero energy on average we would say it's truly empty but there are all sorts of electromagnetic fields that are fluctuating up fluctuating down quantum processes with particles popping into existence and disappearing as they annihilate against each other so it's kind of a seething arena of activity which is pretty different from the placid version of the vacuum that i think we often have in mind when we're thinking about empty space in that language so the empty space inside an atom is a complex arena it is not as simple as the name would suggest okay so priyank dash asked me a question uh dog about my daughter which class is she in she's in um going into seventh grade i guess so it was a sixth grade type chemistry class over the uh the summer i don't know any more detail than that i hope that addresses the question so uh what else do we have here guys um so many questions so little time what should i take um um mshare asks why why why in equals mc squared is light the conversion factor what is unique about light good question and the special thing about light of course is that it's a universal number it's a number that everyone agrees on regardless of their motion or where they are if they correctly calculate the speed of light you get a number that everybody will agree on so the real special thing about equals m c squared is that the conversion between mass and energy is a universal conversion it's not a conversion that fluctuates not a conversion you know when you when you go from euros to dollars that conversion changes from moment to moment as the monetary factors affect the ways in which dollars and euros are selled back and forth sold not selled sold back and forth to one another that's not the case with the conversion with the equals mt squared it's a fixed value once and for all and that's really the importance of the c squared there what's also important about the c squared and perhaps somewhat surprising is that in everyday units c squared is a big number which means if you measure mass in everyday units and then you multiply by c square this huge number in everyday units the energy you're getting in everyday units is big so that should help your intuition that small amount of matter can yield lot amount of energy right we know this from you know the devastating nuclear weapons that were used to conclude world war ii only had you know a couple of pounds couple of kilograms of fissile material that was actually actively engaged in the processes that yield the energy and yet even though there were a couple kilos the energetic output was devastating deadly devastating but other than that the c-squared is not as fundamental as it might seem it's universal it doesn't change in everyday units it's a big number now i think you're also asking so like why is it there and the reason why it's there is that the special theory of relativity which einstein used to show that e equals m c squared the special theory of relativity is not a theory about relativity per se in fact einstein himself wanted to call special relativity in variance it doesn't quite have the same ring as special relativity but to him what we call special relativity was not about motion being relative and things of that sort it was really about the things that are not relative it was the things that are invariant it was about identifying qualities of the world that are independent of your motion and the fact that the speed of light is one of those things is deeply surprising because speeds typically do depend on your state of motion but if you think about special relativity as in variance theory and you think about the speed of light as the ultimate invariant the ultimate number that doesn't change and if you recognize as einstein showed us that equals mc squared emerges from relativity now perhaps it's not that surprising that the dominant invariant quantity c emerges in the dominant invariant equation equals m c squared and the fact that it's c squared the two that it's squared that's just in order to make the units to work out if you write down the natural units for an energy in the natural units for a mass then you have to multiply the mass units by a length squared over a time squared in order to get the natural units of an energy and that's why c squared comes in as opposed to c i hope that addresses the question there's no real full answer to your question but i hope that sort of chips away at the confusion by shining little brief moments of illumination on the question wow that answer had a lot of metaphors i think it's just metaphored myself out no more metaphors for the remaining time that we have together okay so oh what else do we have um uh um caramatt asks why should we know everything and and uh caramatt if that's how your name is pronounced no we we shouldn't and i don't think we necessarily ever will i think it's an ongoing process of discovery to try to understand how the world works but in no way can anyone establish that the human brain has the capacity to understand all the deep truths of reality i think i gave this example before but i think it's worth recounting you know during the first hour if you were here when i was talking to adam reese you know that you heard my dog barking for a period of time my dog is smart i'm pretty we got two dogs i'm pretty sure that was scarlet scarlett is a smart dog maybe she's even listening to the stream right now but there are things about the nature of reality that i pretty much suspect that scarlet cannot grasp i do not think a dog mine or any other can grass schrodinger's equation of quantum mechanics i do not think that my dog scarlet or any other dog can grasp the einstein field equations i could be wrong scarlet and other dogs listening to this dream may be collectively rolling their eyes as under their breath they bark out how silly that human being is we all learn schrodinger's when we're puppies that may be happening i don't think so and yet what this means therefore is that there are smart beings on the planet that are limited in what they can discern about the nature of reality and why should we be any