Your Daily Equation | Live Q&A with Brian Greene

hey everyone it is Friday 3 p.m. you know what that means it's the start of a marathon live discussion Q&A whatever focusing on whatever you'd like to talk about in the world I tend to focus upon things like quantum mechanics and relativity and unified theories and so on but I'm open to questions on just about anything if I don't know the answer I will tell you straight out I'm not a bag of hot air that pontificates on every subject regardless of what it is I'm only going to talk about things that I feel I understand or at least have thought deeply about you may disagree with my perspective on things but I will not spout off on things that I haven't thought about so let's just get right into it and bring up the live chat actually saw something on on YouTube that I thought was a good question maybe to begin with oh yeah so so born to stew song too soon sorry point you soon okay I get it I'm a little thick could you please explain maybe in the next Q&A session what underlies the information paradox of black holes and yeah we did have an episode you didn't need to watch it on black holes it was the last one I think that I did was a few days ago now as now your daily equation has turned into your every other day daily equation or every third day sorry I've just got a lot going on at the moment but there is a puzzle about black holes which is we all know the image that you have a black hole region of space which it has an event horizon surrounding it and according to the classical equations and the general theory of relativity if you cross over the edge of the event horizon of black low you can't get out and that that would be that now Stephen Hawking changed that picture in a way that I should have an episode on which I haven't done yet Hawking showed that when you include quantum mechanics not just our insides classical description of gravity I mean you could quantum mechanics to stuff can get out of a black hole he found that black holes radiate now he found that they radiate thermal radiation was I mean that's just heat heat that doesn't have any information content just the random motion of particles emanating from the surface of a black hole and indeed according to these ideas as the black hole radiates it gets smaller just like when charcoal burns a black piece of charcoal as it burns it gets smaller as it's giving off radiation you know that from your your barbecue the same thing happens with a black hole and therefore if you wait long enough very very long timescales black holes themselves will evaporate now that's very very interesting that's very interesting because imagine you throw something into a black hole I don't care what it is your iPhone your wallet a picture and encyclopedia whatever that entity carries information not only can have actual information like you know phone numbers or the data in an encyclopedia but it has information just by virtue of the relationships between the particles making up the structure it's an organized orderly structure it has information it falls in now the question is as the black hole radiates and evaporates does the information that fell into a black hole does it get back out and that's a very important question that people have struggled with because if the information does not come back out then there's a loss of information and when you carefully look at the equation say of quantum mechanics a loss of information ruins our perspective on how the universe evolves over time if information can truly get lost think about that means you don't have the ability to make predictions about the future because predicting the future requires a complete understanding of the state of the universe in the past but if information gets lost then that complete description will elude you so people were very disturbed about this possibility of a loss of information and it took 20-30 years before consensus really began to come together that information is not lost inside of a black hole at first Stephen Hawking said famously information is lost he said it quite poetically I won't be able to reproduce exactly the language that he used but something like as information falls into a black hole it gets lost within the folds of space-time gone even Stephen Hawking ultimately changed his mind and came to the conclusion that the information would come back and that's that's where things stood say till a few years ago until another puzzle came up and that puzzle is that if information really does come out of a black holes most people think then there is a tension with the edge of the black hole being an ordinary point in space in the sense of there's no signpost there's no singularity there you fall over the edge you won't even know it according to classical general theory of relativity and the reason for that is a little bit involved it has to do with entanglement particles that are entangled across the horizon versus particles outside the horizon that are entangled with each other and thereby carrying information you can't have those kind of triple entangled States it's called the monogamy of entanglement entanglement is not polygamists you can't have three particles maximally entangled with one another and that winds up giving the possibility that the entanglement across the horizon is broken if that's true energy would be released by that breaking of entanglement which might yield a firewall a region of incredibly hot thermal density on the edge of a black hole which would burn up anything that tried to fall in which might mean there isn't even an inside to a black hole it could be that space-time itself ends at the event horizon if these firewall ideas are correct so anyway that's that those are the kind of puzzles that people are still struggling with but you see that it's a ongoing story that reaches all the way back to the early ideas of black holes and on to some of the most cutting-edge insights into the nature of quantum mechanics in the edge of a black hole so thank you for that question it is a good one obviously a subtle one what else do we have her so so professor green Tsai asks my question is following why is it that physicists only talk about the frequency and wavelength of the photon and not of the electron or the quark and it's not completely true when when we talk about the wave function of an electron we really do use that language of wavelengths and frequencies the reason why you perhaps are more familiar with that language for photons is because photons being a particle of light the language of light is in terms of wavelengths and frequencies from the get-go whereas the language of electrons from the get-go is more energy and momentum but according to quantum mechanics there is a dictionary between the energy of a particle and its frequency and between the momentum of a particle and its wavelengths and we do use that language for electrons maybe not with as much intensity or as often I don't want to say it's frequently then I'm gonna make use of the language of the question but we do we do use that language for electrons and quarks just as we use it for photons moses tec / asks professor green can quantum entanglement be used to transport data faster than the speed of light and that's an essential question and the answer is no as far as we know you might think that it could be used because after all if you've got two particles at distant locations and if you do something over here at instantaneously affects something over there well there it is instantaneous influence but when you think about it there's no information in this process because if you're an observer over here at my right hand and another one of you in this chat is an observer at my left hand if you have a whole collection of entangled particles you know spinning up and spinning down and you measure them one after another after an they're at my right hand if you look at the set of data it'll be something like up up down up down down down up up down it'll be a random sequence of up and downs if the particles are in a 50/50 mixture of up and down the set of data at my left hand that you accumulate will also be a random collection down up down down up up up down and so forth the only interesting thing is that these two random collections of results are correlated when this one is up this one's down when this one's down this one is up and so forth but you'd only ever recognize that correlation when you compare the random output at each location and that comparison is a slower than light process you got to bring the information together you got to get on your cell phone you got to communicate so there's a classical channel that has to be brought to bear or I should really say a slower than light channel let me not use that language a slower than light speed channel needs to be brought to bear to even recognize that there is a correlation in the data but over here person over here right person random data left person ran with no information information is only there through the correlations they can only be gleaned discern ascertained by a slower than light process okay let me see what else we have here [Music] how does the graviton hackers lab asks how does the graviton theoretically mediate gravity and it's a question I'm frequently asked because you know when we talk about gravity in the general theory of relativity we talk about warps and curves in space and we imagine that objects are guided by these grooves in the space-time fabric in Einstein's theory that's how we talk about the force of gravity in Newton's theory we just talk about action at a distance one mass here another mass there and they pull on each other without specifying how that pull is actually accomplished so you might say where do gravitons get in on the story and roughly speaking they get in on the story as the transmitter of gravity so when we talk about mass one and mass two pulling on each other you can think about that as the exchange of these little particles these little gravitons and these so-called messenger particles these force particles these force carrying particles carry and instruction when you receive this particle move a little bit closer to the source and so forth and that's where the gravitational tug comes from in the language of gravitons what about the warps encouragin space-time how do you relate that to gravitons and the analogy not really analogy theme I guess it is an analogy it's not a it's a scientific analogy is really with light waves and photons when you think about a beam of light as an oscillating electromagnetic wave that's one language when you think of a beam of light as a stream of photons little pockets little pellets of light little bundles of light that's another language how do these languages relate to each other well if you have enough photons moving in the right choreographed manner they can fill out a wave-like shape much as h2o molecules and the ocean can fill out waterways water is made of particles h2o molecules and yet we know what a water wave is it's when a lot of those h2o molecules move in the appropriate choreographed manner similarly a light wave is photons whose motions are choreographed correctly and therefore if you want to talk about warps and curves in space from gravity same idea gravitons that are arranged moving in the right choreographed manner fill out the warps and curves in the general theory of relativity so that's the the rough language more precise languages use coherent States it's a way of approximating classical descriptions using quantum formalism but in terms of a visualization of what's going on that's a that's a good way of thinking about it so Allie unis son asks why is it the case if you add up all the natural numbers you get minus 1/12 and I wish I can answer that quickly I have been meaning to do an episode on that it's a little bit involved but the bottom line is when you have divergent sums so obviously one plus two plus three plus four plus five you keep on going it's divergent it goes to infinity right but when you have divergent sums we like to play an interesting game which is to find functions that are well defined that are not blown up to infinity and yet if we change a parameter in that function for a special choice of the parameter the function is equal to 1 plus 2 Plus 3 plus 4 plus 5 so it's a way of so-called regulating the sum to make it finite by virtue of seeing that sum as a special case of a general function whose properties are better behaved than the divergent sum itself and you can do that with a so-called Riemann zeta function if you take the zeta function which I think as many of you know is can be written as a divest summation 1 over N to the s when s equals with negative 1 then you're adding up the natural numbers and when you look at the Riemann zeta-function behavior as s approaches minus 1 indeed the value approaches minus 1/12 so that's how that comes about but it's less mysterious than a thing then you would think it's not literally case if you sort of added one post to persuade before and just go up to infinity that it's minus or twelve at the end it's not sort of somehow it's minus 1/12 in any sense that sort of the the finite sums get larger and larger then somehow turn over and head down towards zero then cross here and go to minus 1/12 it's rather a way of defining what that sum means in a manner that's mathematically well-behaved and when you do that you're in essence redefining the sum to be the value of this function at a special location which in the case I just described gives the sum of the natural numbers to be minus or twelve pride asks hi Brian thanks for doing this I was wondering if you're willing to provide your view on the twin paradox of relativity there seems to be some inconsistency regarding the how and the why I wish I knew what you had in mind pride and you can feel free to fill that in I guess in a subsequent post I'm not aware of any inconsistency whatsoever in the twin paradox at all so again what is a twin paradox imagine I have a twin and that twin steps into a rocket ship goes out into space turns around and comes back and traveled really quickly according to relativity when my twin gets out of the ship my twin will be younger than I am less time will have elapsed for my twin than for me and we understand this perfectly well in the theory of relativity theory of special relativity the basic asymmetry I mean the whole puzzle is you it seems completely symmetric I'm watching my twin go a ship but my twin could be looking back at earth and from my twins perspective I am the earth I'm heading that way and then we turn around and we come back and we join up with the twin so it seems to be all a matter of perspective motions relative and from my perspective the twin is going off into space but from my twins perspective I'm the one who's going off into space so how could there be an asymmetry in the ages of the two individuals when they rejoin if the situation seems to be completely symmetric answer it's not completely symmetric only one member of this pair experiences an acceleration when I am being viewed by my twin from the rocket ship my twin sees me go off into space on the earth and then come back I don't experience an acceleration because I don't actually turn around and come back it's is mother twin who slows down the engines on the rocket ship turns around and comes back it is the twin that experiences that slow down and speed up it is the twin that experiences the acceleration that breaks the symmetry and we look at that in detail you find it ensures that the twin who is in the ship will be the one who's younger now if you want to see that in great detail somebody asked this on the previous chat but if you want to see that in great detail go to world science u.com I don't think it's live now or maybe live or it's it's being revamped I think it's coming out in roughly a week so if you can wait a week go to my special relativity course on world science you and toward the end it's free right no money so it's free and you go to the end of that course and I have three modules on the twin paradox one explains it as I just did but the others explain it in more visual ways asking the question what if you had an infinitely powerful telescope you and your twin both of infinitely powerful telescope and you're watching each other during the space journey so you can literally sort of see what's going on through your telescope what will each of you see one of you needs to see the other getting older faster then you're getting older in order for there to be an asymmetry in the two ages and so I go through that both visually and mathematically and I find it very helpful to go through these other explanations because it really nails down what it is that's going on so I would suggest you take a look at that pride and I think you'll find it enlightening but if not you should feel free to ask again here or at a subsequent one of these sessions Luca says hi Brian could you please say Luca you are the best for my fiance to hear what do you think the next big discovery in theoretical physics is and then it raced away so I but I think I got to the end oh and the the great next great discovery in theoretical physics Luca so Luca I I don't know but I can make a guess that the next major discovery by which I will take it to me in a revolutionary discovery that in the future scientists look back on as one of those critical moments of advancement in understanding I think it's going to happen when we finally gain true insight into the fundamental makeup of space and time we've done very well in understanding the fundamental makeup of matter right there was a time when people didn't even believe in atoms but you know by the early years of the 20th century it became very clear that atoms were real and then subsequent discoveries that were all familiar with from school we learned about the neutrons to protons a nucleus the electrons in orbit quantum orbits around the nucleus the quarks inside the neutrons and protons and so on so we've done a really good job at understanding the ingredients that make up matter what about the possibility of ingredients that make up space and time are there any does that should make sense if there are atoms of space and time how must they be put together to yield space and time as we experience those features of the world and once we can gain headway on these questions a truly quantum mechanical particulate description of the nature of space and time I think that our understanding of a great many subjects will take a radical leap forward so that is my best guess on the breakthrough that we potentially will be able to achieve in the future nitro Phoenix asks parallel universe please tell us about its possibility and I can't tell you how many questions about parallel universes I've been getting of late and as I begin to answer that question I'm going