DC Dialogues: A Brief History of Time

good afternoon everyone thanks for coming out today my name is Andrew Lusk I am the person who actually organized this event I'm a Liberal Studies freshman here at New York University Washington DC I'm the executive manager for the DC dialogues program and this was kind of my brainchild my big project for this semester bringing together the sciences for you to see what's going on in space and time New York University DC dialogues is a student-led group that seeks to engage the NYU DC student cohort and the local community in key discussions on politics culture business environment education and specifically specifically for today science on behalf of the university and specifically NYU DC dialogues I'd like to welcome you to this incredible lecture on basic cosmological theory through the eyes of professor Stephen Hawking as presented by NYU's director for the center of cosmology and particle physics and associate professor of physics matthew Cleavon who has joined us today from New York University in New York Matthew will begin the lecture for about an hour and a half followed by a 30-minute break with sandwiches and refreshments upstairs on the first floor lobby we will then return to the auditorium for a Q&A session and continue with the second half of the lecture there will be a brief second break before a final Q&A session and program close again thank you so much for coming and please silence your cellphone's and now please join me in welcoming Matthew clay bond [Applause] can you hear me great well thanks to all of you for coming out today I'm going to describe some of the physics that's in Stephen Hawking's most famous book it's called a brief history of time and I'll tell you some I'll tell you about some recent developments also in these areas which are not in that book before I start let me just say a few words about myself there we go so as Andrew told you my name is Matthew cleamen and I'm the director of the Center for cosmology and particle physics at New York University in New York that's our logo there it's CCPP we chose that because we had quite a number of let's say X Soviet physicists when we started I did my PhD at Stanford and then I went to the Institute for Advanced Study in Princeton which is where Einstein went when he first moved to the u.s. from Germany this is their logo which they're going to update they told me the other day so what do I do I'm a theoretical physicist well when I'm not doing administration and when I'm not teaching I'm a theoretical physicist and that means I don't have a lab and in fact I don't look very much at experimental data so I spend most of the day reading papers writing papers or trying to do calculations and usually failing so this is a kind of science which is fairly unique to physics these days there's such a thing as theoretical biology that's quite new historically really physics was the only science that had theoretical scientists and maybe the most famous example of one is Einstein so Einstein never did any experiments he paid some attention to data but mainly he thought about things and wrote down mathematical equations and tried to solve them and Stephen Hawking is another physicist like that so he also didn't do experiments or work on data I work on the very large the largest things in the universe and the very smallest things in the universe and curiously there's actually a connection between these two so the science of the very large is called cosmology that's the study of the universe as a whole and I work also on particle physics and a theory called string theory and attempts to quantize gravity so just to give you a sense this is my actually old office my former student Marjorie and here we are trying to do a calculation and not succeeding okay so so here's an overview we're gonna start talking just a little bit about Stephen Hawking about his life and his work and then we'll launch into this set of physics topics so Hawking was born in Oxford in the UK in 1942 and in fact 300 years to the day after the death of Galileo he went to Oxford University as an undergrad and then to Cambridge and very early on in his life at age 20 or 21 he was diagnosed with amyotrophic lateral sclerosis which is called in this country Lou Gehrig's disease and that's a disease that's that's not very well understood but it's usually fatal after a few years and so he was told at that time that he had a few years to live he lived a lot longer than just a few years but he gradually became more and more paralyzed to the point that in the some time in the 80s he was not able to speak and had lost essentially all muscular control except some part of his face I'm not sure if it was his eye or something in his mouth that he could move and that was it now he could still operate a wheelchair through the use of this or at least he could he could operate a speech synthesizer through I think again I think it was a wink in his eye from standing behind him it was with some muscle on his face that he had control over but but really that was it his work as will go into was in the same general area as mine so he was interested in big fundamental questions so what's the nature of the universe as a whole what was its origin is its fate and how do we understand the fundamental physics that governs it and these kind of ideas are usually in an arena where they can't be tested directly at least not in the short term so they are certainly subject to constraints from experimental data but they're not the kind of thing you can go into a lab and do an experiment and try to try to rule out that theory or confirm that theory Hawking's career as an academic was let's say meteoric he was very young when he was appointed to the Lucchese and professorship of mathematics at Cambridge University and that's a very old named chair at Cambridge it was held by Isaac Newton Newton was the second Lucayan professor and an 88 Hawking wrote a popular book called a brief history of time which I read as a kid and a lot of other people did as well and were inspired to to learn about physics and he died recently on March 14th which is pi day 314 and also Einstein's birthday and here he is in an airplane that simulates zero-g by diving and you can see he was a very adventurous soul he was extremely determined he wouldn't let his disability stop him and he had a sense of fun I'll come back to that towards the end so Hawking made a number of important discoveries in his career I think probably the most important and he himself said this was his most important discovery was that black holes and I'll tell you more about black holes later and what they are are actually not entirely black so before Hawking it was believed that black holes couldn't emit any form of radiation light any kind of particle that they were completely black inert objects but what Hawking showed is that in fact when you take into account quantum mechanics black holes radiate as though they were hot objects at a non-zero temperature if you heat up a piece of iron it glows red any kind of hot material will glow it will emit light at some level and mucking showed is that black holes do that too is not a discovery that's been confirmed experimentally for the reasons I was just saying and in this case it's pretty hard to have a black hole in the laboratory and if you could it would be very dangerous so it's not something you can just go and check and the black holes that we know of in the universe are very far away and according to Hawking radiates very little they would be very cold so we can't look at them and confirm his prediction that way either but I recently gave a lecture actually about three weeks ago to my colleagues in which I gave seven arguments for Hawking radiation seven different arguments they're all pretty convincing they don't rely on much other than the basic ingredients that go into black holes and they go into quantum mechanics so just about everybody in this area is quite convinced that this prediction was correct I called it a discovery in quotes because it's a kind of mathematical discovery so I think it's it's true and it's had an enormous impact on on research in this area and it's also closely related to something else that has been confirmed or at least that there's very strong evidence for I won't talk about this much but it's closely related to something called cosmic inflation which is the theory of the very early universe and how stars and galaxies came to be so hawking did much more than just that he also proved a number of theorems in Einstein's theory of general relativity that's our modern theory of gravity most of them are related to black holes or cosmological singularities in one way or the other so in in short what Hawking showed is that when matter is collapsing like if you have a cloud of dust or particles in outer space and it collapses under gravity it's impossible for it to stop past a certain point it must form a black hole and inside that black hole there must be something called a singularity which again will come back to you later so he proved several theorems like this some of them with Roger Penrose as a collaborator and he also worked on cosmic inflation as I mentioned and on a theory for how the universe came to be in the first place so I first met him at this meeting this was back in in 2003 at UC Davis so here he is in the front and Here I am a little younger and I met him many times since then I actually interviewed for a lectureship at Cambridge right before I came to NYU and the last time I saw him was on a goat farm in Belgium so so one of his former students Thomas Hertog lives on a goat farm an active goat farm and and had a party for him so we were making cappuccinos directly from the goat and I can tell you Stephen Hawking did not drink when maybe it was not not a good idea given his state of health but in any case he was very active he travelled around the world giving public lectures going to scientific meetings writing papers and he never slowed down as far as I know okay so this is the book this is the old edition the one that I had as a kid and here's the table of contents I can't cover everything in this book in the time we have but I'll focus on