different from the dog why should we necessarily have the mental acuity the mental expansiveness the mental capacity to understand the ultimate deep truths of the world the answer is we may not the answer is we've done really well so far pat ourselves on our collective backs for developing quantum mechanics and using it to understand the electrons magnetic moment its magnetic properties to better than one part and ten to the twelfth i'm not joking when i say that every member of the species deserves to stand up and take a bow for that achievement because we're all responsible for it it is an achievement of a small group of individuals but those individuals could not do the calculations that they do if it wasn't for the mass activity of our brethren over the course of thousands of generations that allows us to be here at all so we should be proud of what we've been able to figure out no doubt about that but it would be hubris to say that this particular physical structure called the human brain has the ability to understand every truth about the world i'm not talking about encyclopedic capacity to store every truth about the world i'm saying that maybe even the deep true laws of physics and perhaps those laws are articulated by one equation we perhaps have been unable to find that equation we perhaps would be unable to understand that equation even if it was revealed to us by some alien super race so there's no reason to believe that we won't at some point get stuck now the amazing thing is we've not yet gotten stuck look what we have been able to figure out not just the electron's magnetic moment we've been able to work out equations that allow us to peer back to a brief second after the beginning we've worked out equations that allow us to understand how time elapses in different circumstances equations that allow us to understand as we spoke about in the first hour how space expands equations that have allowed us to peer with our equations into the far future and understand the kind of fate that awaits us as described in my book until the end of time and as we recounted here that black holes will evaporate between the 68th and the hundredth floor of the empire state building between time scales 10 to the 68 and 10 to the 100 from the big bang we've been able to work out the weird possibility that in 10 to the 10 to the 68 years we might have boltzmann brains something we discussed in answer to another question these are really wonderful achievements we've not yet gotten stuck but we may get stuck it may be the case that we'll have to say wait for the brain to evolve into a more robust structure before the next leap in human understanding can take place is that possible yeah that is definitely possible sherry asks when we say light is a disturbance of the electromagnetic field doesn't mean that the electromagnetic field actually covers all 3d space so it weighs when there is a disturbance as in light and i think that's a fine way of thinking about it shari think about the electromagnetic field when it's in its most docile least energetic state from a classical perspective imagine that the electric and magnetic fields say far away from any sources far away from any electrons far away from any magnets that that field drops ever closer to zero so if there is a large expanse of empty space in the classical intuitive sense the electromagnetic field will drop to zero but it's still there much as you suggest even though its value is zero the field is still there and so when we disturb that field by causing some charges to accelerate sending out a disturbance that ripples through that field there is a field to be rippled already in place if you will and this disturbance just ripples through it so i think that's a fine way of thinking about that shari somebody uh keeps talking about road runner i'm not sure if there's an actual question there but stephen brown asks is a magnetic monopole possible what's a magnetic monopole well let's think about an electric monopole an electric monopole is just say an electron it's just a source of the electric field that has one sign electrons have negative charge protons have positive charge in isolation each of them is a source of an electromagnetic field that radially spreads outward from the location of the particle the electron or the proton or any other charged particle of matter a magnetic monopole would be the analog of that it would be say one particle that say is just a north pole or another particle that is just a south pole giving rise to a magnetic field that emanates from those particles now you may note that you have never seen an isolated north pole of a magnet when you're a kid or beyond if you still like magnets you found that when you buy them they always come in pairs there's a bar say that has a north pole and a south pole there's a horseshoe that has a north pole and a south pole and you say well okay that's just the way it came from the store if i just want to the north pole of that bar magnet i'll just lop off the north pole and throw away the south pole and then i'll just be left with a monopole a single pole and the answer to that as you no doubt no is that that doesn't work because when you try to lop off the north pole where you cut the magnet a south pole appears so you now have a smaller piece of the magnet a smaller bar but it still has the north pole as before but now the new end acquires a south pole you're unable to cut up a bar magnet and isolate a single pole the north pole or the south pole by themselves and so there's no evidence that's actually representative of a fact that there's no evidence that magnetic monopoles actually exist in nature but the question from stephen brown is is it possible and the answer is yes it's possible the mathematics of maxwell's equations of electricity and magnetism absolutely allow for the possibility of magnetic poles magnetic monopoles and in fact the equations are more beautiful if you include magnetic monopoles there's a particular equation called the divergence equation in principle i can write down equations using the system we have here but