to ask anybody who's listening to me who perhaps knows how to contact my wife or my kids if you could ask them to please bring me some water because I had an hour session before this and I am completely parched so hopefully that could be put into works as I at least in this universe maybe that can be put into works so so you may have heard or seeing the headlines blaring out that NASA has found evidence of a parallel universe in which time elapses in Reverse I mean have you seen have you seen those headlines if you have just let me just let me just say yes in the in the chat here because I think they've been getting a great deal of attention because all of a sudden I'm asked about parallel universes left right and center so let me address that one first and I'll address nitros question more generally so what actually happened is in the South Pole there was some anomalous data very exciting some particles called cosmic rays are behaving in a manner that seems to be incompatible with our understanding of how they should be behaving roughly speaking there's evidence at this like Razer kind of traveling in the wrong direction that's weird right traveling in the wrong direction now people have tried to explain this in more ordinary ways the cosmic rays are generally particles coming from outer space the Sun slamming into the earth creating particle Cascades that fall downward could it be that their particle Cascades happening say on the North Pole and the particles are traveling through the earth and emanating from the ice in the wrong direction in that way it's not impossible but certainly the energies required of the neutrinos as these particles are or we would have to have certain value for them to be able to travel through the earth but it seems to be incompatible with the value that's measured there have been more exotic possibilities put forward - maybe there's some supernova explosion that happened in the other side and created just the right particles to pass through the earth and emerge in the South Pole travel in the wrong direction none of these are yet reaching consensus so of course people put forward even more wild possibilities such as maybe there's a parallel universe where time is elapsing the other direction if it elapses the other direction then particles that would be going this way will now be going that way like running the film in Reverse that's an interesting idea it's also a far-out idea it's an idea that would require a tremendous amount of data and a mountain of experimental evidence to really believe it and we don't have anywhere near that data or information yet so it's not that it's an impossible idea it's just there isn't enough support for it so yes thank you my don't want you say hello to the crowd over you did my daughter Sophia she's brought me to waters yeah I'm good thank you I'll see you a bit later on don't get eaten by a bear on the way home so that's that's the story with the NASA headline it's overblown as often these things are clickbait if you will and it's unfortunate because it kind of makes the whole subject of parallel universes turned into a cartoon again it's not that the idea is impossible it's the way it's being presented is a caricature of the actual scientific story that is developing and unfortunately it's told with unnecessary hyperbole now tonight rose question could parallel universes and generally possible the answer is yes look I wrote a whole book on this so I'd like to take the idea seriously a book called hidden reality and in there I go through nine different flavors a parallel universe that modern physics has revealed one is the many worlds of quantum mechanics right in quantum mechanics you can have this result or that result but when you measure 50% chance here 50% here you only find one or the other outcome where's the other outcome one of the possibilities that people put forward is it takes place in a parallel world Wow interesting some people take this idea very seriously do I think it's right I think it's possibly right again I don't think the evidence is yet in place to say that it is right the other possibility that for instance people take seriously is multiple big bangs our universe emerges from one big bang but the fuel the so called in photon field that gives rise to the repulsive gravity that blows the universe open that causes space to swell if that is not fully used up in our Big Bang as the math seems to suggest there be leftover fuel that would power Big Bang after Big Bang after Big Bang universe after universe after universe so I'm just laying out a number of ways in which there could be parallel universes so an answer to a question that you're yes I think it's a real possibility worthy of attention should definitely be within our toolkit of explanation when we encounter anomalies but it will take a tremendous amount of data to convince us that we've actually encountered a parallel universe ok so that's that's the power of parallel universe story what do we have here Christopher asks that slipped away Ali asks any thoughts about Tim Ian's I don't know what Tim Ian's are sorry maybe you want to clarify on that oh gosh just go by too fast here or here's one on bill Paige from Mexico asks the bouncing photon clock shows time dilation for relative motion is there such a picture for gravitational slowing of a clock that's a that's an interesting question so if you remember in one of the earlier episodes or it's pretty commonly used in discussions of special relativity you can have a so-called light clock which is two mirrors that are facing each other and a ball of light that bounces between them and the clock is basically tic-tock tic-tock tic-tock it's the round-trip travel of a photon that is the tick and tock of this particular weird bizarre but very useful notion of a clock if that clock is then set in motion then there's this argument which is Bill is referring to in which the photon now travels on the double diagonal trajectory as the clock moves over starts at the lower mirror bounces up to the upper Merrit bounces down to the lower mirror since the clock is in motion from your perspective you see the photon going in the double diagonal that's a longer trajectory than the straight up-and-down speed of light is constant so if the light is traveling a longer trajectory at the same speed that will take less frequently time therefore slows down on a moving clock a very interesting example for understanding how motion affects the passage of time is there a simple analogy like that for the gravitational slowing of time I don't really know what I'm struggling here to think of the best analogy that I can think up and I don't have as clean an example as the light clock maybe someone in the comments maybe there may be even countered in your reading or in a course some beautiful little example that summarizes the gravitational slowing of time the way I've always described it in more general terms is normally we think about gravity as pulling on matter but in some sense gravity really does pull on the fabric of space-time itself or said differently gravity is the warping of space-time itself so the slowing of time from a gravitational field in some sense is nothing but how should I say it a tautology because the slowing of time is what we meet by grabbing in other words when objects fall you can shown the general theory of relativity they're falling because they head toward places where time elapses more slowly that in essence is Einstein's explanation of familiar gravity as we experience it why do we have a tendency to fall downward and not upward why do we fall downward and not upwards not even just a tendency is what we do we do that because our particles they are under rules that drive them toward locations where time elapses more slowly and time elapses more slowly the closer you get to the source of gravity so in that way it's almost as though if you think about gravity correctly it is the slowing of time you don't even mean the light clock or something to convince you of that that's what we mean by gravity the slowing of time in ordinary circumstances like the gravity near the surface of the earth so I don't know if that's particularly helpful because it does take really understanding the general theory of relativity in a little bit of detail to fully appreciate that but that is is certainly how I think about it ok what else do we have here whoops Ave Ave Adha asks how big would the Large Hadron Collider need to be to prove supersymmetry and Avadhut the answer is I don't know and nobody does and the reason I say that is our theories give us ample reason for suspecting the existence of supersymmetric particles these are particles in addition to the familiar particles like electrons quarks and neutrinos these are particles that differ from those particles by 1/2 unit of spin so for the neutrino that has spin 1/2 there's the neutrino scalar neutrino that has spin 0 for the quarks there are squawks that have spin 0 otherwise their properties would be the same except for their masses and that's the vital thing the theories do not tell us the masses of these particles and that's a drawback because your answer the answer to your question of how big a machine you need is dependent on how massive the particles are the more massive they are the bigger the machine because the bigger the energy necessary e equals mc-squared bigger mass bigger energy bigger energy bigger machine and so the machine in Geneva can probe to roughly about 10 TeV what does that mean so a TV is a thousand GeV a GeV is basically the mass of a proton so the Large Hadron Collider roughly can probe to ten thousand times the mass of a proton now if the supersymmetric particles were in that range from the mass of a proton to ten thousand times the mass of a proton we should have seen them at the Large Hadron Collider and we've not does that prove that they don't exist it doesn't it proves that they don't exist in that mass range but there's a lot of range still unexplored from roughly ten thousand times the mass of proton all the way up to the Planck mass which is sort of the maximum possible mass before these objects turn into black holes themselves and the Planck mass is 10 to the 19 GeV so that's 10 billion billion times the mass per proton so between ten thousand times the mass of proton and ten billion billion times the mass per proton is an unexplored window and so that's the range that you'd have to fully explore to rule out or perhaps to find the supersymmetric particles and that's a huge range and it's very expensive China may build the next machine that maybe will probe the beginnings of that realm between ten thousand and ten billion billion times the mass for proton but we just don't know how big the machine will need to be to actually prove the existence of through detection or rule out by the non observe observation of supersymmetric particles okay hmm all right what do we have here so there's an interesting question it's a little bit involved but maybe as I try to answer it I'll come up with a quick way so Seth Catalano asks what is the geometric intuitive difference between the Ricci tensor and the Einstein tensor I'm having trouble developing intuition for these objects although I understand the other curvature tensors and how to think about that so so the Riemann curvature tensor as you've seen in your own studies obviously you've studied this on your own also in your daily equation is a tensor that has four indices on it and it is the fundamental gadget that allows us to measure curvature and remember how we did that we looked at parallel transport around some arbitrary loop in a space and we compared the direction of a tangent vector moved parallel to itself where parallel is defined by the connection and we compare that tangent vector to itself and the difference between them is basically given by the Riemann curvature tensor contracted with elements that describe the path how does the Ricci tensor differ from that well the Riemann tensor is giving you information about the curvature of the entire space the Ricci tensor you can think about is giving you the curvature of various cross-sectional slices of the space that are determined by the remaining indices on the tensor itself so in some sense the thing about the two indices on the tensor is defining playing in the space at a given location look at the plane spanned by the Mew and new indices on the tensor and the Ricci tensor is giving you information about the curvature in that slice so that hopefully gives you some rough sense of what the the tensor is giving you okay multi muti asks what if our universe is mixing with its anti universe opposites well that is a big question that people struggle with why is there more matter than antimatter in fact why is there any matter at all if the universe treats matter and antimatter on completely equal footing and there's evidence that it doesn't quite do that but there's also strong evidence that to a large extent it does if the universe treats matter antimatter on the same footing then you think that the same amount of each would be produced and matter and antimatter have the property that when they meet each other they can annihilate say into a spray of photons and therefore you really would wonder why there is any matter left at all how did matter and antimatter get sequestered so that they enter matters over there or the matter over here or could it be that somehow there is a small asymmetry and a little bit more matter than antimatter was produced in the period after the Big Bang and the antimatter collided with all of the matter that it could collide with but since there's more antimatter than matter there's some matter left over from that process and you need about one particle in a billion more particles of matter than antimatter in order for that the matter left over could create all the stuff all the galaxies all the stars in the world around us so what's the answer to that and there are various theories that have been put forward but that asymmetry either is generated dynamically the laws themselves create an asymmetry so they don't fully treat matter and anti-matter on equal footing that i think is a perspective that most people take but there's also the possibility that somehow matter got separated from antimatter maybe the annihilation will one day happen when that sequestered state of the world is no longer enforced or no longer the case and that's sort of what Mattie muti is asking if you mix these two together what would happen and what would happen is annihilation what would happen is structures in the world would turn into bursts of radiation and would be gone so if there is sort of an antimatter galaxy out there we just should hope against hope that we don't pass through that galaxy as it would not be fun for any of the structures that define the galaxy as we know it okay Zubair Ahmad asked hi professor would you be convinced with the answer of Bertrand Russell that the universe just is and well there's a lot of what Bertrand Russell says that I am quite taken by quite sympathetic to some of the essays that he's written on thermodynamics and the enemy in the far future influence my own book influenced until the end of time the book that I came out with a few months back and I am pretty much of the opinion that there is no grand design there is no overarching purpose the universe came into existence for reasons that no one yet understands but we're getting closer and closer to it we can explain how the universe evolved from a fraction of a second after the beginning that's impressive in its own right that's amazing but we've yet to really understand why there is something rather than nothing and until we can answer that question we have an incomplete story reality but certainly based on everything I know everything I've accounted everything I've studied over the course of whatever thirty years of being a professional physicist everything to me suggests that the universe is just the way it is that there is nothing that designed the universe there is design in the structures of the universe and I think we have a good understanding of how a generalized version of evolution by natural selection can yield those kinds of organized complex collections of particles we understand how gravity has the ability to pull together ordered structures like stars and planets out of what otherwise is a chaotic mass of particles so we understand at least in rudimentary form how structure can come into being and it's just the blind laws of physics that are at work so from that perspective there is no overarching intelligence there's no overarching design it is just the laws of physics acting themselves on the particles of matter and that's all there is to it again that's incomplete I don't know where the laws come from I don't know where space and time come from I don't know why there's something rather than nothing there are theories they're ideas these are open questions so modulo that uncertainty I would say that I agree wholeheartedly with the notion that the universe just is okay what else have you got here nhave ball asked professor please explain about Hawking radiation and I can say a few words about that it emerged in 1974 from a brilliant paper by Stephen Hawking and in this paper Hawking puts together gravity and quantum mechanics in a fairly rudimentary form but a form that is good enough to answer certain kinds of questions not every question in quantum gravity could be answered by the approach that Hawking took but he was able to analyze physics near the edge of a black hole using this blending of quantum mechanics and general relativity and what he found is that look an ordinary quantum mechanics without black holes we learn that Heisenberg's uncertainty principle in some sense tells us that particles can erupt out of the nothingness a particle and antiparticle and erupt out of empty space they can live for a very short period of time they then encounter one another annihilate and return to empty space now that process we believe is happening all the time throughout empty space particles erupt through this quantum fluctuation they live for a very short period of time they annihilate once they encounter each other a short time later Hawking re-examine that process not in ordinary space but in the space just outside the edge of a black hole and he found that oftentimes what happens is just like what happens anywhere else in space the particles are up to find each other disappear but they also found a new possibility that can occur near the edge of a black hole which is this when the particles are RuPt into existence in the space just outside the edge of a black hole one of the particles can get pulled into the black hole the remaining particle which is outside the black hole does not have a partner to annihilate with any longer so it rushes off out