a good part of it so we'll start off discussing the nature of space and time and what it means for the universe to be expanding and then we'll talk about black holes and the fact that in fact they're not black according to Hawking then we'll talk about the arrow of time and eventually a theory for the origin of the universe and what it might mean for the unification of physics and we'll see how we do there's going to be two questions sessions so please if you have questions about anything save them up and and ask them then so we'll see how we do in terms of time but but don't hesitate to ask in those questions sessions if you have any questions okay so we'll start off talking about space and time so modern science in a sense starts with Isaac Newton about 300 or more years ago 350 years ago oops so what Newton did is show how mathematical equations and he invented those equations he invented the equations and the technique you needed to solve them could be used to predict the motions of planets and other objects near the surface of the earth and this caused a revolution in in human thought it sparks the Enlightenment arguably because all of a sudden you had someone who could solve an equation and predict with great accuracy the motion of something as far away and exotic as a planet in the sky so that was something that wasn't possible until Newton and his universe was a kind of clockwork universe there was no randomness Newton's laws were absolute if you say where something is and how fast it's moving at some time that's enough to predict where it will be at any future time so his laws were deterministic and his rules for the solar system were like some elaborate device which just moved the planets around in these predictable patterns space and time for Newton we're an absolute frame like a piece of graph paper or something which is just fixed and on top of which events happen but space and time themselves weren't things that could change or they weren't interesting they were just an absolute frame in which you could describe the motion of things and Newton is most famous for three things so first of all he invented calculus I'm sure you all love calculus it's an incredibly useful mathematical technique because it lets you solve equations for all sorts of things so everything from the motion of planets to problems in industrial engineering it's a very powerful and useful mathematical technique and it involves infinitesimal quantities which was the main sticking point the reason it took this long for it to be discovered and involves a limit in what you think about smaller and smaller intervals in Europe liveness is given more credit than Newton for discovering this but in any case both of them contributed and Newton also wrote down the laws of motion Newton's three laws that if you've studied physics in any level you've heard of and he wrote down a theory of gravity he also worked on optics and a number of other topics which are important as well but it's for these three accomplishments that he's most famous and again this combination was so powerful that it revolutionized the world because it showed that you could predict the motions of planets very precisely so what did Newton's theory theories predict first of all that the planets go around the Sun not the other way around and the earth goes around the Sun and that the planets move in closed ellipses so they don't actually move in circles necessarily although that's a possibility they move along an ellipse like an oval and they move with the size and shape of the ellipse is determined by the mass of the Sun and by the initial position and velocity of this planet they don't move with constant speed they move faster when they're closer to the Sun and slower when they're farther away and all these things had been observed already by my various astronomers of the time but there was no understanding of why they were true so it was known that planets seemed to move this way but but not not why and not how you could explain that and so Newton's theory finally settled this ancient debate between heliocentrism and geocentrism whether the earth was the center of the solar system or whether the Sun was it's funny to say the earth was the center of the solar system but that's what the debate was about and this was a very serious debate not so long before Newton's time Giordano Bruno was burned at the stake for espousing a theory that went against the teachings of the church and Leo himself was sort of put under house arrest and not not allowed to to talk about his theories openly so in those days it was it was dangerous to be a physicist anyway that framework stayed with us until much much much later centuries later when Einstein came along and what Einstein did is he had what's called a miraculous year annus mirabilis in 1905 he wrote four papers each one of which would be more than enough to win a Nobel Prize and each one of which was in a different area of physics it's never happened again one of the things he did in this year was fundamentally alter our notions of time and space so according to Einstein first of all space and time are not really separate you should think of them as unified into one structure which he called space-time and secondly time does not flow at the same rate for everybody or for every object it's a little bit like saying that if I take an object and I rotate it or if I rotate myself relative to an object you can imagine yourself floating in outer space and looking at something so there's no obvious up or down what you might say is the height versus the width versus the thickness of that object would change as you rotate it or as you rotate yourself relative relative to it right if you don't have a notion of up or down you don't know what's height and what's width and what's thickness when time is unified with space they mixed together in a way that's sort of like that so the duration of an event depends on the frame of reference of the observer now a lot of people when they hear this they think okay this is just a question of perspective I mean after all when you're excited or scared and time seems to slow down or if you're waiting in the doctor's office or something like that it really slows down right so a lot of people think this is just a matter of perspective it's nothing more than that it's not it's a physical thing has physical effects which in Einstein's time were very difficult to measure but today are rather easy to measure still not something you do at home but it's something you can do in any scientific laboratory or even if you're a surveyor or an engineer working in certain applications so what am I talking about so here's a famous example and this is of course not an experiment that's been done exactly like this but I'll tell you in a moment what has been done so imagine two twins so of the same age obviously one of whom remains on earth and the other of whom gets into a rocket ship which blasts off and flies very fast through space and then turns around or makes a big circle and comes back to earth if you exaggerate this so that this rocket ship goes very very fast there's a there will be a very big difference in the age of these two twins when the rocket comes back to earth and lands and the traveling twin comes out and greets the twin that remained on earth this twin will be young and this twin will be old so this is not a matter of perspective the biological age of these two twins will be different now again this experiment is hard to do because we don't have rocket ships that can travel fast enough and in any case the biological age of something isn't something you can a person isn't something you can measure precisely but we have very accurate atomic clocks so these are clocks that keep time incredibly accurately and you can literally take two clocks which are synchronized together put one on a jet plane fly it around the world have it land again and you'll see that less time has elapsed on the one that flew around the world so this is done again it's not this experiment it's done all the time because it's already been done there's no reason to do it many times but there's no problem in doing it these kind of clocks have much more precision than is necessary to do this experiment so it's it's something we know for sure is correct so that was Einstein in 1905 and for the next 10 years Einstein worked on a theory which would be even more revolutionary and it was called he decided to call it general relativity this theory that I that I told you about with the twin paradox it's called special relativity Einstein was working for ten years on this theory of general relativity he understood that he wasn't done that there was something more that he could add and he wanted to explain gravity in a totally revolutionary and different way so he wanted to think of gravity not as a force at all but rather as a consequence of geometry Einstein's idea was that space and time not only are relative in the sense that time can flow at different rates for different observers but that space and time are some sort of flexible dynamical thing something which doesn't always take the same form but reacts to the mass and the energy that's near it so this is a cartoon which is in some ways is very misleading but it's supposed to show you that the space near a massive object like the earth is warped according to this theory of Einstein's it's curved and what Einstein said was that so the space is curved and objects move on the shortest paths they can move on in this curved space so something that looks like an orbit so the moon orbiting the Earth or a satellite orbiting the orbiting the Earth is actually the moon or that satellite going in a straight line all right now how can that possibly be it's going in a circle it doesn't sound like a straight line well imagine you fly from Washington DC to Beijing if you plot the route of that that flight on an ordinary map it doesn't look like a straight line it goes nearly over the North Pole on its way from Washington to Beijing the reason it does that is not so that you can look down and see some polar bears it's because that's the shortest distance that's the shortest path that you can follow on the surface of