i'm not going to bother and the divergence of the electric field according to maxwell is proportional to the density of the electric charge the density if you will of the electric mono charges the divergence of the magnetic field is usually written down as zero because maxwell's equations assume that there are none of these magnetic monopoles but if they do exist then the divergence of the magnetic field would be proportional to the density of the magnetic monopoles just like the divergence of the electric field is proportional to the density of the electric monopoles and that would be a beautiful symmetry between electricity and magnetism which is almost but not quite realized in the the versions of maxwell's equations that we currently hold to be true so nobody knows if magnetic monopoles exist there was a discovery or an announcement on valentine's day i believe it was 1982. don't hold me to that date but you know roughly roughly 28 years ago there was an announcement by a group that they had found a magnetic monopole there's a big spike in the output from a detector that the group used and they thought they had it but no one has been able to reproduce that discovery of a magnetic monopole and therefore most people today believe that that was an erroneous announcement that magnetic monopoles are not real but are they possible an answer to stephen's question absolutely they are possible no evidence yet uh dominic asks how does string theory seal the bond between general relativity and quantum mechanics i don't understand and look uh that's a deep question and there's a mathematical answer to that question which is the one i indicated earlier when you quantize the motion of a vibrating string within the resulting equations you find einstein's equations of gravity so right there there's the unity the bond the union the coming together because in the motion of a vibrating filament quantum mechanically there is einstein's equations wow that's cool i don't know if that gives you much intuition though for why string theory succeeds in unifying quantum mechanics and gravity where a theory without string theory doesn't succeed and i can give you some intuition for that so one of the reasons why it's hard to quantize einstein's theory of gravity is this einstein describes space in terms of warps and curves in the fabric okay right you have the sun it warps the earth you know those are the familiar images that come to mind when we think about einstein's theory quantum mechanics because of the uncertainty principle always talks about a certain amount of uncertainty which translates into a certain kind of jittery behavior undulating behavior in any physical system now when that physical system is the fabric of space those jitters cause the fabric of space to widely undulate and on very short distances the undulations in the fabric of space are so spectacular according to the mathematics that einstein's vision of space as nice gently curving geometry and gently curving geometrical shapes that image is shattered that image breaks down and so it's the undulations from quantum uncertainty that runs smack into the central paradigm of general relativity that the fabric of space is gently curving how does string theory resolve that tension well here's what it does string theory says you can't ever get smaller than the smallest ingredient in your theory strength base says it doesn't make sense to talk about the fabric of space over distances that are smaller than the size of a string and basically we choose the size of a string to be sufficiently large that the jitters that arise at that scale while non-zero are not big enough to shatter einstein's image of a gently curving geometry the jitters get bigger the smaller you proceed into the micro world and we cut off the jitters by saying you can't go smaller than the size of a string that's how we put gravity and quantum mechanics together in strength here and that's really a fairly accurate description of the the actual mathematical story of how of how this works i hope that gives you some intuition for how that works blue smoker notes that you can't handle the truth some people can't certainly those like jack nicholson had trouble articulating the truth and thought that others couldn't handle it i don't know who the jack nicholson of string theory would be we'd like to think that we are intrepid explorers willing to accept any truth that might present itself to us okay that was a long waste of time to an offhand remark that i found in the youtube comments so let me now get back to business here arjun asks if the speed of light is constant no matter what then how does a prism work and my answer to you are john is that we're sometimes a little bit loose in our language when we talk about the speed of light being constant and not changing what we mean is the speed of light in empty space the speed of light through a vacuum not the speed of light through a material substance we have long known that when light transitions from going through empty space and going through a material object like a prism in your particular case we've long known that the speed of light does change and we have measured to fantastic accuracy the speed of light not just through empty space but the speed of light through a whole variety of different substances and you're right a prism works by virtue of the fact that different wavelengths of light have different speeds in a prism in some sense and that causes the frequencies of white light the various wavelengths to separate and undergo different bending and that's how we separate light into its component colors and that does rely upon the speed of light not being constant in a material medium something we've known about for a long time not all in contradiction with our claim that the speed of light is constant because what we mean by that is the speed of light through empty space so sometimes we're loose language but i hope that clarifies that confusion and yeah snell's law and people are chiming in with their own understanding of how light and glass light and a vacuum works satyam asks