into space and must it actually must do that because if one falls in the other must go in the opposite direction to conserve momentum so therefore what that means is at each and every location on the edge of a black hole this kind of process can take place which from the outside looks like particles streaming from the edge of a black hole we don't see the partner that falls in more over the partner that falls in you can show has negative energy almost like negative mass which means as the black hole absorbs that negative energy particle it in heft it decreases in heft so you've got radiation coming out from the edge of a black hole as the black hole itself gets smaller and smaller as I mentioned before it's like a piece of charcoal it's like a charcoal burning as the charcoal burns it gives off radiation and the charcoal itself gets smaller and smaller so in some sense Hawking radiation is just like that radiation emitted from quantum fluctuations around the edge of a black hole resulting in observers far away seeing that radiation and at the same time recognizing if the black hole itself gets smaller and smaller that's the basic idea of Hawking radiation so new Raj Verma asks if Earth orbit is a slight ellipse to conserve momentum and speed increases when it's closest to the Sun to be changes there's an acceleration why can't we feel this acceleration and I would even go further Niraj right now the earth is spinning around on its axis the earth is about you know twenty five thousand miles in circumference I know you guys hate miles I still I was brought up a mile so I still think that way when I'm just talking about things in loose language so the earth is about twenty-five thousand miles around and it takes 24 hours for it to complete a rotation so the earth is spinning around at a thousand miles an hour Wow that's pretty fast and of course that's in circular motions that's an acceleration all right why don't we feel that want to feel any of these motions and the answer is that these motions are quite small the accelerations are quite small even though it sounds like they're quite big they're quite small relative to the intrinsic pull of Earth's gravity and therefore the intrinsic pull of Earth's gravity we do feel that all the time and according to Einstein that is equivalent to an acceleration so we are feeling acceleration all the time right now I'm feeling that acceleration on you know the seat of my chair if you guys are sitting you're feeling in the same way if you're standing up you're feeling it on your feet and so the idea is that the changes to the acceleration from the processes that you're talking about are so slight that that we're not sensitive enough to feel those changes but we are subject to accelerations all the time not just from the elliptical orbit but even from the rotational motion of the earth let's see that's see Peter asks a hot professor Green could the Big Bang be some sort of phase transition and absolutely it's possible you know a phase transition you understand that from ordinary real world experience you go from ice to water that's a phase transition from solid to liquid you can go from liquid water to gas that's another phase transition in general in physics say in the fundamental physics of the universe phase transitions are associated with a change of the value of a field like the value of the Higgs field we're to change value that would be a phase transition and when you're looking at the swelling of space in the Big Bang at least in the inflationary theory you need the value of the field to have a certain quality it has to have a certain size and has to be uniform within a region of space so if you imagine that that field may have had different values prior to the inflationary expansion and then only through a phase transition acquires the uniform value in space that allows the repulsive gravity to take hold then yeah the inflationary expansion could well be the result of a phase transition where the value of the field jumps to the value necessary for that repulsive gravitational push to come into play okay what else I'm gonna take a look over so haven't looked at in a while at the YouTube thief not the YouTube the the Twitter or Facebook feed if a photon I can't claw asks if a photon is fired directly into a black hole does it pick up speed beyond the event horizon and the answer is no if you appropriately measure the speed of light the speed of a photon and you do that in a local measurement which is how you measure speed something's racing by and you measure the distance that it traverses over a period of time as measured locally you will always find that speed of light is something a dum-dum the speed of light constant the value does not change and that's the only definition of speed that you should use when talking about the speed of an object you don't want to use clocks that are far away you don't want to use rulers that are far away you want to use clocks and rulers that are situated where the object whose speed you're measuring is actually located actually moving and if you do that then the speed of light and the general theory of relativity is the speed of light constant doesn't change um Margaretta Dahlgren from sweden mama sounds like a familiar name wend if I know that person has my exact future existed from the beginning of space time and if so in which size shape and from what sense as my exact future existed from the beginning of space time and to that answer I would say no I have to qualify what I mean by that so to mechanics which we believe is the fundamental architecture of physical reality quantum mechanics doesn't predict definite results of how things will evolve toward the future quantum mechanics only predicts the probabilities of how things will evolve toward the future and so even in the most simple of circumstances like the motion of an electron we never predict exactly where it will be or what its motion will look like we only predict the probability of these qualities of the electron recognizing that quantum mechanics allows for a kind of openness in the future now the probabilities of the quantum mathematics are determined by Schrodinger's equation with the same kind of deterministic aspect that we are familiar with the determinism of Newtonian physics the differences Newton determines the outcomes quantum mechanics determines the probabilities of the outcomes so what I would say to Margaretta is the probabilities for your particles moving this way or that or engaging in this manner or that or coming together in this grouping or that the probabilities were determined say all the way back at the time of the Big Bang or just after the Big Bang but those probabilities do not actually determine the behaviors that you will undertake they only determine the probability of those behaviors so your future has existed in a probabilistic sense in the sense of the various possible futures in principle are determined by quantum mechanics way before you're born but that's different from saying that your actual future was determined it isn't under your control but it is a random outcome of quantum evolution but with the probabilities of this or that outcome fully determined by the equation so a little bit subtle it's a little bit more subtle than you might suspect so the answer somewhere in between you don't control your future but neither is it the case that your future could not have been otherwise your future could have been otherwise one of the other outcomes allowed by quantum mechanics could have been the one that actually took place okay what else do we have so Ivan Flores if the universe is expanding what is it expanding into and it's a natural question I think that probably everybody on this chat has asked themselves at one point or another and the point is that when you are talking about the expansion of more familiar objects what expands in the world well you know bread expands my waistline expands right when things expand they expand within a pre-existing environment so they are expanding into something and therefore when you think about the expansion of the universe the natural thought is that space itself must swell into a pre-existing environment but things that are true for objects in the universe need not be true for the universe as a whole and this is an example of that because our belief is that the universe is not expanding into pre-existing territories not like there's some unclaimed land unclaimed space out there and the universe does a land grab and grabs hold of it as it expands rather the idea is as space expands it's truly creating new space creating space that did not exist until the universe expanded so the new space that is within an expanding universe is created fresh it's not stale space out there that is now reclaimed that's the difference so it's not that the universe is expanding into something is that universe is creating the very space that it then occupies as part of the spatial expansion again on any of these questions if if you feel that I've not adequately addressed it or if you disagree with my answer I'm more than happy to hear well see your response and and address it if I can Paul C asks hi professor Greene I'm really enjoying your new book thanks and I thank you back Paul see what do you think about Leonard Susskind's EPR equals er theory and I should say it's it's it's Leonard Susskind and whan maldacena so as I understand the history of that idea Lennie and Wong were in conversation and one day Wong sends Lenny a cryptic email which says EPR equals ER and that was sort of the beginning of that way of potentially thinking about things so what does it mean first of all so EPR that's the einstein-podolsky-rosen paradox is what is often called but it's not a paradox at all it's simply the recognition that there can be these two distant particles that are entangled and we've spoken about entanglement here before where you do something on one and it affects the other so there can be threads of quantum entanglement that link distant particles er that stands for Einstein and Rosen and is referring to another paper that Einstein wrote in the very same year Einstein wrote his EPR paper in 1935 he wrote as ER paper the R is the same Rosen in both cases also in 1935 and the ER paper has to do with wormholes so wormholes we discuss a little bit here earlier this possibility of tunnels within the fabric of space and what Lenny and Wong conjecture and give evidence for is that the connections established in ep are those quantum entangled connections from quantum mechanics in some sense are equivalent to the wormhole connection that comes out of the general theory of relativity that's a stunning claim right wormholes that come a classical phenomenon the general theory of relativity EPR entanglement is a feature of quantum mechanics and they are conjecturing that secretly these two ideas may be the same and it is a natural consequence of deeply thinking about black holes and the radiation the particles that emerge from a black hole and the ways in which that radiation can be entangled on the outside with partners on the inside and what that might mean for the geometry of the environment that entanglement may actually be nothing but a wormhole connecting the product on the outside and the protocol on the inside so so it's a it's a beautiful idea it is one that I find quite compelling and convincing what it's actually fully telling us about the nature of space and time and black holes I think is still yet to be a story that's fully written but it is a major advance and it is an important way of thinking about how the classical concepts of wormhole and the quantum mechanical concept entanglement can come together in fact we're gonna do just by way the advertisement we are going to do a world science festival program and all the world science festival programs are going to be coming out quite soon you may have seen the Kavli prize announcement program that we did on Wednesday I think it was but we're going to be turning to our regular world science festival programming and start to turn those out over the coming weeks and one of the programs that we're going to do is this EP r equals ER and Lenny's Huskie and I've told today is going to be in that program which is great and some of the other leaders in quantum mechanics and string theory and general relativity will also be in that program so keep your eyes open for that and we're will have a full gushin of this idea of er equals e PR that's coming up so thank you Paul C for that question let's see what else we have here Siler gaming asks follow up on the idea of expanding universe and thank you sada I'd like follow-ups if the galaxies are moving away from each other how does it only signify that the universe is expanding and-and-and-and you're right I mean it could be the case and imagine that the universe is not expanding and what happened is space is static it has always existed not expanding and imagine that you have a lot of matter at one location within that pre-existing realm and the matter just explodes and it gets flung outwards and somehow the bits of matter can coalesce as they're spreading outward into galaxies or the galaxies would all be rushing away from this initial explosion could that be the explanation of what is going on and most physicists have long concluded that that can't be the explanation for what's going on and the reason is all of our observations suggest that there is no special location in space that the universe is so-called homogeneous and isotropic on sufficiently large scales which means that our view of galaxies all rushing away from us is the same view that an observer on another planet in another solar system and another galaxy would see using powerful telescopes every thing is rushing away from everything else as opposed to everything is rushing away from one central location and in order to have a situation which everything is rushing away from everything else you need space itself to be expanding and again the balloon analogy it's hackneyed but it's pretty darn good right so if you if you have pennies on the surface of a balloon I know you're rolling your eyes you've heard it so many times but this worth having it in mind again as you blow air into the balloon and the surface of the balloon stretches every penny moves away from every other penny on that surface it's not as though there's some special penny from which all galaxies are rushing away and it's the only penny that can claim to be the penny from which the others are rushing every penny can make the same claim if you think about it put yourself on one of the pennies because it's the entire surface of the balloon that's stretching every penny looking out along the surface of a balloon will see all the other pennies rushing away and that is our belief regarding the nature of the motion of the galaxies and that is why we explain them in terms of the expansion of space itself it also fits right into the general theory of relativity that's what Einstein's theories shows so by virtue of the fact that we have tested Einstein's thirty theory over and over again I mean Einstein's theory is kind of incredibly ridiculously resilient it successfully explains not only the observations that started to happen in the 1920s but a hundred years later it explains the observation of gravitational waves in fact predicts it 100 years later explains the black holes that we now can actually take photographs of so Einstein's theory is just spectacularly resilient and so when we realize that Einstein's theory can explain the observations of the motion of the distant galaxies in this very simple beautiful economical way in terms of the expansion of space when we see that we realize that the evidence is kind of overwhelming so does that prove it it doesn't prove it it's very hard to prove things in science is the evidence overwhelming that the expansion of space is the explanation for the motion of galaxies it is absolutely is but definitely a good a good question to ask all right so pool ball asks the follow-up to that why is it therefore that a galaxy does not expand to and the answer is that galaxies are held together by the attractive version of gravity and that attractive version of gravity as of today is stronger than the repulsive version of gravity that for instance is driving the currently observed accelerated expansion of the universe why are our bodies not expanding again our bodies are held together by electromagnetic forces and the electromagnetic force is holding my particles together they are stronger than the swelling of space and that's why we hold together so if you are expanding it is not because of the expansion of space look for a different cause now having said that we don't know for instance about the dark energy which is driving the accelerated expansion of space we don't know the long-term behavior of that dark energy and it is possible and people have written down models in which the repulsive gravitational push get stronger and stronger over time and if that's the case then what I just told you which is true today may not be true in the future in the future it may be the case that the outward swelling of space is stronger then the force is holding a galaxy together and if that turns out to be the case in the future then galaxies will expand so pool ball the answer to your question is in the future if the dark energy increases in strength galaxies will grow even stars will grow and ultimately explode planets will explode as the repulsive gravity drives constituent particles apart even atoms may be ripped apart by the repulsive gravity by the force driving the accelerated expansion in the future so whereas today we hold together our structural integrity is held intact by the forces that hold us together that may not always be true and the far future the swelling of space may blow everything apart that is a the possibility that people do indeed study okay what else here so so abaca Farooqi asks how does reverse in the direction of velocities reverse the direction of time and I would say it does not reverse the direction of time so if you have for instance a wineglass and I like this example because we did it in a nova program of mine years ago we allowed the wineglass to fall off a table and smash we then stopped the film and I ran around in some subjective non real version of time I ran around as the shards were hanging in midair and I reversed the direction of each of their velocity vectors and with those reverse velocity vectors we then allowed the system to carry on forward and of course what happens with the reverse rassa T vectors is all the shards of glass jump together and reassemble into the pristine glass itself I would not call that a reversing of time though I would call that the observation in the ordinary direction of time of a physical process that we never see unfold in