the earth from Washington to Beijing so straight lines how do you a straight line well the shortest distance between two points is a pretty good definition straight lines on a curved surface like the surface of the earth they don't look straight they seem to be curved and Einstein's great realization was that maybe you could have a theory where mass and energy heavy objects like the earth would change the geometry of space-time in such a way that objects actually just go in straight lines but that mimics all of the results of Newton and Beyond makes more predictions so that was einstein's idea it worked and again we now have strong observational evidence that it's true I'll mention a little bit of it in a moment and among other things this theory because it predicts that space and time can react to the presence of matter and energy it predicts what are called gravitational waves so if I go like this I'm moving some mass and energy around and I'm creating a little bit of sound you can't hear it but there's a little bit of sound if you stood near me you'd hear that because I'm causing ripples to propagate through the air when I make that motion I'm also creating according to Einstein gravitational waves which are ripples in this fabric of space-time which spread out at the speed of light and and move away from from from this event okay I think this is the only equation in the talk but there had to be at least one so this is Einstein's equation or really equations it's a bunch of equations all together and I'll just tell you that what it says is that geometry so the stuff on the left-hand side of the equality here relates to the geometry of space-time it tells you how it's curved and how it's warped so geometric curvature that's this left side equals this is a constant C is the speed of light and G is Newton's constant of gravity it tells you how strong the force of gravity is so geometry equals some constants times this capital T is the energy of whatever's in the universe energy and mass of whatever's in the universe so if you have some energy or mass for instance the earth that's on the right-hand side the left-hand side can't be 0 or wouldn't be equal to the right it has to be equal to something which means the space is curved and that's where this that's what this picture was trying to illustrate that the space is curved near the earth because of its mass now Einstein was able to write down these equations because there's really only one set of equations you could write down they are very close to unique and it took him 10 years to do that because he didn't understand the correct mathematics to be able to do it at that time it existed this mathematics it's called Romani and geometry or non Euclidean geometry it's the geometry of curved spaces so that mathematics existed but it was relatively new and not very many people knew about it so it took lines done quite a while to learn enough of it to figure out what equation he could write that would make sense but eventually he did in 1915 and then having written down this equation he couldn't solve it so one of the strange things about being a theoretical physicist is that you often have a precise theory you have a set of equations which in principle tell you everything but you can't use them because they're too hard to work with they're too hard to solve what I mean by solve well for example I say okay I want to have the earth and I don't know another planet or the earth of the Sun and I want to know what they're gonna do well I know what they're gonna do they're gonna fall together or they're gonna orbit but I want to see that come out of this equation so I put that in and then I try to solve it and it's really hard the problem of just two objects orbiting each other and general relativity is so difficult that there is no exact solution to it and in fact it wasn't done even on a computer until about 15 years ago there's lots of approximate solutions to it which is a good thing it turns out to be easy to see well not that easy but relatively easy to see that these equations predict almost the same thing as Newton would have but finding the exact thing they predict is very hard the reason it's so hard is because of these gravity waves that I mentioned so it's really not a problem of just these two masses orbiting each other it's the whole space-time which can have these ripples and waves in it anyway so those are adsense equations they're very hard to solve and among the achievements of Stephen Hawking was establishing some characteristics of those equations I mentioned these singularity theorems before so Stephen Hawking along with collaborators George Ellis this is George George Ellis here and Roger Penrose and several other people they were able to establish some very foundational and fundamental results about Einstein's equations and remember this was in the 70s and Einstein wrote his equations down in 1915 so it really was a very hard problem to prove much about this and again many of many of them Hawking's results are related to black holes which won't come to you how many of you have seen interstellar yeah more people should see this movie it's really good well I'm gonna spoil it for all the rest of you sorry at least one part of it so there's a part of this movie where there's a moon or a small planet which is orbiting a black hole and I feel really bad about this sorry about this some of the characters go down on this planet which is covered with this ocean and I won't tell you what happens to them but they spend a few hours down there and then they fly back up to their spaceship which is orbiting around that planet and it's further away from the black hole out of its gravitational field where they meet another crew member that they left behind and a few hours have passed for these characters who went down to this to this planet and many many decades have passed for the person remaining up in the spaceship and of course they're all shocked and and surprised by this and in fact in watching the movie I immediately started calculating whether this could really happen because it seemed like exaggerated and it is but maybe it could happen just barely but as a plot device it's great and and the basic idea is correct time not only flows differently for objects that move that was special relativity it also flows differently for objects at different heights or deaths in a gravitational field so if you spent your life high up in the mountains you would live just a little bit longer and be just a little bit younger sorry the other way around you would live a little bit shorter you time would flow faster for you so you would be just a little bit older than your twin who lived down at sea level or at the bottom of the ocean or something the size of that effect is tiny so that again could never be measured with biology but with atomic clocks it's really easy so again you take to synchronize clocks you send one up on top of a mountain or in an airplane you keep the other one down here don't put it on an airplane because then it's moving that makes it harder just send it up on top of a mountain wait for a while and then bring them back together and they will read there will be a they will be a mismatch between them the one that was lower down last time will have passed so this is so well understood and so well relatively easy to do that it's even being used for a kind of surveying if you want to know you know what the altitude is at various points on earth this is one way you can do it you just take a clock and you let it sit there for a while and then you compare it to another clock somewhere else a reference clock so it's really remarkable that these ideas which have still nowhere near been absorbed by by most people that time runs differently at different heights in a gravitational field it's a crazy idea but it's used routinely in this kind of thing and in fact it's integral to the GPS system so this is some cartoon of the GPS system there's some number of satellites orbiting the Earth you need at least four of them in the sky at any time if you want to know where you are and how high you are and the way it works is your phone or your GPS device receive signals from these satellites and it compares the the timing so each satellite sends out a signal and like it's noon it's noon and one second it's noon in two seconds whatever it sends out these signals repeatedly your phone receives them and it's receiving them from multiple satellites and they're saying different times because the satellites are different distances away and it takes some time for the signal to arrive and then your phone compares these differences in time and it can figure out where it is by a kind of triangulation but those satellites are high up in a gravitational field and they're moving so actually both of these two effects are important and if you didn't take them into account the whole system would cease to work within a few minutes so these effects were taken into account by the engineers that designed this so just to emphasize these are not these are effects that one can usually measure and must take into account for a system like this to work okay so the last thing about Einstein's theory so it predicts that space-time is curved it also predicts how gravity acts on light this is called gravitational lensing we'll come back to that when we talk about black holes it predicts black holes and again we'll come back to that shortly and lastly it predicts that the universe as a whole cannot be static it can't just be sitting there and not doing anything it has to be expanding or contracting so let's explore what that means what does it mean to say that the universe is expanding okay so I think the best way to explain this is with an analogy and like all analogies this one is deeply flawed it's it's good in some respects and it's terrible in some other respects and the main problem with it is that it's a two dimensional world instead of a three dimensional world if you're wondering what a dimension is well if you want to specify where you are you could say I'm at this latitude and this longitude and this altitude right that says where you are you need three numbers that's because our world is three dimensional