what happens if the cosmological constant value changes right now will there be an immediate effect and it depends on how dramatically the cosmological constant were to change you know if in this region of space that we're in right now if the if the value of the dark energy were to suddenly grow by many many orders of magnitude then yeah the world would change the world would radically change it could it could change by such a large factor that material objects might not even be able to hold together any longer the repulsive push of the repulsive gravity from a cosmological constant might below material objects apart atoms might be ripped apart electrons that we were talking about before in answer to someone's question i can't remember the name any longer but the question about are atoms mostly empty space so you've got electrons going around the nucleus if it's mostly empty space if the dark energy or the cosmological constant were to change in that region of empty space and become fantastically large it could blow electrons away from the nucleus it could rip atoms apart so it all depends on the scale of the change but yeah if the scale of the change is large enough then that is something that we would feel that would be something that would change reality as we know it all right we're getting toward the end of our conversation here but i'm willing to go another 15 or so minutes to cross the three-hour threshold which we've only done once before partly because the first hour was um made so much easier by that conversation with adam reese so i've only had to talk non-stop for two hours that's always nice uh but um let me see if i can find some good questions that uh yeah they're scrolling by so region asks can the future influence the past is that what the delayed quantum choice experiment proves some people say that region written ch i don't think that's the case at all i'm a pretty firm believer in causality i'm a pretty firm believer in the past influencing the future but the future not influencing the past in that kind of causal sense of signals being sent back in time but there are these funny experiments and the delayed quantum choice experiment is is one of those where it seems as though a decision you make in the future affects the past what's the quantum delay choice experiment well we all know about the double slit experiment where you can choose to measure the substance that you are examining in a manner that highlights its particle-like qualities or it's wave-like qualities and if you highlight its particle-like qualities then it behaves like a particle if you highlight its wave-like features then it behaves like a wave that has interference pattern so you get different results depending upon what you do in the past the delayed quantum choice allows you to delay your choice of which quality of the object you study in the past until apparently enough time has gone by that the particle itself had to commit to either being a particle in a wave and then it seems as though what you do in the future can change that commitment can change the past and that's the language that's often used but that's not what's happening so if you want the detailed explanation look at my book fabric of the cosmos i believe it's chapter seven but in a nutshell it's simply the case that your future action influences the experiment in a way that looks as though appears as though you get the same result as if the password changed but that's very different from saying you change the past you change the experiment at a given moment in the future and you change experiment which influences the results and it just so happens that you influence the results in a way that looks like what it would have looked like how you influence the past but did you actually influence the past no i do not think that you did okay um a name in arabic that i don't know how to pronounce says does a quark have an effect on space-time fabric and yeah a quark has energy and what we really learned from albert einstein excuse me for one second what we learned from albert einstein is that any energy at all influences the space-time fabric so a cork has very little mass very little energy so it has very little impact on the structure of the space-time fabric but it does and i'm influencing the space-time fabric right now but i'm not influencing it very much that's why this camera and this monitor they're not falling toward me the way they fall toward planet earth planet earth influences the space-time fabric in a far more dramatic way it has more mass the impact on the space-time curvature is greater i have little mass and my impact on the space-time curvature is less but i am influencing i am impacting the space-time curvature no doubt about it that is what einstein teaches us so similar question to the one before about the electromagnetic field rom asks is light everywhere in empty space and then the second question that rahm asks is what are the odds that professor green will answer my question huh well i haven't actually answered your question yet so i don't think people can calculate those odds but i am about to so is light everywhere in empty space in a sense yes depends exactly what you mean by light all you know in everyday usage light refers to very specific vibrations of the electromagnetic field that our eyes because of evolutionary pressures of having evolved in an environment that has a sun our eyes are well attuned to responding to the wavelengths that the sun emits our way our eyes are not well attuned to see x-rays or infrared rays or gamma rays right those are a form of light but our eyes aren't particularly good at seeing those wavelengths but if by light you mean the general property of an undulating electromagnetic field regardless of which frequencies those undulations are undulating at then in a sense light is everywhere because the electromagnetic field extends everywhere there are places where its value may be incredibly close to zero so close to zero that it's as if it were not there but a zero field is still a field and that's a quantum mechanical notion because the