that temporal order so a glass smashes a glass can unsnap the unmasking is not happening in Reverse time if I reverse the velocities it is looking as though the smash is being reversed it's looking as though I've run the film backward but if you're reversing velocities you're watching the physical unfolding in ordinary time it's just that you have set up the initial velocities to do something that we never see in the real world which is to unbreak a glass but that unbreaking process itself is taking place in the ordinary direction of time it's simply that you reverse the velocities allowing the ordinary time evolution to do something incredibly unordinary incredibly extraordinary which is to recreate a smashed recreate a pristine glass from a smashed one so it's very important to take the distinction between reversing time going backward in time and watching a physical process unfold in the reverse temporal order - two different things okay so what else do we have here so I don't know let me go over to some of my questions from Twitter Oh so Alexander from Twitter asks professor green why didn't you use the Hamiltonian operator when you talked about the Schrodinger equation and I would say Alexander that I didn't use that language but I did introduce the Hamiltonian operator I didn't want to use highfalutin mathematical terms when they weren't necessary and I do realize that for some people it winds up becoming more confusing to not use the high level language because you've studied the subject in a class that you use the high level language and when I don't use you're like well wait how does what he's talking about relate to what I previously learned so so sorry about that for the few of you who are well versed in in these ideas that I don't always use the technical language but when I introduced Alexander this idea of minus d2 by DX squared right I had minus H bar squared over 2m d2 by DX squared plus V of X that is the Hamiltonian operator nonrelativistic quantum mechanics so I wrote it down but I didn't label it so to connect it up with things that you may have learned elsewhere my operator differential operator minus H bar squared over 2m d2 by DX squared plus V of X all that acting on the wave function side that is the Hamiltonian operator I just didn't didn't use that language so I hope that clarifies what I was going on about in that episode on quantum mechanics alpha-omega spy's says I enjoyed listening to you the laws of physics do not care whether you are vegetarian or not so how do you feel whether your choice of being a vegetarian comes from the freedom of your will and I do not tribute my choice to not eat meat actually to be a vegan not just a vegetarian I do not attribute that to freewill as you know from either reading my book you know chapter 5 I spend a lot of time on freewill because if you're trying to understand what it is to be human on planet Earth within the context of the cosmological unfolding you better spend a little time thinking about freewill because free will is for many people the very definition of what it means to be a human being we have the freedom of choice the freedom of will that's not available to say a rock or a dandelion so do I believe that that sensation of free will is real I do not I think it is grossly misleading I think we do not have the freedom of will that our intuitions suggest that we do so when it comes to any of my choices vegan vegetarian choice a spouse did I freely ask my wife to marry me I did not I don't know if she's watching right now actually I can see her window from here and she seems to be not reacting to what I just said so I suspect that she's not listening or it's on mute or something but I do not believe that I freely asked my wife to marry me because I don't believe that I freely make any choices it felt as though I was making that choice it's an easy one in case she's listening but I do I did feel I did have the sensation of the freedom of will that we all have and the sensation is real but the actual act of asking my wife to marry me was the result of particles in my brain moving in a particular manner that resulted in my mouth and vocal cords making certain motions that my wife interpreted as me asking her to marry her and none of those particle motions were under my control what would it mean for particle motion to be under my control it's absurd I don't control the motion of particles how would I do that the laws of physics determine particle motion and what I think and feel is the result of that motion I don't even know what it would mean for me to control the particle what does that mean doesn't make any sense how can I a collection of particles control the motion of the particles I don't have some kind of extrasensory force capacity electromagnetism weak strong nuclear forces gravitational forces those are the forces that depend on not dependent those are the forces that yield the motion of particles so the motion of particles depends on those forces in those forces I don't control them neither do you so even beyond vegetarian/vegan free will none of it do I believe emerges from the traditional notion of the freedom of choice so zeros skull asks god god god exists as an idea in the mind of everyone who has such an idea but it is different in every mind therefore imaginary well I get what you're talking about zero skull it's actually a point that Richard Dawkins is fond of making Richard Dawkins emphasizes that there are many different religions practiced on planet Earth and even within a given religion as you point out zero skull many people have different perspectives on what the nature of God is and if we're not all agreeing if all the religions are not agreeing then that suggests that there isn't any fundamental truth that they are tapping into and in fact you know Richard likes to say that most religions deny the viability of other religions right most religions say that their view of God in the world is the right one and that the other religions are misguided and therefore in some sense every religion articulates the belief that the other religions are irrelevant to truth and Richard likes to say as an atheist all he does is what every world religion does but he goes one step further he denies the reality of one more religion in any given perspective and thereby takes it down from one to zero and it's an interesting argument and I have a certain degree of receptivity to that way of thinking about things but you know it's possible it's possible that one religion is right and all the others are wrong right and it could be that within that religion there is one view of God that is right and all the other views are wrong that's not logically impossible it's not logically ruled out by the fact that many people say different things many people say different things about you know why a ball flies through the air now if you're a physicist and you know physics you give one answer but you do a you do a poll of Americans about why a baseball flies through the air and I'm sure you're gonna get a lot of answers that are utterly wrong and that doesn't mean just because many people have different perspectives that there is no truth about the motion of the ball there is and we approach that truth through the language of physics so I think it's a little quick to say that just because there isn't universal agreement on what God is that God doesn't exist too fast on the other hand I think there are other arguments that give strong reason for trying to understand the object of world through the language of science not through the language of religion and if you want to talk about that was I'm happy to but even though I'm of a similar mindset to say Richard not identical but similar another mindset I think that resonates with your question I don't think that you can reason in the manner that you did and in that way rule out every religion just because everyone doesn't agree on what the right religion is ali al-taher asks do you love your wife yeah I sure do love my wife do I have freedom in that trust I don't am I happy that that I that I'm in this situation that I have a wife that I love yeah but do I have any choice in that fundamentally no I don't there may not in the slightest and I happened that I'm not a rock that isn't able to experience the kinds of emotions that a more complicated arrangement of particles is able to I am is that sometimes painful sure is it sometimes wonderful absolutely am i thankful for the range of reactions that this kind of collection of particles is able to undergo compared to say a rock which kind of doesn't do much but sit there yeah much more interesting what's Reviver it but do I have control over the motion of those particles and therefore control over the motion of particles yielding the reactions and sensations that I experience now I don't just how it is okay well I'm exhausted too dad I guess it's cuz I did an hour before this I'm gonna for you guys any of you guys were there but I did this reddit public access network I did that for an hour before this so how long we been talking we've only been talking hour and a half but I'm at the two and a half hour mark right now I'm starting to feel it so I don't know how how long I'll go today but as long as I as long as my voice holds up so let's keep going here then I'm gonna try to get this scroll from there's a follow up I so follow up with question it with the exclamation mark sai yok says what if we are also getting expanded along with the entire universe as they're also part of space there was just a reference object in this University compare it so that that's that's certainly true you know if if truly everything is expanding at the same rate then you'd have no benchmark to measure anything against anything else but that would be sort of an unobservable kind of expansion the observable sort of expansion is the expansion that we witness where it is not simply that the very definition of a meter is now two meters or a little later it's defined to be three meters or it's probably the reverse you know if you want to truly expand what we what we called three meters in the past is now a meter so that length in some sense is the new standard so you're right but we're not that interested in unobservable qualities of world you know sometimes people talk about you know the the buddhist god what's the word co on got the right word i think it is you know the puzzle what is the sound of one hand clapping you know that one you can solve by the way and if you guys had visual i'm sure that you would show me the solution but what is the solution to the issue the sound of one hand clapping it's one hand clapping can you hear that there it is done we saw that one let's move on but there are questions about the world that are fundamentally unanswerable not that one but you know things changing in a way that's completely the same for everything you know if we were to say that time time just sped up by a factor of two but for everything every process that would be an unobservable change to the world right because the very standard of time would be subject to that increase in speed so unobservable so that those are just not that interesting to us though the ones that are are interesting are the measurable changes and those are the ones that we talked about we talked about the expanding universe the expansion of space oh here's a very interesting question Cambridge breaths Cambridge breaths asks hi Brian what is your opinion about Freeman Dyson's theory that nature will never allow us to measure a single graviton in fact the very observation will turn that process into a black hole and and I wonder how widely Freeman wrote about this idea I read about it I think in the New York Review of Books where Freeman loved to publish these wonderful erudite reviews and articles and I think it was in that publication that he noted this contrarian perspective that not only did he say that gravitons could not be measured he basically said they may not even exist his view was maybe don't need to put gravity and quantum mechanics together he said look quantum mechanics does its job in the realm of particles molecules atoms and so forth general tivity does its job in the realms of gravity matters stars and galaxies and he basically said it's misguided to think that there has to be a unified theory that puts gravity and quantum mechanics together and by the way if you don't put gravity and quantum mechanics together you don't have a graviton the graviton is not something you find in a classical textbook on Einstein's general theory of relativity you find the graviton only in circumstances where one is using quantum mechanical ideas in a situation that involves gravity but if you believe Freeman Dyson and you don't have to put the two theories together then there isn't such a thing as a graviton now why don't we take this idea more seriously well look it's possible it's true but the reason we don't take it seriously is because we're aware of physical situations in which as far as we can tell gravity and quantum mechanics have to be blended together for instance the R equals zero location of a black hole so somebody actually on on YouTube I remember seeing this question I think it's Bernard Mick Garvey was asking he said look you know when you solve the equations of Einstein's general theory of relativity and get the black hole solution it's only valid outside the mass responsible for the solution so are equal to zero that singularity maybe you just can't ignore it maybe it's outside the realm of the solution and sure you can say that but still they are equal to zero location as far as we can tell is a real location we want to understand the physics at that real location it's true that the Schwarzschild solution is not valid there that's what that singularity means the article to infinity but what is valid at R equal to zero and we think that you need to put quantum mechanics together with Einstein's theory that's why the Schwarzschild solution is not valid at all right the de xira doesn't take quantum mechanics into account so according to Freeman Dyson maybe you could just ignore it much as Barnard is suggesting but as far as we can tell there's a real is a real spot in space-time called R equal to zero and without quantum mechanics and gravity coming together understand it so that's why you need to blend them together similarly the T equal to zero moment of the Big Bang it's another circumstance in which the math breaks down you could just ignore it and say well whatever the universe starts not a time equal to zero we're gonna ignore that just start it later on sure you can do that but if you want to truly understand how the universe came to be and not just lock it away in a black box called ignorance you've got to go deeper you've got to go to T equal to zero for the Big Bang and at T equal to zero you've got to put quantum mechanics and general relativity together because it's an incredible amount of matter crushed to a very small size small size quantum mechanics incredible amounts of matter general relativity so these are the reasons why we're not receptive to Dyson suggestion that we just keep the two theories apart and if you put the two theories together then indeed you have gravitons in the gravitational story could you ever go about measuring them as you note in your question were they in some sense turn into black holes yeah maybe but black holes can be detected right astronomical black holes you saw a photograph of them a black hole in the laboratory it would just be an unusual particle if it had a mass of a particle so again there doesn't seem to be any ironclad reason to suspect number one that graviton it's don't exist number two that they couldn't be detected so bottom line is I'm not predisposed to I even predisposed my reasoning suggests to me that Dyson's account is not right okay si monster asks what is gauge symmetry holy smokes that's a tough one dad is a tough one um I don't even know if I'm gonna try to answer that question so oh my goodness how do I answer that in an ordinary language that's that's a hard one um you know what si monster if my brain was more fresh I suspect I might be able to come up with a quick explanation but I'm I'm going on two and a half hours for the other one and my brain is not quite as fresh so I am going to put off that question ask it next week and next week I'm gonna take it early on and see what I can do with that but it's a hard question very hard question it's hard question to say now I'm mathematica of course oh yeah so let me do this to answer your question I may answer it mathematically not something our ordinary do but this is called your daily equation after all so good so what is a gauge symmetry so when we write down the equations of for instance with standard model of particle physics we find that those equations respect certain symmetries those symmetries are generally Li groups which are built upon Li algebras and the symmetry group of the standard model of particle physics is the group su 3 + su 2 cross u 1 and that group can be thought of as a group of transformations that act on the quantum fields in the standard model Lagrangian so what does a gauge symmetry a gauge symmetry is the transformation of the fields in a given Lagrangian by a given Li group that leaves the Lagrangian of the theory invariant or it most changes it by a total derivative in time and such a symmetry is called a gauge symmetry for historical reasons but that's what a gauge symmetry is okay so captain frantic says stand back he's doing math didn't understand far back it's not that bad so what else have we got here [Music] Yvonne or Ivan Flores asks professor how did cosmic inflation flattened the universe and first of all Ivan or Yvonne I'm sure I don't know what the right pronunciation is when people talk about the universe being flat that can be a confusing idea from the get-go flat usually for me brings to mind the picture of a pancake or if you're in France a CREP you know something that's really thin and flat and that's not what we mean when we talk about the universe being flat what we mean by the universe being flat is that the universe doesn't have spatial curvature and it's hard to think about spatial curvature in a 3-dimensional sense because when we talk about curved objects in space you know we can picture that right here we go here so this is actually here's a curved object people recognize that actually the cylindrical part of there's not actually curved it's simply the way it's embedded in three-dimensional space but here's like a actually curved space this is uh this is a you know a nice mug beetles monk given to my son Alec by two fellas we work with in the UK named Al and Al and that's a curved object in a three dimensional space but to picture the curvature of three dimensional space itself it doesn't exist within space and it's very hard