but suppose somehow our world was two dimensional there's a book called flatland which imagines this so there is no third dimension just doesn't exist for us you only need two numbers to specify where you are okay except that now if you can imagine that although it's a two dimensional world it's curved it's the surface of a round balloon and then you may say well how can you have curvature without this third dimension fine have a third dimension then you're going to need a fourth or a fifth dimension to understand Einstein's mathematics you don't actually need that but if you like it you can keep it so you could say there's a third dimension but you can't get into it you're stuck whoops you're stuck to the surface of this balloon and no matter what you do you can't get off it you're not even aware that this third dimension exists you just live on the surface the surface happens to be curved and okay the balloon is really big so it's hard to tell just like people used to think the earth might be flat even though you can see a ship disappearing over the horizon but people used to think the earth might be flat because it's so big it's not that obvious that it's curved same thing here all right so you live on the surface of this balloon the surface is your whole universe now imagine that this balloon is increasing in size if that's happening then if you look at a friend of yours at some distance away you'll see them receding away from you not because they're moving relative to the surface of the balloon but just because the balloon as a whole is growing and so the space in between you and that other person is increasing in size so these are some galaxies or something and they're getting gradually further and further apart okay now I'm sure you've all heard of the Hubble Space Telescope that's named after an American astronomer named Edwin Hubble and he was working in the 1920s and back then he had one of the best telescopes in the world back at that time the u.s. didn't have very many great theoretical physicists but it had a lot of money to build experiments and telescopes and so forth so he had one of the best telescopes anywhere in the world and looking through it he could see multiple galaxies so here's a picture of a galaxy so this is taken with the telescope far better than the one Hubble had access to Hubble could just see a kind of smudge and it wasn't entirely obvious what it was there was some debate back then over whether it was really a different galaxy or whether it was some cloud of gas within our own galaxy but it got resolved around that time anyway this is called the pinwheel galaxy it has this beautiful spiral shape happens to be oriented so we're looking at it face on it's quite flat like a plate but we're looking at it face on this is an image from the Hubble Space Telescope it's a small part of the sky it's a tiny tiny part of the sky but the Hubble Space Telescope can see very far and you can see many galaxies in this image there's one there and that's one that's one there's one over there there's one over there this is the star that's pretty nearby so there's some stuff that's nearby but there's also a lot of galaxies in this image the universe is full of galaxies each galaxy has billions and billions of stars if you're old enough to remember Carl Sagan billions and billions of stars and there are billions and billions of galaxies so Hubble couldn't see this well by any means but he could see 50 and as time went on more like a hundred or 200 galaxies and he noticed something very strange when he looked out at these galaxies he noticed I lied there's one more equation he noticed that the further away something was the faster it was moving and not in some random direction moving away from us away from Earth so in this equation V is a velocity and specifically it's the speed away from Earth D is the distance and H is a constant which I'm not sure what Hubble called it but it's now called the Hubble constant so velocity or speed has units of distance over time this is distance so this has units of 1 over time this H so he noticed that the further away something was the faster it was moving away from us so here's a plot from his paper this is distance this is velocity and you can see a not very linear relationship these circles and dots are his data points they're kind of a big cloud so it's not a very tight line but it's not too bad if you're used to looking at this sort of thing a linear relationship between distance and velocity is a reasonably good fit and I mean notice for instance that's so zero anything below this line means it's coming towards us only the very nearby galaxies down here only a few of those are moving towards us every other galaxy is moving away from us and that alone is strange if galaxies were moving around randomly then some of them would be moving towards us roughly half and half would be moving away right and also in other directions but but the component of their speed towards or away from us half of them it'd be towards and half of them it would be away but instead all of these distant galaxies are moving away none of them are moving towards so something strange is going on by the way we now know what causes the scatter why it isn't a perfect line and there's there's two things one is just experimental error so not being able to measure things perfectly and the other is that galaxies don't uniformly move like that they move in all directions but the further away you go the clearer this tendency gets and if you average over lots of them you find a very nice line like this so it's as though the universe is running away from us right that's a very strange thing or you might if you think about it a little bit come up with another explanation so suppose there was a big explosion at some point in the past in outer space forget about gravity and so on but just an explosion that throws out a bunch of particles or galaxies in all directions at different speeds if that were to happen the more distance something is the faster it's moving away from us because that's how it got that distant right so so there was this event this explosion and the things that were moving fast they went further because they're moving fast and things that were moving slowly didn't go very far so when you look around now the further away something is the faster it's it's moving away from you and the closer something is the slower so that's very reasonable except for one thing why would we be at the center so this is almost going back to the Ptolemaic system in astronomy where the earth was the center of the universe why would we be at a special place so here's this is just a shearing explosion so these things that are moving that are far out they're moving very fast that's how they got far out things near the middle are moving much more slowly but why would be why would we be at the middle okay well let's come back to this balloon so this surface of this balloon has a really interesting characteristic there is no center to the surface right if the balloon has a center it's inside the balloon but you're not allowed to think about that you're a flatlander you're in two dimensions you just live on the surface the surface has no center no edge nothing there's no point on the surface that's much different from any other point and not only that wherever you are on this balloon you'll see nearby galaxies moving away from you slowly and further away galaxies moving away from you more quickly as the balloon gross so everybody will see the same thing everybody will see an expansion which makes it look like they're at the center even though they're not because every point is the same as every other point another thing that this analogy helps to illustrate is that this constant H doesn't really have to be constant it can change with time it depends on how fast the balloon is expanding and it doesn't have to expand at the same rate for all time now one very important thing to realize about expansion of space it doesn't mean everything is expanding you are not expanding the earth is not expanding the solar system is not expanding even our Milky Way galaxy is not expanding that's because you and all these other things I mentioned are bound together by forces that are much stronger than this overall tendency so it just doesn't affect them at all the only things that are expanding our distant galaxies or clusters of galaxies have very very large things not not smaller scale objects so you can put that into the analogy if you want you can imagine instead of drawing these galaxies on the balloon you can glue pennies or something to it and then blow it up and then the pennies move apart from each other but the pennies themselves obviously don't change size so the pennies are galaxies and okay the space between them is the balloon so that helps the analogy a little bit right a big flaw in this analogy is that somebody's blowing up the balloon and if they stopped it stops growing the universe isn't like that it's more like as I'll mention in a moment an object with momentum with inertia so you give it a puff of air and then it just grows and it keeps growing maybe it slows down maybe it eventually turns around but you don't have to keep blowing to keep it to keep it growing it doesn't have a tension the way a balloon does that makes it want to shrink okay make its problem with this analogy it was in too few dimensions so good luck but you have to add a dimension to all of this there's one analogy people sometimes make which is a loaf of bread with raisins those are the pennies so the loaf expands and the raisins or the galaxies they move apart is the loaf expense but that's also bad because a loaf of bread has a crust there's an edge to the universe we don't think there's an edge to our universe so you really need to imagine something that's like the surface of a sphere but in one more dimension and this is called a hyper sphere sometimes or a three sphere it has the characteristic that if you go in any one direction you'll eventually come back to where you started if you could see all the way around you'd see the back of your head you can imagine that so yeah but it's in three dimensions okay so I'm not going to say much more