zero field will still fluctuate about zero from the quantum mechanical processes of quantum mechanical uncertainty in a classical world if a field has zero value there's nothing that commits you to saying that the field is there the difference between a field having value zero and being absent there's no observational consequence so it's really a matter of choice but in quantum mechanics there is a difference between a field having zero value and a field not existing because a field that doesn't exist can't quantum fluctuate a field that does exist and has zero value can still fluctuate about that zero value and that's the picture that quantum mechanics takes us to so in that quantum mechanical sense the electromagnetic field aka light does exist everywhere hope that addresses the question okay so danny bittman is 25 too old to go back to school to study physics and no although you have to really be realistic about what your goals are maybe you are the next einstein danny and all you need to do is unlock your inner physicist and all wonder of spectacular result will follow that's possible of course it's unlikely it's unlikely that any person is an albert einstein i'm not an albert einstein you heard nobel laureate adam reese in the first hour who won the nobel prize at 41 years old said that he's no einstein so so look you have to manage your own goals and expectations but if your goal instead is just to deeply understand the workings of nature understand the fundamental equations that guide the universe if you perhaps want to contribute whatever you can contribute maybe getting a job maybe not getting a job which is another thing to bear in mind if you're willing to sort of take all those those risks and still find it worthwhile at the end of this journey if you don't get a job and all you do is commune with the universe at a deeper level because you understand its inner workings you understand the equations of einstein and schrodinger and find men if you if that's enough then i would say go for it but you know people still need to eat and if you're not independently wealthy danny i'm not sure you know oftentimes when you get a degree in physics you are very marketable to other fields unfortunately i can rattle off a number of names of my phd students who got the phd and now work in wall street or work in some kind of hedge fund stock investment capacity i get it you gotta eat and it's hard to get a job in the academic world not impossible but the point is if you hunger for that connection to the mathematical underpinnings of the world if you hunger for that connection to the physical processes that you can study as an experimentalist then yeah 25 is not not too old to achieve that again if you're einstein like i said you can achieve anything even if you start late but if you're a mere mortal like most of us it's hard to get a job and it's hard to succeed in that academic world sense but that should not put you off if you hunger for it sufficiently so samir knows that even einstein worked at the patent office and that that's true too that's a little bit different einstein didn't start late einstein followed the usual trajectory went to school i mean he missed a little bit of school for a period of time but he was not particularly old when he got his degree in physics he did kind of irritate some of the faculty members because he had no patience for people who weren't as smart as he was and that irritation led those faculty members to not work very hard to get einstein a job in the academic world and so it was einstein's boyhood friend well school friend really marcel grossman whose father gets einstein a job at the patent office so yeah even einstein spent time out of the academic world all right so we got about five minutes more if uh if you still want to hang out here um what other questions uh roadrunner says you aren't a mere mortal i don't know who roadrunner's referring to but if it's me i can assure you i'm a mere mortal in in in every category that that you might imagine and the mere thought of that not being the case is causing my wife who may be listening to this in one ear as she listens to a podcast in the other to roll her eyes to roll her eyes no doubt um let's see uh actually i haven't gone to the uh the youtube list in a while let me just see if there's anything there that i prop should have uh answered um loop quantum gravity from youtube asks are gravitational waves useful in detecting extra dimensions and also strings and and it may be it may be you know because as as gravitational waves propagate through space they're causing space to ripple that's what a gravitational wave is and if there are more dimensions of space in the three that we know about those extra dimensions in principle can affect the precise details of the rippling perhaps even affect the speed of the ripples or perhaps even might cause ripples of different frequencies ripples are different wavelengths to travel through the higher dimensional spatial expanse at different speeds so it is conceivable that gravitational waves may give us give us insight into nature of string theory and extra dimensions nothing that i've seen yet that really brings that idea home in a convincing way in an observational way but yeah that is a real interesting possibility for how gravitational waves and their discovery just a few years ago may play a vital role in understanding the next chapter of fundamental physics okay what else i'm going to do another youtube one for a second uh mv asks on youtube could or maybe this is the world science no it's from the world science festival page i'm sorry that wasn't youtube at all i got it wrong this is from the world science festival page as opposed to the youtube page because you can be watching the stream i think you watch on youtube as many of you are as like i said 774 of you are right now but you can also watch this on the world science festival page or the world science you page as far as i know i think it's all the same but they're different communities in these different places so you know if you want to hang out with one group