to picture so we often use two dimensional analogies and two dimensional analogy for a curved shape would be say the surface of a basketball right that's a curved surface and a flat version of that would be the crap or the pancake where it's not got that curvature its flattened out but you have to scale it up to three dimensions so when we talk about flattening the universe it's not like flattening the universe into a pancake it's removing any spatial curvature that the three-dimensional spatial section the slice of the universe of the universe might have and so now the question is how does cosmic inflation get rid of curvature in that sense again I'm going to use the two-dimensional analogy because it's so hard to picture curvature in 3-space but scale this up to three dimensions to really understand what's going on let's take that basketball the surface of the basketball is curved two-dimensional but let's use it so we can picture things if you make the basketball larger and larger let's say you make the basketball as large as planet Earth then you'll notice that the curvature has been reduced right as I look out on the surface of planet Earth yeah there's some little indentations and so forth but overall it looks flat that's where we get the flat earther perspective that I don't know if we spoke about in this chat or if it was in the previous one that I was in just before this but if you look out at the world the surface of the earth looks flat even though we know that it's not simply because it's so big so the lesson is when you take a surface and you make a bigger and bigger and bigger it becomes flatter and flatter in any local region making something bigger makes it appear flatter now apply that to the three dimensions of the universe what is inflation do it makes the universe grow by an enormous factor in a short period of time by the same reasoning if the universe had any curvature spatial curvature when it was expanded by a huge factor that curvature is diluted just like the curvature of a basketball is diluted when you make it as large as the earth the curvature of space itself is diluted as it gets larger through the swelling of inflationary expansion so Ivan that's the answer to your question that's how cosmic inflation flattens the universe doesn't flat this way it flattens if I'm making the scale so much larger which diminishes the amount of curvature in any local region good question though so anu asks a related question why would cosmic inflation want to get rid of the curvature so very anthropomorphic but I use that language too so I can't criticize you for that that would be throwing bricks in a glass house but I'm not exactly sure what you mean a new by why would cosmic inflation want to get rid of Kircher I'm gonna interpret that to mean why would physicists be happy that cosmic inflation gets rid of curvature so I hope you don't mind my rephrasing and the reason why we're excited and happy that cosmic inflation can dilute curvature is for the following reason in the pre inflationary version of the Big Bang before inflation came on the scene we faced a bit of a problem and the problem was in order for the evolution of the universe in its expansion to yield the amount of curvature that we now measure which is quite small compatible in fact was zero but it's quite small for the universe today to have a small amount of curvature in the ordinary pre inflationary version of Big Bang you would have had to tune the curvature of the spatial slices of the universe in its early moments by something like one part in 10 to the 50 because in the pre inflationary version of the Big Bang curvature grows over time it doesn't dilute in the way that I have explained and if it grows over time then it had better be so close to zero in the early universe so that after the amount of growth that it's had since the Big Bang it can still be a small number the small number that we currently measure people don't like that businesses don't like a theory that requires you to tune quantities to say 50 decimal places it just feels unnatural it feels it feels artificial the beauty of the inflationary theory is because of this rapid swelling and hence dilution of the curvature regardless of what curvature of the universe had in its earliest moments it will be diluted to nearly zero by today through the inflationary expansion ah that's beautiful we love theories like that we love theories where you don't have to fine-tune things early on where it doesn't matter hardly what the curvature was early on within reason because the dynamics dilutes it to the small value that we currently witnessed that's why we're happy that inflation gets rid of in the language of on ooh ha si gets rid of the spatial curvature okay so yosh Paul asked should we take Penrose diagram seriously yeah definitely take them seriously as a mathematical tool for analyzing the global structure of space-time the beauty of the Penrose diagram is it can take points that are at infinity arbitrarily far apart and through a clever rescaling bring them into a finite looking distance in the Penrose diagram allowing you to see the full structure of the space-time so don't take it seriously say in terms of the distances that you see on a page reflecting the distances out there in the world know that rescaling is not part of reality but that rescaling is a powerful mathematical tool so you can understand the reality so in that sense you can take it seriously and should take it seriously so Shahab olofi asks where does energy come from and the most honest answer is I don't know nobody really has an answer for why there's something rather than nothing which is a equivalent kind of question but there are interesting theories that suggest that because the gravitational potential energy can be negative remember what you learned in school the potential energy of gravity goes like minus GM over R - right gravitational potential energy is - which means it can be arbitrarily negative so imagine a circumstance in which the energy that we see in the world around us that positive energy is exactly balanced by a negative amount of gravitational potential energy yielding a total energy including both contributions of zero then we wouldn't have to answer the question of where does the energy come from because it's zero it's just broken apart in an interesting manner positive that we see balanced by the negative gravitational potential energy that we often don't take account of in our intuitive accounting of the energy in the universe that's one possibility we don't know that that's the answer but at least it's one possibility in which you wouldn't have to answer the question of what's the ultimate source of the energy because the total mountain be zero and you've just divided it up into some positive and some negative in a useful way they could ask what what divided it up and that can take you into inflation and cosmology and other things with that sort but it refrains the question in interesting a different way Allah sorry Alaa I don't mean to be sacrilegious Ya Allah o Allah sorry my question is when the fabric of space-time bends how can a 4d space contain it anymore in analogy as if we bend paper it will no longer be sufficiently it can only be contained in the plane so yeah you're absolutely right if if you lived in a two-dimensional universe and then you bend the surface if you want to embed it in a Euclidean space in an ambient environment then you're right you can no longer embed it in a two-dimensional environment but bear in mind that to a little ant walking on that piece of paper whether it's bent or curved that ant is still experiencing a 2-dimensional reality so to the end the ant doesn't need to embed the space in a higher dimension sometimes we find it useful mathematically to visualize a curved space by embedding it in an ambient environment but you don't need to do that you can talk totally intrinsically when you're talking about the geometry of curved space-time environments but you're right if you want to embed our universe if our universe has some interesting complex not complex in the sense of imaginary numbers but some intricate curvature then yeah you'd have to embed our universe in a higher dimensional space if you want that higher dimensional arena to say be flat in the sense of having no curvature and you wouldn't need a higher dimensional space to do that and sometimes it's useful to do that but sometimes it's not sometimes just talking completely intrinsically we are the ants walking on the piece of paper we are the ants inhabiting an environment that does not need to be embedded in a higher dimensional environment that's fine if they're higher dimensional beings that like to visualize things the way that we do they might think about embedding our universe in a higher dimensional expanse but we don't have to do it we're perfectly happy dealing with our universe intrinsically not extrinsically so by chaco says Methuselah and yeah I don't know what that means in this context but actually I just reread George Bernard Shaw's beautiful play on Methuselah and it's called back to Methuselah and I encourage you to check that out and read it it's a very interesting survey it's almost like it's almost honestly it's almost like my book until the end of time because it begins at the beginning and it goes hit again but it does it through a fictional account of characters dealing with different versions of mortality and immortality so it's kind of a nice companion to my book in a fictional setting so if you if you if you like that sort of stuff you should definitely check out it's old but you can still get hold of it back to Methuselah by George Bernard Shaw like I should have such a son that book to the class I'm teaching right now on my book mm-hmm yeah I think I may do that Hammad Ali asks how can we access another dimension and one possibility if there are other dimensions is through high energy collisions at the Large Hadron Collider our hope is that when you slam protons against protons that some of the debris that's created through the collision might be able to leave our dimensions and enter another dimension for instance a dimension that suggested by string theory how would you know that well if you slam particles together and some of the debris leaves our dimensions and goes into another dimension that we don't have direct access to we would notice that process by virtue of there being less energy after the collision than before the collision so you add up the energy of the say two colliding protons you add up the energy of all the debris that you can get your hands on that the detectives detect and if the energy of all that debris adds up to a number that's less than the energy that you started with that could be evidence of what you're asking that could be evidence of accessing another dimension it could be evidence of particles wafting out of our dimensions and entering into other dimensions we have never seen a process like that unfortunately it'd be hugely exciting to have a whole mountain of data supporting the idea of these missing energy signatures supporting the idea of extra dimensions but unfortunately we don't have any such data of that sort yet in fact I was thinking now that you asked about extra dimensions we have a video that we created about extra dimensions I don't know if any of you've seen my string theory video where I'm in an environment and all these things happen around me oranges grow strings vibrate violins grow out of my chin allow me to explain the basic idea of string theory if you have seen that video and and liked it let me know because we have another video in the same visual look but on extra dimensions and that might be we've never released it may be fun to release that now that many people are home hanging out searching for interesting things to look at and maybe we'll release that in the next couple of weeks and it's sort of a fun example of the of thinking about and imagining extra dimensions of space okay aashiq rayel asks professor grant according to dr. Tyson which I presume is dr. Neil Tyson because there's other Tyson's in the in the worlds of physics if Earth turned into a black hole moon still rotated around Earth is that mean curvature of space-time don't depends on its mass density darn I'm having trouble parsing that one a little bit but I'll interpret it to mean the following if you have a spherical body like the earth it gives rise to a certain gravitational field around it and that gravitational field in the general theory of relativity is reflected in the space-time curvature in the environment outside earth now if you were to squeeze earth down to the size of a black hole you have to squeeze it down to like a couple centimeters across or something the only change you know to be gravitational waves but let's imagine that you just sort of somehow replaced the earth by a smaller earth the gravitational field in the environment outside the distance from the Earth's center to its surface right so roughly whatever four thousand miles or something the gravitational field out there wouldn't change at all the only thing that would change is you could now travel closer and closer to the sources of gravity because the Earth's surface would no longer get into your way if you crushed it down to a centimeter across so if your question is that the gravitational field due to a spherical body only depends on its mass the answer is that you you know I'm not talking about things like rotation and electric charges over the answer to that yes there's a powerful theorem called birkhoff's theorem and birkhoff's theorem tells us that the gravitational field outside a spherical body is just dependent upon its mass so in that sense I think the question you're asking is if you crush it down to a tiny little nugget of the same mass would it ultimately yield the same gravitational field far away and the answer is yes absolutely yes okay so so Kerr CaroMont asks why are we so smart other animals do not even get close well I kind of have some sympathy with that every time you know I've been accused strangely enough I don't know I've been accused by some people of not giving animals enough credit not giving animals enough credit regarding their language capacities and I kind of feel bad that I give that impression I do think a difference between us and animals with the level of sophistication of our language I have read studies that have tried to analyze the information content in the utterances of other species and by any measure that seems reasonable it's clear that the information content is less than the information content in the utterances that we humans make you can just sort of analyze the variation in the sonic waveform and the variation in ours the realm or the spread of variation in the sonic form allows for it to carry more information than the spread in the wave form of other animals whose utterances have been analyzed is that airtight could there be some other quality of their utterances that carries information and maybe they know more than we do or have much more sophisticated communication than we are aware of it's possible it's just I don't see any evidence for that and when I say I I don't do these studies linguists do these studies so maybe there are other studies that I'm not aware of that have given evidence that in the animal kingdom besides human beings there is a level of communication that I'm not aware of and I'd be interested to know of those references if you have any but putting those possibilities to the side there is something special about our mode of communication there's something special about what we can do with our ways of analyzing the world both through ordinary language and through the language of mathematics and why are we able to do that I would say the reason we're able to do that is because we're the beneficiary of generation upon generation upon generation of evolutionary advancement evolution by natural selection has allowed us to head off in a direction that is quite unique and quite special and I don't know that dogs are able to solve on Stein's field equations I don't know the cats or fish or dolphins are able to analyze Schrodinger's equation to get the spectral lines of hydrogen maybe they can or maybe they do other things that are so spectacular that along a different axis we are the less intelligent species and they are the more intelligent species again I don't see any evidence for that but the bottom line is evolution by natural selection is a powerful force that allows molecules and structures to get ever more refined in their organizational structure and we have so exquisitely organized that we don't have to limit our language and our thinking to the moment most animal communication that I am aware of is for the most part focused on issues of the moment a predator of the moment an opportunity of the moment a danger of the moment what we humans can do is we can use the power of language to rise above the now and really think about the past and analyze the future contemplate what was and imagine what will be I don't think elephants can do that I don't think elephants can predict that the Sun will swell to a red giant in 5 billion years Dave Emeril I don't think dolphins are able to calculate that the lifetime of a solar mass black hole is about 10 to the sixty-eight years I don't think they can calculate that I don't even think they can contemplate that am I wrong I don't think so but I'd love to hear views if you think otherwise so mr. castle asks is there a quantum mechanical equation showing why we can't predict the time for radioactive decay for a single atom and and there is I mean it's right there in the Schrodinger equation with the interpretation of the wave function given to us by Max Born so this was a subject of at least one of your daily equation episodes and in the Schrodinger equation and in the wave function is intrinsically a notion of probability intrinsically a notion of probability that's what the wave function is it's a mathematical articulation of the probabilities of one or another outcome and because it's in the language of probability all you can ever say is there's a 50% or 30% or a 90% chance or a 1% chance of this that or the other thing happening and radioactive decay is one of those things so you can only predict the probability of the decay happening as opposed to when it will definitely happen and that is the nature of the beast in a world in which probabilities are so intimately interwoven into the nature of reality so Lauren crew asks a follow-up based on that thought I'm not sure which that thought is but probably something earlier your thoughts and consciousness yeah no you've told me what thought Thank You Lauren so based on that thought your thoughts and consciousness in string theory do you think there is a much larger and single consciousness