about it it's not easy to picture but there is such a thing it at least it exists as a mathematical object and that's one of the possible geometries for our universe and this three-dimensional sphere it can grow just like the two-dimensional sphere all right so let's talk a little bit about the fate of the universe so now that we know that Hubble's observations are explained by the expansion of the universe we can ask what's going to happen in the far future what's the fate of our universe and there's this famous line from Robert Frost that the world may end and fire or it may end in ice and this nicely encapsulate the possibilities so [Music] roughly speaking there there are three options so one is that the universe is going to keep expanding forever it may slow down in this expansion or not but it will keep expanding forever another possibility is that it will reach a maximum size a maximum extent and then it will start to collapse and the third possibility is sort of right in between these two there's a close analogy which works even at the level of the equations if you throw a rock up straight up off the surface of the earth or fire a bullet so you give it an impulse so that it's moving up and then no more force is acting on it and forget about air resistance then there's three possibilities well if you really throw a rock up in the air obviously it's gonna fall back down but if you were Superman or you lived on an asteroid with very weak gravity then you could throw a rock hard enough so that would not come back down it would escape from the gravitational pull of the thing you're on and if it doesn't encounter anything else travel forever the whole time it will be slowing down but it'll never come to stop it'll never come to rest and return so there's a critical speed you have to throw it to make that happen it's called escape velocity so if you throw something faster than escape velocity it escapes slower than escape velocity it falls back right at escape velocity it sort of stops infinitely far away that's the case that's right in between so the universe is just like that there's three possibilities that can expand forever it can wreak elapsed or it could be right in between and because Einstein's theory relates geometry to gravity it actually predicts three different geometries for these three different cases the spherical case is the closed case where the universe V collapses there's a thing called an open universe it's like a negatively curved sphere it looks sort of like a saddle that's the case that expands forever and in between is a space that actually has a flat three-dimensional spatial geometry without curvature if the universe expands forever then eventually stars will run out of fuel stars burn sort of they undergo a kind of fusion reaction but eventually they'll run out of fuel for that reaction and they'll go out after a while that we know stars left the night sky will get darker and darker everything will get colder and colder and that's the end so that's called heat death very cheerful and if you lived in this in this world for long enough you wouldn't see anything in the night sky planets would get very cool because there'd be no no sons to warm them up okay so that's the ending a nice option if the university collapses then well it depends on how long this takes for this to happen but let's say it happens maybe before all the stars have burned out then these stars in these galaxies will actually come back together if I get closer and closer together don't start smashing into each other the universe will heat up it'll get hotter and hotter after a while it'll be full of plasma it'll be like the inside of the Sun everywhere and nothing can stop that so one of the things Hawking proved is that once the starts to happen you cannot stop it in fact it goes faster and faster because gravity gets stronger and stronger pulls everything together and after a certain finite amount of time the density and the temperature and everything else will reach infinity this is called a Big Crunch and it's an example of a singularity it's a point where Einstein's equations break down by the way in our universe it seems that it will expand forever as far as we can tell in case you're wondering it will end a nice all right so the plan is that at some point yeah not yet well yeah it's at some point we'll take a break there's some food and refreshments upstairs and this seems like a good stopping point so why don't we take a break now there's food and refreshments upstairs and Polly will will sort of usher everybody back when we're gonna restart and we'll have a question and answer session and then we'll continue with the lecture all right and then there'll be one more brief break later on okay so we're gonna resume and we'll start with a QA so if you have questions please raise your hands and a mic will come to you and just just wait until the mic comes okay what I'd like to know is is the whole universe laid out and we're just encountering it as we pass through time is it already there or is it developing at the present moment yeah that's a good question so I think you're asking is everything predetermined maybe is there okay so the question of of why we feel like we're traveling through time why we have that sensation is a very tricky one it's not something that physicists understand very well I would say I am gonna come to the arrow of time which is a closely related thing just just after black holes so let's hold that that thought and if I haven't answered it we'll have another Q&A session at the end that'll be longer so we can come back I think you were you're next yeah you you you mentioned that galaxies aren't part of the expansion of universe but isn't the truth they expand a little bit but because they're so small it's respect to the rest you know first I don't really expand that much and plus the gravity didn't in the galaxies right I guess it's true that if you took the same galaxies and you put it in the universe that wasn't expanding it would be just a little bit smaller okay so yeah but we're hoping this stretching it would be hard to measure okay thank you I've seen inner stellar four times I good you know trying to calculate the mystery of that universe I was in treat to hear you say the that gravity has a relationship with time so does that mean in microgravity we age very quickly so we're going so our astronaut core they're going faster and time is slowing down but they're in microgravity and what's happening with that side of things right it's a good question yeah so in zero gravity there's a certain rate of aging which is faster than it would be if you're in the gravitational field but it's not sort of infinitely faster so in other words someone in you know interstellar space and basically zero gravity will age a little faster than someone on earth but not that much faster it's actually a pretty small effect you really have to get very deep down into a gravitational field like an interstellar when they go close to the black hole because they land on this moon that's orbiting it before it becomes noticeable so it's not that there'd be a problem with someone that's that's in very very weak gravity situation they wouldn't age much faster than anyone on earth now that we've actually been detecting gravitational waves how does those observations affect the theoretical people well they're very interesting I'll come to them in in just a moment but yeah that's a very exciting development it's like opening up a whole new window into the universe yeah it's it was predicted so it's not a big shock but it's it's like being able to hear all of a sudden when before all you can do is see when you said that we're not expanding because atoms and molecules are bound together force is much stronger than gravity what forces are much stronger what are they mm-hmm that's a good question actually it's funny all forces are much stronger than gravity gravity is by far the weakest of all forces so there are three other forces in nature than we know if the one that's most important for binding us together is it's called the electromagnetic force and that's what binds atoms in the molecules inside atoms there's a force that binds the nuclei together and the quarks inside the protons and neutrons that's called the strong force and there's a third force called the weak force which is not very important for much of anything it's important for radioactive decays and so forth but it turns out that all of those forces are much stronger than gravity so the simplest example of this imagine you have a refrigerator magnet so you have a tiny little magnet and with that magnet you can lift a paperclip let's say so the paperclip is being pulled on by the entire gravity of the earth right you have this massive incredibly heavy object pulling as hard as you can with gravity on the paperclip and this tiny magnet is stronger so this shows you that that magnetism which is part of the electromagnetic force is a lot stronger than than gravity in the sense that a much much smaller object can exert an equal or greater force than the entire earth thank you huh given the the tools that Hubble had at the time how was he able to determine that that galaxies further away were moving faster away than those close yeah that's another great question so he had to measure two things he had to measure distance and he had to measure recession velocity it turns out that the second one is actually the easier one and the reason the recession velocity and the reason is that you know when a car goes by you honking its horn it goes near in a roll so when it's coming towards you the sound is shifted up in pitch and when it's moving away from you it's shifted down in pitch which is just because when it's coming towards you the sound waves are kind of piling up on each other and when it's moving away they're getting stretched out so that same effect that's called Doppler shift that same effect applies to light so if an object is moving towards you the light of the mitts will be a little bit bluer and if it's moving away it'll be a little redder you don't notice this because it's a small effect for your eye to see but it's easy to detect and especially for a distant