or another feel free to do it okay so mv asks on the world science festival page could dark matter be the cause of repulsive gravity and the answer to that question is no as far as we know no because here's the deal particles that clump together into larger collections of particles clumpy things like particles give rise to the usual attractive gravity it's only when the energy is completely uniformly spread out through a region of space that einstein's equations show that repulsive gravity can emerge and so particulate matter which is the quintessential kind of clumpy stuff is exactly the wrong source for repulsive gravity it's only the spread out energy field that dark energy is a linguistic summary of that's what we mean by dark energy it's just energy that's uniformly spread out and we don't see it dark we don't see it energy is just another word for uniformly distributed it's a uniformly spread out energy that gives rise to repulsive gravity that that is the key quality that gives it there all right maybe one or two more questions before we wrap it up um trevor iron wolf says only 800 viewers for the green man yikes most of the world must be asleep yeah i don't know only 800 viewers oh you know i like talking to you guys so even though it's irrelevant even though we're a small tight-knit community of now 782 people it's uh it's good to talk about yeah over time i notice that over time people watch these live streams i'm surprised i thought that people would only really want to watch them in real time like we're doing right here but people enjoy watching them after the fact i think some of the sessions that we had some of the questions that you asked some of the sessions have well over a hundred thousand views in a few weeks so that's fun you know spreading the word somebody asked how do you spread the word of of science and that this is this is one of the ways of doing that of course um harveer asked did you ever make any did you ever have any collaborations with mathematicians when you're involved in string theory and absolutely so one of my most proud moments my most proud discoveries was a discovery a while back where we showed that the fabric of space can rip apart topology change or the more precise kind of topology change it was called a flop transition flop is not like the word the work was a flop it's a it's a mathematical term for what can happen to a certain kind of two-dimensional sphere embedded in a higher dimensional space i won't go into the details of it but the way this result the way this discovery came about was i was at the institute for advanced study in princeton and i was sharing an office with somebody that i knew a mathematician named david morrison and dave morrison and i he's a mathematician i was a physicist we decided to lecture to each other every day every evening actually so every evening i'd give him a lecture on physic and he'd give me an evening on electron mathematics and we need to go back to our our separate rooms and study what we learned from our mentor in the other field and little by little through this mentorship we recognize that we could combine our expertise together with another mathematical physicist named paul aspinwall who's now at duke david morrison's at ucsb we were able to combine our understanding from the math and the physics to prove that within string theory the fabric of space could rip and this result would not have emerged if it wasn't for this collaboration between someone who was deeply embedded in the world of math with paul asmo and myself who are deeply embedded in the world of physics so yes exactly right all right so we've we've passed i think the um the three hour mark so i'm gonna wrap it up with that a couple quick things before we end again as i mentioned let us know if you enjoyed the first hour conversation bringing in a well-known person from the world of physics or mathematics or some other field as we go forward usually someone who's done a world science you master class so go to world science u and and check out the master class of adam reese he did a wonderful job giving a more detailed explanation of the things we spoke about in the first hour and i think that we'll often do that going forward have a guest not every week that we have a your daily equation or whatever we're going to call these sessions going forward but oftentimes i think we're going to shoot to do that it's kind of fun also let me know what time you'd like us to meet as i mentioned early on this time slot fridays from whatever three to six eastern daylight or eastern standard whatever the heck time we're on right now isn't going to work well for me after september i have to teach my course at columbia if you guys want to take my course at columbia i don't think columbia would allow that though uh but that course meets from 2 p.m to 5 p.m uh on friday so i got to choose a different time slot on friday make a recommendation or a suggestion or a different day maybe thursdays from two to five whatever and you know if there's a dominant time slot that happens to emerge in the comments section i'll certainly take that into account otherwise again thanks for hanging out for the last three hours those of you who have been here the whole time those of you who have not you can watch the recording of this as we post it and uh and i'm not sure exactly when the next live session will be maybe next friday or the friday after but until then take care this is brian green signing off thanks you

Channel: World Science Festival

Views: 632,516

Keywords: brian greene, Adam Riess, Nobel Prize, Johns Hopkins University, supernovae, Breakthrough Prize, Nobel laureate, Live stream, #livestreaming, #livestream, #live, #streaming, Albert Einstein, daily math lesson, Your Daily Equation, brian greene interview, brian greene until the end of time, professor brian greene, professor brian greene interview, brian greene book, professor brian greene until the end of time, brian greene string theory, number theory, daily series, math series

Id: HZhHovgJ680

Length: 185min 19sec (11119 seconds)

Published: Fri Aug 07 2020