and no is my answer I'm not saying that there can't be and again I'm not dogmatic and I and I and I hope I don't come across as dogmatic even when I forcefully describe my point of view I describe things forcefully both verbally and in written form because I get excited about these ideas and there are certain ideas that I feel very strongly about because they so directly emerge from our understanding of the physical universe and the ingredients making up the physical universe but I'm not dogmatic but when I think about consciousness Lauren I think about it in the incarnation that I am aware of and the only incarnation of consciousness that I am aware of is my own consciousness and it takes place in a brain and then I extrapolate from that to allow for consciousness to exist in other brains and by other reports of people in the world around me they seem to confirm that that indeed is what's going on other people claim to be conscious I don't know that's true but I'm willing to admit the possibility because they have a brain much like my own since my brain allows me to be conscious I will admit there's a likelihood that their brain allows them to be conscious good ok and then I look at animals dogs and cats and their behavior seems to suggest that they have a certain degree of conscious awareness as well and they also have a thing much like mine slightly different form in their heads called the brain of their species is what I'm willing to allow that their brains are also centers of conscious awareness but I don't see any evidence for any kind of conscious awareness that stands outside some kind of brain or some kind of physiological system that we would call alive I want to even be a little bit more broader and therefore the notion of some kind of universal consciousness that presumably would inhabit space and time as opposed to inhabit a brain is not something that I'm led to posit because I don't see any evidence for any kind of conscious awareness outside of the motion of particles inside an organized structure called a brain you might call that limited thinking I call it thinking based on the evidence based on the facts based on our understanding of how the world seems to work I am open to the aliens coming down and saying you guys are so limited in your thinking or or or Brian Greene you're so limiting your thinking why didn't you listen to Lauren Crewe it was more expansive and and they're thinking and that could happen it could happen but I just don't see any evidence for it and therefore I entertain it as an interesting mind tickling possibility I don't entertain it as something that I want to integrate into my worldview at least not yet I hope that addresses the question Laura and feel free to follow up if you have other thoughts that have not addressed there so progress asks we'd love to hear your thoughts on the opto nians and what role this discrete number system could play in the bigger understanding of our universe I haven't seen this subject touched and for those of you who aren't familiar with the octo nians but hopefully are familiar with imaginary numbers so imagine a numbers you know introduce this new number called I where I squared is equal to minus one and that proved very useful in physics it's not that we have devices that measure imaginary numbers but the mathematical tool of imaginary numbers is a very powerful one we see it at work in quantum mechanics we see it at work in the wave theory more generally and so the natural question is if the move from real numbers to imaginary numbers was useful are there other moves to other number systems that might be equally or even more powerful and impactful and indeed there's a natural progression which is where we get to inspire progress is question you can go from the imaginary numbers to I guess are called the Hamiltonian or Cayley numbers you know I J and K for I squared equals minus when Jace critics my case good his minor items Jake was K items K equals minus J and so forth and that's an interesting number system then there's the another one the octo nians so imaginary number you just have I and minus 1 so I squared is minus 1 so you said we got two numbers there for the next step you have for I J and K and I squared equals minus 1 Z minus 1 I J and K you have the octo needs we have 8 such ingredients that come together in an interesting structure and might that of B have my kind of physical relevance and and people have actually studied this people have gone into all sorts of interesting discrete number system p-adic numbers based on the prime P if you are familiar with those there's a P attic version of string theory that people have developed but I don't I don't really I don't know that these have had any great success they're interesting mathematical structure and maybe someone can correct maybe there is some deep use of the octo nians in physics I can't really think of I'm probably missing something obvious and just having a moment of brain old-age amnesiac here but I don't really know of any good example of the octo nians where I can say to you aha if it wasn't for the octo nians we would never really have been able to analyze X someone tell me about X's and I don't really know of an example but I do know there had these you know fun P attic examples of string theory again I wouldn't say that they've done anything deep for us just yet but the fact that they exist is certainly interesting so I saved the jury's out the other thing that the jury's out on that people often ask me about is you know there are different kinds of topologies different topologies different notions of nearness and different notions of open sets that you can make use of in even you know higher dimensional spaces that we study or in four dimensions they're different differential structures do these have any role in physics I don't know again and people have written down papers with using different differential structures different notion of derivatives they've written down papers using different topological structures on higher dimensional spaces but I've never really seen anything come out of it where I can say and this is the deep insight into our world that has emerged from these studies so again jury is still out I hope that answers the question inspire progress um let's see what else we want to deal with here so Jo Siebert asks if there is a limit to how fast a particle can move through space is there also a limit to how fast it can move through time and what I would say is I think the best way of answering that question is there is a limit in fact a very precise number that describes how fast a particle moves through space-time rather than breaking it up into space and time separately it's more profitable to think about it particles motion through space-time and in a very real sense the speed of a particle through space-time is always equal to the speed of light always equal to the speed of light so there's a fixed and never changing speed through space-time that describes the velocity of particles through space-time itself now when you break that up into space and time and as interesting consequences because if there's a fixed speed through space-time then if you're not moving through space then all of that motion through space time is through time and that is the rate at which you age if however you're like a photon and you're always moving through space at the speed of light then there's nothing left to move through time and that comes this poetic notion of photons not ageing photons experience in some sense all the time at once if you are a regular person like me and you get up and you start to walk through space then you are diverting some of your fixed amount of motion through space-time into that motion through space and there is less speed left over to go through time and that explains why your motion through time slows so if I'm watching you and you go up and walk well before you walk I would say all your motion is through space-time you're moving through space-time at the speed of light as you get up and start to walk I say you now have diverted some of your previous motion through space-time into motion through space and therefore you've got less left over to move through time and therefore I will conclude that your motion through time will slow down and indeed it does slow down that is the slowing of time for an object in motion that to me is the most fruitful way of thinking about the relationship of motion through space and through time marry them together consider motion through space-time and then you get the conclusion that I just described so captain frantic says is it's sort of like more you go Easter less you move north and yes that's exactly the analogy that we use that I use in elegant universe and also did visually in fabric of the cosmos now so if you're moving at a fixed speed 100 km/h in the northward direction and your car can only go a hundred kilometers per hour if you veer off to the east then you've diverted some of your northward motion into eastward motion so your progress in the northward direction your speed in the north or direction will slow similarly if you've got a fix and unchanging speed through space-time of the speed of light and if you then start to move through space your dev are diverting some of your motion in space-time into motion through space so less is available for for your motion through time and your motion through time will slow exactly the same thing with space and time as you suggest captain frantic with space and space all right so I think guys I need to I need to exercise more to be able to do the full three hours that I did last time because I'm now with the hour that I did prior to this session I'm at 3 hours and 8 minutes and I just don't know that I have the energy left to keep going but I'm gonna do a few more just for the heck of it right that was like a false ending to this session right you all thought it's all over it's time to go to sleep but actually I'm gonna do just a few more my second wind just coming up alright yeah good keep those questions rolling do not stop all right sir what do we have here can dimensions be relative saintly physics asks in a sense yes right in terms of the motion through one can affect the passage through another with space and time so in that sense yes but in in any other sense you'd have to really articulate what you mean by relative I'm not sure exactly what you are referring to quantum jump says can we access the reddit I think so it was a reddit public access network it was set up by Random House so it's the penguin Random House subreddit or something like that subreddit called social distancing all these words don't really mean a whole lot to me but if you search on to it I would imagine that you can find it all right Lauren has a follow up on her consciousness question what about in relationship with string theory all particles are vibrating at specific frequency could that be occurring in and out of our brain hence in one mind and the answer to that again would be I would say no if string theory is correct a big F in its own right then yes the particles in your brain would properly be described in the language that you use as vibrating filaments inside your brain and the interaction of those vibrating filaments is what generates in a way that we cannot yet articulate with any precision whatsoever your conscious awareness but just because my particles of my brain are vibrating strings and just because your particles in your brain or vibrating strings it doesn't mean we are one brain it does not mean we are one thing I like the poetry of that I like the sensation that you have in mind that we're all sort of part of one universal cosmic consciousness that arises from the vibrating strings of string theory but it doesn't mean anything as far as I know it sounds nice and if it makes you feel good I have no objection to you thinking about the world in those terms but my point is if consciousness is what I think it is and what many researchers think it is physical processes taking place inside of a brain then your brain is one center of consciousness my brain is another center of consciousness and they are separate centers of consciousness they are not the same they are not one all right follow-up from last week how do you re on re re on ORN that's sort of like in Game of Thrones is not her name Ariane and I just started watching Game of Thrones so I'm like years behind but anyway follow from last week how do you reach the extra dimensions via equations but I guess we mean it's how you describe the actually question also hook the tofu turned out well last week you realize that last week when I went home after our marathon session my son had been watching this and he held me to cooking the tofu he said look dad you mentioned you were gonna cook tofu to everybody who was watching so I'm gonna hold you to it and I was so exhausted but yeah I did I made tofu and black bean sauce I made one piece of deep fried tofu for my daughter who does not like black bean sauce and everything else oh yes it turned out turned out quite well so thank you for asking now to your question in terms of the extra dimensions how do we describe them with extra dimensions I'm gonna show you if that's possible I'm gonna try to bring my iPad up to what we're looking at here let's see if it's working so I got to turn this on a little bit brighter so I can actually see what I'm doing I have the the brightness down low so it doesn't run out of juice because I'm almost out as it is but let's see alright so I'm gonna try to do this I'm gonna try to bring up my my screen so here is my share my screen is this gonna work start the broadcast I think it's gonna work yeah alright alright let's see three two one come on broadcast and now if I bring up my notability which should be somewhere you can see all my stupid apps where's my notability oh come on and now it's not scrolling there it is where's my notability oh damn it this thing is like it's hiccuping amount of guys I'm gonna give it one more second I'm gonna give up come on come on come on alright my iPad has died on me guys it's no longer responding so anyway I'm gonna stop this broadcast darn I thought it's gonna be all fancy with you and do my usual iPad split-screen but I can't get it to work I think I've shut off that broadcast who tell me if I have I think I have um so here's how you do it with with maths X Y & Z is usually how we mathematically described the common three dimensions of experience X gives you a distance this way Y gives you a distance this way Z gives you the distance this way how do we go to higher dimensions we add new letters X Y Z and W I'm not being facetious here that W is a coordinate if it's a fourth independent coordinate it describes a space that has four dimensions if I have X Y Z W and T let's say T is time now I've got four dimensions of space in one of time if I want to have 10 dimensions of space and one of time let me do X 1 X 2 X 3 X 4 up to X 10 and time and by writing down the equations that govern the geometry the shape of these dimensions we can have arbitrarily curved or curled up dimensions and that's all that we do now if you ask me can I picture higher dimensional shapes that I can articulate using the coordinates and equations I can't I'll be flat-out honest with you I've worked on higher dimensional shapes in string theory for 30 years can I picture them no can I write down equations that allow me to analyze them yes can those equations give me an understanding on par with visualization I think they do but if you ask me in my mind's eye the way I can picture a basketball and a donut can I picture a higher dimensional shape no now are there people who claim that they can there are so there's a famous story I don't know if it's true of the mathematician at Princeton Thurston teaching a class and I don't know something like six dimensional spaces and he describes something to the class and he looks at them and they all looked kind of baffled and he doesn't understand why they're baffled so Thurston goes to the corner of the room to think about how to better explain these things and he comes back and says hi know why you are baffled by my explanation the process I'm talking about is much more easily visualized in 12 dimensions you know is that a true story I don't even know can he really couldn't really picture things in higher dimensions I don't know I can't so that's uh that's my perhaps own limitation all right what else do we have Navi Bal asked if our universe is 13.8 billion years old how can astronomers find starts older than 13.8 billion years show me those stars show me I don't know what you're talking about I'm not aware of stars that are older than the age of the universe so if you have evidence for that send it on look to take a look at it let's see Kim Conger asks how do we know that there are more three spatial dimensions we don't Kim we don't in the mathematics of string theory the math as far as you know seems to require that there are more than three dimensions of space but we don't know that the strength is right I'm a skeptic of string theory I've said it before I'll say it again and you should be a skeptic of string theory too don't believe anything that doesn't have observational or experimental support it's an interesting possibility of extra dimensions it naturally emerges from string theory we've looked for evidence for them and we haven't found any yet it doesn't mean that they're not there it doesn't mean that we haven't acquired the data establishing that they are there so how could we establish them well look I gave an experiment before slam the protons together at the heart Large Hadron Collider and find missing energy signatures in just the right pattern that agrees with what we'd expect if there are extra dimensions soaking up that energy draining it from our dimensions that would be really interesting but we haven't found those signatures so could we prove or could we acquire increased evidence beyond pure mathematics for the existence of extra dimension sure have we not yet that's the state of play a cure to dev why do most physicists accept the assumption of eleven dimensions and why not twelve or thirteen or 26 or ten and blah blah blah yes so first of all most physicists do not accept the existence of 10 or 11 dimensions most string theorists allow for that possibility and work within a framework that involves those extra dimensions but we're not saying that they're there in the sense that they've been proven experimentally they've not been proven experimentally now you might say why do we look at the particular number of extra dimensions that we do instead of 13 or other numbers that Kurt to dev harita lists that's mathematics so in the mathematics of string theory there are equations that tell us the number of dimensions we don't just make this up we don't just say