galaxy that's moving very rapidly away so the way he measured recession velocity is by looking at the light from those galaxies and and measuring how red shifted it's called red shift and how red shifted it was and from there he could get this this recession velocity but the distance is actually really hard to measure and that was where he struggled and in fact he was off by a very large factor it was off by about a factor of 10 distance is hard to measure because well if you imagine you're in a big black empty space and there's a light bulb you want to know how far away it is and with no points of reference it's tough if you know how bright the light bulb is intrinsically like it's 60 watts it's an old incandescent bulb at 60 watts you could figure out how far away it is by how bright it seems to be right the further away the dimmer it'll look but you don't really have a good 60 watt light bulb in the universe in other words you don't have an object where you know exactly how bright it is at least Hubble didn't so he had to use something called Cepheid variable stars which are stars that that pulse there's a relationship between their brightness and the frequency of this pulsation but it wasn't a very good measurement and he was pretty far off and now we have various ways of doing it but none of them are they're all much better than what Hubble had but still that's one of weakest points the best thing is called a type 1a supernova which is it's called a standard candle it is a bit like a 60 watt light bulb but even that is not not perfect so yeah it's it's very hard to measure distance have we been able to measure yet for sure whether antimatter falls in a gravitational field and a second question where do you think the missing antimatter is okay so it's a matter is similar to matter but with opposite charge and if it encounters matter it can annihilate and turn into energy according to Einstein then according to every modern theory in physics it will fall it will behave exactly like ordinary matter gravity is totally universal and the thing attacks on is the energy of something and any matter certainly has positive energy for example you can annihilate an anti-electron with an electron and you produce usually light but with twice the energy of one electron so it has exactly the same energy as ordinary matter so if it if it didn't fall or if it if it went up or something it would violate everything we think we know about gravity with that said I'm not sure whether there's a direct test of that because to make antimatter generally requires a really high energy event like smashing particles together and gravity just doesn't have much effect in those situations as for where the missing antimatter is it could be that there's just more matter than antimatter so the good question is essentially there seems to be a symmetry between matter and antimatter and yet we live in a universe that's full of matter and has very little antimatter so where's all the antimatter one possibility is that that symmetry between matter and antimatter is actually broken in the early universe so that you just produce a lot more amount of an antimatter another possibility is that where we live there's a lot of matter somewhere very far away there might be a lot of antimatter but there's not a very good understanding of that it's one of the open questions why don't we take one more question and then then we can move on hi thanks but you said there's no edge to the universe I saw somebody a picture of they used the balloon and said it's like that so you just go around or something but yeah as far as we know I mean of course well maybe it's not not of course maybe it's not obvious but there's a certain distance that we can see in the universe and it's like a horizon so if you're an island in the middle of an ocean you know there's a horizon and you really can't see beyond that there's something like that in cosmology and it arises because light travels at a finite speed so you know when there's a fireworks display you hear the sound after you see the light but even the light takes some time to arrive at your eye that means if you look at something distant you're seeing it as it was in the past when it emitted the light if you look further and further away you're saying further and further back into the past and if you go far enough back into the past the universe becomes opaque so now it's pretty transparent if you beam a laser in outer space it'll keep going for a really long distance before it runs into anything because space is empty back then it wasn't empty it was full of plasma and in fact it was like a solid or like the inside of the Sun if you beam a laser pointer at the Sun it doesn't come out the other side so the universe was like that far enough back in time so you can't see further away than the distance that corresponds to that amount of time going back in time which is about 13 billion years so you can see pretty far but but that's it okay so if there's an edge and it's further away than that it would be hard to tell so we don't know for sure but there's no reason for there to be an edge there's no evidence for it to be there and it's perfectly mathematically consistent according to Einstein anyway to have universes that have no edge and no Center just like the surface of that balloon okay well thanks for the question says we're great so like I said there's going to be at the very end of the program roughly between 4:30 and 5:00 another Q&A session so if you think of more questions save them for then okay so I want to tell you about black holes and then about Hawking radiation pockys prediction that black holes are not exactly black but before I can tell you about that we have to understand what a black hole is in the first place all right well as I was just saying light moves at a finite speed it doesn't move infinitely fast it moves in American units at this speed 186,000 miles per second and according to Einstein nothing can go faster than light there's a kind of universal speed limit which is the speed of light by the way the speed of gravity waves is the same as the speed of light and it's not really that there's something special about light any kind of radiation it all moves at the same speed now sounds just to give you I'm gonna make an analogy here for Sam so sound in water it's a bit faster than sound in air but much more much slower than light it's like five thousand feet per second okay so now imagine you have a river and let's say there's a waterfall somewhere and the river can be very straight and with the same depth until it gets to the waterfall and as it approaches the waterfall the current is getting stronger and stronger and the water is flowing faster and faster and so at some point close to these false the water is moving very fast and now this is totally unrealistic but let's imagine that close to the Falls the water is moving faster than the speed of sound in water okay you can't actually have a river like this know what nowhere close but there are in fact experiments sort of like this where you have a fluid and you can make the fluid itself move move faster than the speed of sound in that fluid and people are trying to do this experiments okay so here you go so if there's a place in this river where the current is moving faster than sound that's this red line in Washington DC you know a lot about red lines so here's a red line across this river and so closer across the spread line closer to the falls the water is moving faster than the speed of sound in water so what does that mean it means if you emit a sound over here it'll actually get carried with the water so you can emit it trying to have it go up the stream but because the stream itself is moving faster than the sound it'll get carried over the falls okay so now imagine that you're you're living in this river and you're living you know upstream up there and you're trying to investigate what's going on over here you know there's something going on because whenever you send something that way it doesn't come back and you can't hear anything from over here and if you send a friend over there they never come back so here you have some fish and they're thinking hard about about what happens what happens if you cross this red line so okay so so well we know what happens because we can see so I don't know what sound fish make if anything let's say they squeak I don't know so if there's a fish in the river squeaking and she's getting closer and closer to this to this line then because the rivers growing faster and faster relative to the speed of sound the sound she's making will will take a longer and longer time to move upstream and a sound admitted right on that line will kind of hover there right it'll be moving upstream and the water will be going the other way and they'll just stay stationary relative to the bank and it sound emitted past that line will never cross back over it it'll go over the falls okay so that's the idea of a horizon this kind of horizon so this would be a sound horizon it's a line or it's actually a surface sort of through the water beyond which no sound can can go back upstream okay so that's like a sound horizon now imagine that you have a very very massive object or a very dense object so an object that has an extremely strong gravitational field it's possible according to Einstein for the gravitational field to be so strong or for the space to be so curved near that object that not even light can escape it's a bit like saying that the freefall velocity of things falling into this which is like something being carried along with the current and the river exceeds the speed of light close enough to this object and you may say I just said you just told us nothing can move faster than light that's right and if you do an experiment locally so if you're in a lab that falls into this thing you won't actually notice much that's unusual but if you try to shine something out of it it won't make it not even light will get out just like in the river so so there's this certain special radius around this very dense object where if you're inside that radius a light signal and it doesn't again it doesn't have to be light it could be a radio wave it could be microwaves it could be gravitational waves but no form of radiation can escape and if no form of radiation can escape nothing can escape because nothing can move faster than light or any other kind of radiation all right so it's called an event horizon because an event that takes place past it or inside of it well now like no information will come out there will be no information about that event it's hidden behind this event horizon okay yeah so okay so imagine you have an object like that it's so dense that it's surrounded by this spherical or roughly spherical event horizon so nothing inside it can emit any light that you see so how are you going to see this object is it totally black well the answer is it's not necessarily totally black in the sense that something which is falling in but hasn't yet crossed the event horizon may be emitting light so this is like our Fisher sisters who is moving towards this red line and and squeaking or whatever and the squeaks are escaping upstream until right when when the fish crosses that that line so stuff that's falling into the black hole before it crosses the horizon can emit light that you can see and actually what it looks like is kind of interesting it gets dimmer and dimmer as it approaches and redder and redder because there's a Doppler shift that affects it so the squeaks of this fish would get fainter and fainter and pitched down more and more approaching that line for a black hole the object falling in emitting light gets darker and darker and redder and redder and eventually it just disappears and you don't see it anymore so that's what happens to stuff falling in but if there's a stream of stuff falling in all of it emitting light as it goes or some kind of radiation as it goes you see that right so as I said no information can escape from the interior because not light can't escape and so nothing can escape so it's an event horizon now according to Einstein all of this is according to Einstein inside this black Hall there's a singularity actually I should say it's according to Einstein's equations but Einstein had trouble accepting the existence of black holes he didn't really believe in them like I said they're very hard to solve and so although people very early on discovered black hole solutions and Einstein's equations it wasn't understood what they meant or what they were for quite a while and I'm not sure when Einstein thought at the end of his life but at least for a while he didn't believe that they made sense so it's not really according to Einstein it's according to Einstein's theory that there are these black holes and there's the singularity at the center so when we were talking about the Big Crunch where the universe ends in fire when it collapses on itself there's something like that inside a black hole there's a place where the density is infinite and it's not exactly a place a place is something you can avoid you can't avoid the singularity at the center of a black hole any more than you can avoid the Big Crunch singularity in the universe that reeked elapses once you've crossed the event horizon you're gonna hit the singularity there's no way to avoid it it's not really a place it's more like a time it's a time where everything becomes infinitely dense now normally when you're falling in a gravitational field you don't really notice it so astronauts orbiting the Earth for them they could be in the middle of empty space and they would feel exactly the same thing they're in freefall the Space Station or Space Shuttle or rocket or whatever is in freefall around the earth which carries it in the circular path so astronauts on the space station float around just like they would if there were no gravity at all they're in freefall however if the gravitational field is strong enough it'll have a different strength here than it does here and so you can't really be in freefall and remain the same shape as you are in other words if this part of you wants to fall faster than this part of you you'll feel a tugging this is actually like a tidal force it's what what causes tides on earth so close to the Sun of a black hole near the singularity those tidal forces get extremely strong and they tear everything apart and it's called spaghettification so here's an artist's conception so don't fall into a black hole okay so so what's a black hole according to Einstein it's a completely universal phenomenon any time energy or matter its concentrated enough so anytime the density is large enough an event horizon will form and again Hawking contributed in a very significant way to this by proving that an event horizon must form in a black hole must form and there must be a singularity using Einstein's equations under certain circumstances where things are already collapsing so it looks sort of like a black ball of some size out there in space the size can depend does depend on the mass of the black hole so a black hole has a certain mass it's however much mass fell into it to form it and it can actually have a spin so if it forms from a rotating cloud of dust for instance which is pretty typical and then it will have a spin it's still black but what it does is actually pull the space around it in a kind of swirl so if you fall near it you'll get pulled around and you'll end up orbiting in that way before you fall in so it does that it's a little bit like water draining into a into a drain in a bathtub and in principle it could have electric charge if you somehow threw a lot of electric charge in there the black hole could have an electric field but that's it according to Einstein black holes have mass spin and they can have charge and those are their only characteristics otherwise they're completely black and totally inert apart from whatever might be falling into them now in our universe there are as we now know many black holes and this is something that has become more and more clear in recent years and it's now confirmed beyond any doubt there are many black holes in our universe it seems that most galaxies have large black holes at the Centers at their centers very large black holes masses of millions or sometimes even tens or hundreds of millions of Suns so it's like that black hole has a mass that's as much as a hundred million or ten million or a million times the mass of our Sun so there seems to be a so-called supermassive black hole like that at the center of nearly every galaxy there are also much smaller black walls and I'll come back to that in a moment now in many of these cases and in particular for the black holes of the Centers of galaxies there is what's called an accretion disk of matter so the center of the galaxy is a very busy place there's lots of stuff in there it's pretty dense and so that stuff is falling into this black hole as it falls in it forms a flat disk sort of like the solar system but it gets pulled into the black hole and as it does that it heats up and it glows so you can detect it I'll show you an artist conception of that in a moment there can be other black holes which are just floating around in empty space and those would be very hard to detect not totally impossible but very hard one way to detect them they're not emitting any light there's nothing falling into them but they have an effect which is sort of like a lens on the Stars behind them so if you have a field of very distant stars and you have a black hole that floats in between you and those stars you'll see the strange kind of warping rippling thing going on with the field of stars behind it that's called gravitational lensing and that's because the gravity of the black hole is affecting the light which is coming from those distant stars and causing it to travel and curved paths okay so this is a conception of an accretion disk this is not a black hole at the center of a galaxy necessarily it's a black hole in orbit around a large star and what's happening here is that material from the star is being pulled into the black hole so the star is accreting onto the black hole and this is a fairly typical situation we believe that there are binary systems or two objects one of them is a star and one is a black hole and when the black hole gets close enough it starts sort of tearing away at the star and and pulling material into it and this often produces an energetic jet which shoots up in the plane that's in the in the direction that's perpendicular to the accretion disk and when that jet happens to pass across the earth you can see it so this is to illustrate this other effect of gravitational lensing so here's a black hole it's not pulling anything in so it's really black but there's a dense field of stars behind it and you can see that while this field of stars doesn't look for a uniform there's a kind of ring around here so where is this ring coming from since we have this nice whiteboard maybe I can draw a picture of it so if you have a black hole here here's the black hole and you have a star here and everything's in a line you're over here here's your eyeball observing this thing so light that's coming directly from the star towards you just falls into the black hole and you don't see it okay so that doesn't come back out but light that started off going this direction will actually get bent around like that and light coming this direction will get bent around like that and so what you'll end up seeing it's as if there was some object over here and some object over here and this is happening in all directions so you see this ring it looks like there's a ring in place of That star now this only happens when the star is directly behind the black ball from your point of view so it's kind of it doesn't happen all the time it requires them to be lined up but in this image which is just the it's not a real image it's a computer simulation of some sort but you can see that there's this ring it's called an Einstein ring and it's coming from that effect and there are real images like that where you see it's called strong lensing you see this this effective okay so you can detect a black hole even if it's not emitting any light and even if it's not pulling anything into it by this lensing technique looking for this lensing the other way you can detect black holes