hey you know tens nines for Elevens even better why not 12 or 13 it's mathematical investigations that lead us to particular numbers of extra dimensions and those investigations have not led us to a unique number there are versions of our mathematics in the old days that suggested 10 dimensions of space more refined versions of those mathematical equations suggest 11 dimensions some insightful physicists have found reason to allow for the possibility of 12 in the early days of string theory when it didn't have so called fermions only have bosons the theory suggested that there were 26 space-time dimension 25 of space in one a time so we have been led to consider a lot of possibilities for the number of extra dimensions but as yet there's no evidence for any of them in the sense of concrete observations or experiments it's only mathematics that leads us to these possibilities so that is the state of play as of as of now may change all right so Joe Siebert asks a follow-up can the rate in which we age when we're at rest be considered a maximum speed through time in the same way that the speed of light is a maximum speed through space absolutely absolutely so when I am looking at you was a Joe sorry the question just rolled away I think your name was Joe I'm sorry if it's not so Joe as I look at you can't really show you but if I imagine you sitting still then I can imagine the rate at which you are aging and I don't mean you know some very personal biological age I'm talking about how quickly your clock is ticking forward in time and that is your biological aging although people aged biologically at different rates and that's not what I'm talking about here if you then get up and start to walk Joe I will if I carefully measure things conclude that your clock is ticking of time solver so you're aging slower so yes the maximum rate at which I will say that you are aging is when you and I are not moving relative to each other and if we are moving relative to each other the rate of which I will say you are aging is less than the rate I would have said when you're stationary so good excellent conclusion love that we are we are of one mind of the Lauren I'm talking poetically I do not think that we are one mind literally okay some more stuff before we wrap it up what do we got here Andrew Hanny ass is length contraction a consequence of the relativity of simultaneity and you know in some sense yes so all of the pieces of special relativity fit together into a very tight logical pattern and where you begin and where you end is a matter of taste and you can begin your thinking about relativity with the relativity of simultaneity and recognize that observers in relative motion will not agree on what events happen at the same moment and with that analysis you can naturally extend it to realize that not only is it that clocks don't agree but you can work out the precise discrepancy between simultaneity aha then you've got time dilation and once you've got time dilation then you've got length contraction because you can measure lengths with a stopwatch if I want to measure the length of a train that is rolling by me I can work out or if I'm told it's speed relative to me I start the watch when the front of the train passes me I stop it when the end of it passes me I multiply that duration times the speed and I've got the length of the train so once you got time dilation in clocks then length contraction will follow it because if you're measuring the length of a train on a clock that I say is measuring time more slowly than my own clear you really get a smaller length and I will because your clock is ticking time more slowly so I guess is the answer and you can sort of have the logical arrows in special relativity emerge in whatever order you like the fact is they'll all exhaust the interesting insights but just through a different order different way of thinking through the logical connections so meirin Chi Kendall asked quakin does is the block universe compatible with quantum mechanics and and it sort of is you know the block universe you may know guys is this notion of past present and future all existing at the same I can't even frame it same time but I'm talking about time so but past presidents you're all existing I don't even have a time it says no temporal quality so the past normally we think is back this way the future that way we're right here but this whole block of time in some relativistic sense just exists is that compatible with quantum mechanics well yes it is in the sense that in quantum mechanics the only difference is we talk about probabilistic outcomes but if you think about like the entire wavefunction of the whole shebang just meandering along by the Schrodinger equation then in that notion of reality so the block is a different kind of block but in that notion of reality again you can lay out the whole history of the wavefunction by just evolving it through Schrodinger's equation so the block is a little bit different it's a block of the probabilistic outcomes at any given moment in our version of reality but if you elevate reality to be the domain where the wavefunction lives and it's evolving through time then yeah the blockiness of that reality is just as blocky as the classical version focusing upon positions and velocities of particles except now we're talking about the wavefunction and the probabilities that it describes so in some sense the answer is yes okay Prabir dosh has what is the no-hair theorem well that's the theorem that as you continue to watch these your daily equation episodes I will have less and less hair as the years roll by and the theorem is that at some point I will have no hair that is a version of the no hair theorem but of course it's not what you have in mind what you have in mind is the idea that black holes have no hair what does that mean that means that every black hole that has the same mass charge and angular momentum every such black hole is identical according to the classical description of the general theory of relativity so much as you know in some sense you know all you know bald heads look the same all black holes look the same if they have the same mass charge in angular momenta there aren't any distinguishing qualities black holes don't have hair in the sense of having local distinguishing qualities that make once it's black hole different from another now these ideas were developed a long time ago and they have been challenged in very subtle ways where there are kinds of hair that people now believe that black holes can support actually I've been working on this myself come to think of it I showed years ago that black holes can have a certain kind of babka gauge theory a certain kind of gauge theoretic hair but it wouldn't be stable in the sense that if you kick the black hole at all those singin qualities will radiate the way to infinity but there have been more subtle things to do with various unusual symmetries that are relevant for this discussion so there is a sense in which black holes may have a certain kind of hair even though the old classical notions of the more coarse kind of hair have been eliminated through those no hair theorems so fabulous asked why don't physicists combine all the physics equations and formula with general relativity and see what happens we've done that and what happens is not nice the equations all break down the equations give infinity as their answer if you just try to naively put quantum mechanics together with the general theory of relativity that's why we're motivated to develop theories like string theory theories in which you put gravity and quantum mechanics together and they don't yield nonsensical mathematical results so good idea we've tried it the most naive way fails more sophisticated complex ways seem to work although we've yet to test these ideas through observation or experiment so Anand ask is everything predetermined and again no I'm not saying that everything is predetermined in the sense of the actual outcomes that happen even a single electron the wave function gives us probabilities for what's going to happen are those probabilities predetermined yes in the sense that they are determined by the mathematical equation Schrodinger's equation that just evolves everything forward in time so if there's nothing else but physics and I don't think there is anything else but physics in terms of the fundamental evolution fundamental change of how the universe progresses from moment to moment then it's not that the outcomes are predetermined but the probabilities of each possible outcome those are pre determined so King Lamar says tachyons travel faster than light what and yeah you're right is what most people think the takin's are not real but they are hypothetical particles that would travel faster than the speed of light they would up end notions of causality if we were playing catch with a tachyon baseball there would be observers who would say that you'd receive attacking on baseball before it threw it to you that's weird but nevertheless if these things were real that's what would happen but most people do not believe that they are real so bogdan asks what are your open questions and yeah I mean open questions that is um that's what science is all about I'm sure answering questions is great but it's the open questions that really fire us up and I think the deep open questions are the nature of space and time you know in in your daily equation in the series whether you've watched the previous episodes or not I've said a lot about space and time I've spoken about some of the things that people have asked here today I've spoken about length contraction have spoken about time dilation I've spoken about the curvature of space-time in the general theory of relativity but the weird thing is if you said to me yeah you're using the language of space and time all over the place but you've never defined it you never defines face you never define time and how - say - you're right I don't really know how to define them nobody really does we can speak about their properties and in that way we refine our understanding of space and time but I do not know a fundamental articulation of what spaces or what time is moreover I don't know if space and time are fundamental it could be that space and time are built up of finer ingredients I like to call them atoms of space or atoms of time in the amended earlier and if we could find the atoms of space or the atoms of time or even find the right mathematical language for really talking about them some of my colleagues in the loop quantum gravity community believe that they have the language you're doing so I'm less convinced of that but one day I think we will have consensus on the right language to talk about the ingredients of space and time and that's when I think things will take a radical step forward I think that's where the big jump in understanding will happen so that to me is the deep open question sort of in fundamental science but you know outside of physics bogged in the open questions that really fire me up are what is consciousness like Lauren is asking what is consciousness I'm giving you my perspective on what the answer to that ultimately will look like I think the answer ultimately will come down to consciousness is this physical process I mean like aha I get it that physical process the motion of those particles in that particular combination in that particular choreography that's what it means to feel this or that emotion that's what it means to feel this or that aspiration AHA will begin to understand it at that deep level I do think that whatever ten thousand years from now or whatever is how we will understand consciousness do I know that for sure I don't could it be otherwise possibly so really working out as best we can the nature of consciousness I think is again one of those deep open questions exciting deep open question so hannah geist marin ellis asks do you think you have sacrificed many things in your life would you undo something looking back well we all have to make dramatic sacrifices each and every one of us in different domains that we experience you know i-i've been blessed I don't have great hardship in in my history like everybody else I've suffered difficult situations you know the death of a parent is always a difficult situation and I've gone through that and that's tough you know but if I talk about sacrifices I think most of my sacrifices were sitting in a library or sitting in my office 10 12 hours a day trying to make progress in physics and I don't I didn't view it as a sacrifice at the time and I don't even know I would describe it as a sacrifice today but it definitely definitely was a choice and it was a choice of not doing something else and you know now you can bring him free well it did I freely make that choice let's put all that to the side it certainly felt like I was making the choice freely and I'm gonna use ordinary language of human discourse even though you know my view that behind the scenes I didn't have any choice at all but using the common language of human discourse just that we can communicate one another it was a choice and it was a choice at the exclusion of other things and so you look back in your life and you do wonder what trajectory your life would have taken had you made different choices and and made different sacrifices but I'm I'm I'm I'm thrilled and happy to have had a life it's not over yet I hope I've had a life that focused on questions that really matter to me you know I look friends of mine went into Wall Street are really rich I'm not really rich at all my son sometimes says me can't you take a few years off and just go to Wall Street get really rich it's not my thing not that I wouldn't like money like everybody else but the things that matter to me are these questions I mean I love talking about these things and thinking about the nature of time and space and relativity and quantum mechanics and unification and string theory and black holes and the big banging cyclical cosmology of that holds any water and I love those ideas and those ideas to me make life worth living so there are a lot of sacrifices to make headway in this arena to contribute some small amount to knowledge but I consider them all worthwhile because in the end of the day you have a short finite life and you want to spend your life doing something that focuses upon things that you feel matter and to me the ideas and bringing these ideas to the general public as we're doing right here to me it feels like that matters and in the end you know in your short period of time if you can say that you did something that you felt mattered that improved people's lives in some small way or or brought joy to people by providing information in a way that was successful and exciting and and visually interesting I can't do the visual side of it other folks who work with me do all the visuals out of it then yeah it feels like it's worth it John prova Titus asked Percy Greene did you study solid-state physics during your degree and did you enjoy it I started a little solid state physics in a graduate course in quantum mechanics there was some part of it that focused on solid state physics and I guess actually went to Oxford as a graduate student we did some solid-state physics I didn't really enjoy it sorry I mean I didn't hate it it just felt more applied than suited my perspective I am drawn more to the big questions of like the origin of the universe that I am - the physics involved in certain materials and it's just a matter of taste but I've always just been fired up by those questions that to me feel bigger more philosophical maybe that's a way of saying it I'd like physics questions that border on the philosophical questions because those are the kinds of questions that we can think about even without technical training and get interested in I mean we can all think about what is time we can all think about like how would a universe begin we can all think about why is there something rather than nothing I'd like those kind of questions but we can't all think about you know the electron band structure and this or that material you have to have the technical training to talk about that and so that's why I think the former questions are more they grabbed me more than 10 questions say in solid-state physics but you know everybody's different and you have to follow wherever your passion leads let's see what else we have here max have Lobeck ask is there any possibility that there are also huge dimensions so huge that we're not able to see them and and I it is an interesting question is one that people ask now and then you know when you say that something is perhaps curled up so small that you don't see it then ask yourself well could it also be so big that we're not aware of it and when it comes to dimensions there is a version of that story that I'll come to in half a second but the most naive version I don't see how that works so you know if I can see along a dimension as long as it's big enough for me to send light beams along that direction then regardless of whether the dimension is you know a meter or an infinity of meters it doesn't matter my light will traverse that dimension and I'll be able to access it so the most naive version of dimension being so big that we can't see it I don't know how to make sense of that now having said that there is a version of string theory any was we discussed before this brane world scenario I discussed it with slices of bread I think is the analogy that I used where we can't see off of our slice of bread we can't see off of our dimensions light itself is constrained to our dimension and that allows the other dimensions to be quite large and yet invisible to us not invisible because they are large per se but invisible because regardless of them being large light can't traverse their expanse so sort of in-between to what you're asking there can be big dimensions that in principle we can't see because what we used to see namely light is unable to travel in those directions so so sort of a yes and no on that um Rob Vandenberg asked where do you stand on time being a human construct and I don't know I don't know part of me feels that the temporal quality to the world is fundamental but that's partly because every theory that's ever been developed has time within it and the successful ones have time within it and so we're led to an intuition that time is within the cosmos I certainly allow for the possibility that those atoms of time that I mentioned a little while ago might be such that when they're not configured in the right way there isn't even a notion of the conventional human construct of time and that then would of course lead to the possibility of realms where there is no time and and and then you then you ask yourself when those particles are configured in the right way is that time woven into the fabric of reality or is it something that we notice as a pattern and we impose upon reality in order to organize perception so it's a very hard question it's time that we something we impose on the world in order that we have some organizational framework for the events that we observe or is time fundamentally they're hard to say hard to say so Ariane asks about a time machine I said I've already discussed that earlier with the wormhole so I won't go back to that Candice asks Candace will is the discovery neutrinos linked with the presence of parallel universes and I would say no so the existence of neutrinos does not in any way shape or form require anything but our universe and so speculations about other universes that may come from data associate with neutrinos is interesting to kick around but in no way is there some kind of logical link between neutrinos and other universes none whatsoever Jonathan Salas asks do you think we will ever reach the technological capacity to observe strings and well it partly well largely depends on how big well it depends on whether shrinker is right so if string theory is wrong and the answer is no no matter what our technological capacity will be we will not observe strings so they don't exist if string theory is right then the answer depends on how big the strings are in the most conventional formulation of string theory the strings are the Planck length it's about 10 to the minus 35 meters that is so spectacularly small that is hard to wrap your head around how small that is an analogy I like to use is that if you were to take an atom and scale it up to be as big as the entire observable universe that's a huge magnification then under that same magnification the Planck length or the size of a string in the commencement formulation would grow to be roughly the size of a tree so a string is to an atom as a tree is to the observable universe that shows how fantastically small strings are even on atomic scales and so it's hard to imagine the technology that would be able to directly probe such an incredibly small scale it doesn't mean they won't exist but hard to imagine what in the world it would be now having said that there are versions of string theory which the strings are bigger in some versions of string very much bigger and in any of those theories proved to be the right one then yeah it could be that the observation of strings might be right around the corner might be within our lifetime I think that's extremely unlikely but just to say there are mathematical versions of string theory in which they are not as tiny as in the conventional formulation therefore Jonathan they would in principle be susceptible to observation perhaps in the not-too-distant future that would be a wonderful development I would not hold your breath waiting for that to occur though so Cambridge breaths as an interesting follow-up question why can't we predict what is the next thought that's going to occur inside of our brain and you need to bear in mind that when I talk about say the lack of free will I don't frame it in terms of my being able to predict what you're gonna think next and I don't frame it in that way for a very good reason the ability for me to predict what you're going to do next is logically distinct from my claim that what you do next will not be the result of your free will those are two different statements and the ability for me to predict your next thought is one that runs afoul of our basic understanding of quantum mechanics again I can't even predict whether the electron will be here or there if the wavefunction is the probability waist is 50 percent chance here 50 percent chance yeah I cannot predict where it will be and if I can't predict where a single electron is going to be based on the insight given to me by the wavefunction of quantum mechanics then I can't predict the state of your brain made up of trillions of particles I can't do that that's not what quantum mechanics allows me to do now even though I can't predict where your particles will be and what they'll be doing I can say that whatever they do will be the outcome of an evolution described by Schrodinger's equation not by you you can't get in on the evolution only the mathematical articulation is controlling it only controls probabilities but it still controls them with ironclad grip and so whereas I can say with some confidence that you don't have free will I can't say with any confidence what you're gonna do next so in that sense you could say you're free no one's ever gonna predict what you're gonna do next but that's a very modest kind of freedom I think most of us feel freedom by saying we are the source of our decisions and actions not that nobody can predict what we're going to do that's not enough we want to feel that we are the ultimate authors of what we do that's what's necessary for the usual conventional notion of free will and that's what I'm claiming runs into significant tension with our understanding of fundamental physics all right a few more before I will sign off here do astronauts drink coffee in space ENS I don't know I though I do know some astronauts I can ask them if you're really curious I imagine that they do Harvard grad thank you for your time and congratulations Harvard grad did you graduate today Harvard grad I believe today was graduation anything else wait here's a garden says you were free but the universe already knows what you're going to do no I'd say it's completely opposite you're not free and the universe does not know what you're going to do so I'd say that comment garden is maximally wrong I don't mean that it as an insult I'm just saying it's completely opposite from what I would say is the case Claudio Fernandez what come on ongoing technological endeavors excite you and how can they influence physics well there as you may know is a wonderful collection of developments in artificial intelligence editing toward generalized artificial intelligence and you know I've spoken to some of the folks who are leading the charge here and some of them come from the world of physics and want to go back to the world of physics and are working out ways in which they can turn these artificial intelligence developments set them to work on problems and fundamental physics and how interesting would that be if an artificially intelligent structure was able to give us insight into some of the unsolved questions in physics wow that's what some of these folks claim is going to happen in the not-too-distant future I don't know if it'll put us physicists us us hopefully intelligent but not artificially so physicists out of business but maybe maybe they will that that would be a that'd be quite interesting all right let's see what time is it Alexandra wants to know what I'm gonna cook for dinner tonight I'm not gonna commit to it because as I said my son may be watching and if he is I don't want to be locked in I don't have free choice on a free will in deciding what I'm gonna what I'm gonna do for dinner let me just one one or two before we wrap it up so Georgio's George I Giorgio a see if only probabilities are predetermined how can we Square this with Einsteins block universe again bear mine Einsteins block universe is a completely classical description of the world it's a description of the world that takes into account relativity but does not take into account quantum mechanics and if you want to take quantum mechanics into account too then you have to modify the block universe in my description in terms of probabilities attempts to do just that Harry Potter was a harry potter question under that's coming from maybe you guys can tell me my you know it's raining here I know if you can see it it's gonna for the next day and a half my son wants me to agree to watch all Harry Potter movies all eight of them in the next day and a half while it's raining outside that make any sense to you guys I haven't seen any Harry Potter movies are they good he wants to bring me into the Harry Potter and universe before I go somebody tell me whether I should do that but let me just find a physics question in the interim everything's going by so fast Oh King Umar said light speed is slower in water then what do you mean by the speed of light being constant and yeah when we talk about the speed of light being constant we mean that it's speed through empty space through a vacuum as a fixed number of fixed speed we've long known that when light goes through a medium its speed can change because the medium affects has the light propagates so you're right the speed of water is speed of light in water is slower than the speed of light in a vacuum and but that doesn't affect anything that we talked about because the statement more precisely is the speed of light in a vacuum is constant quantum jumps asks from Austin Texas have you found quantum computing hell Bodie no Scott Aaronson I dunno Scott Aaronson very nice guy incredibly smart guy and quantum computing well I would say that it has not helping me in any of the physics that I've done but I full well imagine that as quantum computing develops it will maybe it gets blended with generalized artificial intelligence to yield a new way of doing physics can I imagine that I can so yes I do think that one day now and by the way I should also say when when you say quantum computing I took it to mean the actual devices that people are developing and building to do quantum computing but quantum computing as a theoretical subject has had enormous influence in extending our understanding of quantum mechanics so in that sense yes the people who have spent a lot of time developing the theoretical architecture of quantum computing have advanced our understanding of quantum mechanics in important ways let's see what else we got here anything what time is it now it's 553 so I'll wrap this up in seven minutes and that would be our new my new record of four hours with the the reddit that went before this let me just see a still test paratus how smart is Ed Witten goddamn smart incredibly smart frightening ly smart you know I worked with him at the Institute for Advanced Study many years ago and you know you'd stand in his office and we'd all be looking at the same puzzle on the blackboard but Edie would be like he'd come out with some insight be like oh my god where did that come from where did that where did that insight come from back a friend of mine spent a summer at the Institute for Advanced Study in an office that had a wall adjoining Edward Witten's wall and they were like the only two people there most people are traveling and he described to me how he was desperately trying to work on his research projects but all he could hear through the wall was Edward Witten typing out one masterpiece paper after another as if he was hardly needing to sit down and do the calculations he was just writing these papers in real time as these thoughts were gestating in his miraculous brain so yeah it's probably a little bit hyperbole there but having worked with him directly I can tell you it's a kind of mind that is so rare the kind of mind that is just so attuned to the workings of mathematics and the workings of physics that the insights in physics are delivered and derived and arrived at with seemingly no more effort than breathing whereas for most of us it is a struggle most of us you sit down with the equations and it's a slog to try to find something new and and an insightful to extract from the equations but some minds are just so well structured and organized to deal with the ingredients in mathematics and physics that it's just completely different a completely different process as far as I can tell and it kind of makes you wonder is it worth being a physicist when there are people like Edward Witten in the world and the answer is yes because however smart a given individual is there's just so many things that they can focus upon and the wealth of interesting problems in the world is so enormous that there is a place for everyone there's a place for people are going to focus on observations on experiment even theory theoretical ideas that are just not the ones say that are being flattened by the bulldozer of a mind like an every Witten so there is there is room there is room for everybody and that's a that is ultimately a comforting an important idea to take on board fully James Weinberg please comment on the Big Bounce theory yeah cyclical cosmology it is gaining more adherence in recent times due to the work of Paul Steinhardt and collaborators and it's worth attention and maybe it's right and only time will tell so I'm all too thrilled that alternative cosmological models are being developed let's see Brad Dibble says make sure you take your Louisville straw a slugger when you head to the house bears don't mind the rain it is so much it's really interesting though it's so much lighter than last week I thought it was dark when I when we when we ended last week which must have been around the same times in just one week it really changes that much sort of hard to imagine but um but yeah you're right bears do do like the rain Rob and uber how often do you have a Q&A with colleagues well we always kick around things we have seminars and questions and answers all the time but they're usually focused on very specific technical issues it's rare that they brought an out to the range of things that for instance we speak about here what else do we have still have two minutes to go before the witching hour Dean asks how can any mass exist inside zero volume inside a black-owned Dean or not Dean dan D aan den you're absolutely right that's the puzzle the mass falls into a black hole it gets crushed because you can prove that there's no force that can withstand the pull of gravity and therefore in in this environment and therefore the mass gets ever smaller it gets crushed to an ever smaller size so you save yourself what in the world does it mean to have mass and say zero size is that even possible and that's what you mean by singularity it's dividing by zero right the density is the mass divided by the volume if the volume is truly in a zero volume space then then you've got an infinity in there so that's not exactly right the Center for black goes more like a moment in time but it does give you a feel for the extreme physical environment at the center of a black hole now it's possible again it's classical it's possible that in the quantum description the singularity gets fuzz out and what was zero is no longer zero that may be the way it altom Utley comes about or there may be a more radical impact of quantum mechanics as described by folks who have been developing the fuzzball picture of black holes but but my point Dan is you're exactly in the right when you say what could that mean mass in a zero volume that's the kind of puzzle that we face and which drives us onward to try to make sense of it because it doesn't make sense as it is articulated John pro vittatus has a follow-up question in the last half a minute here thanks for your earlier answer professor do you believe someone can be wise without being a genius like say Albert Einstein absolutely yes absolutely yes you know there was time I'm gonna be honest here I've been honest the whole time I'm something need to say that but there was a time when I kind of thought that the only that the real geniuses were scientists and that is such a limited view of of what genius is genius is so broad and even your question is a good one wiseness and an erudition and insightfulness are qualities that can exist in minds of radically different sorts minds that perhaps we wouldn't characterize as genius whatever that actually means which is a very broad broad term and there's so much insight that we can get from so many different experiences of the world and that's really the point there are so many ways of living in so many lives that have been led in so many perspectives and so many personalities and so many different combinations of experiences inside the different brains that have lived on planet Earth that there is so much that we can get from each of those Minds even if they're not going to come forward with an Einstein field equations of the general theory of relativity they're not going to come forward with say the the ninth symphony of Beethoven there's so much that we can get from the individual experiences of individual Minds that have lived on planet Earth that it's a tragedy when those insights don't find a way of being brought out to the world so it's just to say respect and human kindness and an ultimate interest in the wealth of experience and wealth of insights is what we should all have because it's not all coming from one mind if Albert Einstein was the only human who had lived we have an impoverished world genius aside the world would be impoverished and the richness of the world comes from the richness of human experience broadly defined so with that let me wrap things up and welcome you to again join your daily equation which will be happening sporadically going forward but I'm gonna try to keep these Friday sessions as long as I can join or subscribe that button I think it's over there this subscribe button for world science festival and again as I mentioned before in the coming weeks we're gonna have more of our traditional world Science Vessel program dominant done in a non-traditional way which will be fun some of the programs that we're talking about a program on cosmology called back to the beginning I think we're going to do that we're talking about a program on quantum entanglement and wormholes which is basically EPR because er question that somebody raised here today we're talking about questions about does math reveal reality I think we're gonna have a program on that but talking about talking about all sorts of things programming dreaming so so definitely stay tuned to this channel where those programs talking with world experts will be rolling out in the next few weeks and keep an eye out for your next your daily equation and join us next Friday for another marathon session and also you can join the reddit a 2 p.m. I don't know if I'm doing next week maybe just a week after but in any event I'll see you here next Friday at 3 p.m. I'm now off with my Louisville Slugger to protect myself from the Bears I'm off to cook dinner for the family enjoy your evening or wherever you are take care you

Channel: World Science Festival

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Keywords: brian greene, 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, astrophysicist brian greene, professor brian greene interview, physicist brian greene, brian greene book, professor brian greene until the end of time, brian greene physics, brian greene joe rogan, brian greene string theory, number theory, daily series, math series

Id: n2szWY9xSzU

Length: 181min 30sec (10890 seconds)

Published: Fri May 29 2020