and this is something which very recently has been in the news so you may have heard about it is that I mentioned that that Einstein's theory predicts the existence of gravity waves these ripples in space-time and any motion of matter or energy produces gravity waves but because gravity is such a weak force as we were just discussing the amount of energy that goes into those waves and the effect those waves have on any kind of detector it's very very weak so to have any chance of detecting these gravity waves you an extremely sensitive detector and it was only in the last decade that such detectors were produced and came to be so the only detectors in existence are are part of an experiment called LIGO which stands for laser interferometer gravitational observatory or something like that so this is an aerial view of one of the facilities of LIGO and what these lines are are kind of long shed inside the shed there's a tube which has a vacuum inside there's two of these arms at 90 degrees inside this vacuum tube there's a laser beam going down the middle of it and at the end here which I maybe cut off a little bit of the picture but at the end there's a mirror so this is an image of this mirror there's a mirror hanging over there which the laser beam is bouncing off of and then coming back and what you do is you send laser this way you send laser light this way these arms have the same length and if nothing is happening the light will come back and you can interfere these two beams of light with each other and you'll produce a pattern that looks sort of like ripples on a pond or something and it's like a fringe eep sort of pattern and why is this interesting well imagine you changed the length of one of these arms a little bit then this pattern shifts and you see it evolve and change so if something changes the length of one of these arms or makes this mirror move this mirror is suspended from essentially it's hanging from a string but it's a very fancy string that has lots of vibration absorption but if something makes this mirror rock back and forth that changes the length of that arm and you'll see that in this pattern so basically it's a way of measuring the difference between two distances which are the two lengths of these arms very precisely so when two black holes merge they orbit each other they emit gravitational waves that's the form of energy going out from them and the emission of that energy causes them to fall together a little bit so now they're orbiting a little closer and a little faster they go faster faster faster faster they finally merge and at that moment they emit a burst of gravitational waves that's very strong and this instrument LIGO was designed to measure that so when this bursts of gravity waves arrives on earth it deforms the detector a little bit it makes everything first get a little longer this way a little longer that way so it makes everything kind of wobble including the mirrors in LIGO it's a very weak effect you'll never feel it yourself but LIGO is incredibly sensitive then it can measure this tiny difference in this tiny deformation if you want so a tiny change in length relative length of these two arms now it's so sensitive that it can detect trucks driving by waves hitting the beach people that work there are instructed not to run because in the control room or anywhere near earth because their footfalls can be measured so if they get excited about something they can't run across the control room to tell anybody and there's actually two well now there's more than tune that now there's there's several but originally there were two of these facilities one is in Louisiana not far from the ocean or not from from from the the Gulf of Mexico and waves were a problem there the other one was in Washington State and logging trucks were a problem there but the way that this experiment works is that they would only look at an event if both detectors detected it and not at exactly the same time but almost exactly the same time the difference being the time it takes for the gravity wave to propagate from one to the other because most of the time it's not hitting them at the same time takes a tiny fraction of a second but gravity waves move at the speed of light so they don't hit them both at the same time so if a logging truck drives by one it's very unlikely that an almost identical signal will be measured at the other one at exactly or almost exactly that same time so this is a powerful way of discriminating a real of a real event a gravity wave from something else and this was the first event they detected this is just noise here this is the signal it may not be very obvious looking at it that the signal is all that different from the noise but the chances that this signal would occur at both of these detectors with just the right offset in time are less than one in a million so they do a lot of statistics to measure that and moreover this signal is precisely what was predicted by Einstein's theory actually I told you guys a while ago that this problem of just two objects orbiting each other can't be solved exactly and wasn't even solved on a computer until about 15 years ago the reason that was such an important problem was to predict what LIGO should see and fortunately it was solved in time it was self like I said about 15 years ago by France Pretorius and so that allowed this prediction to be made that they confirmed this so that first detection was in September of 2015 the first number is the year and this is the month I think this means on the 14th that's when it was detected it was announced sometime later because they spent a long time making sure that it was a real event in fact this experiment is so careful with making sure that the events are real and significant and that they're not having false positive discoveries that they would even inject fake data like events that aren't real in such a way that nobody working in the experiment would know if it was real or not except for the three leaders of it and then they would allow the collaboration to get excited and prepare a paper and have everything ready and then they would reveal that it was fake just to test this whole stream of how it works and to make sure that people didn't sort of become biased by thinking that this had been a real event if the three leaders of the collaboration had died in a plane crash or something I'm not sure what would have happened but in any case they're really very very careful about all this so everything is done blind they don't know what's real and what's not everything is automated nobody is picking and choosing events and since that that date they've detected now what is it six of these of these mergers this one wasn't quite significant that's why it's dashed but the rest are all detection there are all significant detections and what is this vertical access mean well these are the two black holes that merged with each other this one had a mass of about 35 Suns and this one had a mass of about 30 times the Sun and what they produced is a black hole with a mass of about 60 times the Sun which if you look carefully you'll see is not the sum of these two it's not the addition of these two it's a little less and that's because a lot of energy was away in gravity waves and that's what was detected so several Sons worth of energy equivalent was radiated away in gravity waves that's an incredible it's an enormous amount of energy and these are other detection so these all involve black holes that are several times the mass of the Sun up to maybe thirty times the mass and it's actually an interesting question where these come from it wasn't expected that there would be black holes in this mass range but now they've been seen okay so that's what black holes are according to Einstein or at least according to Einstein's theory they've now been detected in a variety of ways I think they're definitely out there and again according to Einstein nothing can escape from these things because once you fall in you're trapped behind this event horizon you can't get any information or any kind of message out or at least that's what was believed until Hawking came along what Hawking realized is that when you combine quantum mechanics with Einstein's theory of gravity things that would have been impossible are no longer quite impossible so a good example of this that's closely related to talking radiation is a phenomenon called quantum tunneling so quantum tunneling is the following physics imagine you have a rock or something which is trapped in a valley or trapped in the bottom of some sort of not just a valley but a like a lake bed so it's really down in some kind of minimum that rock is prevented from escaping from that lake bed and getting out into the countryside beyond because there's some mountains surrounding it on all sides right so it has to climb up to the top of this mountain and get back down again to escape and if it's just a rock sitting there it has no way of doing that now obviously somebody could pick it up or push it and get it to the top and then it could escape so if you add energy to the system the rock can escape from this lake bed but without adding energy it can't get out it's impossible now a black hole is a little bit like that so there's a barrier which is the event horizon through which nothing get out according to non quantum classical physics it's like having a barrier which is kind of infinitely high you can't get out no matter how much energy you have it no matter how fast you're moving but in this situation there is a probability according to quantum mechanics that the rock will just find itself over here and this is called quantum tunneling it tunnels in a sense through the barrier even though this would be impossible in classical physics and even though nobody gave it any energy it just happens now again this is something which has been observed in fact of all the things I've told you about quantum mechanics is by far the most established quantum mechanics is at the basis of every operation in your in your phone for instance so all of solid-state electronics relies crucially on quantum mechanics it's at the basis of modern chemistry it's used in all sorts of different technologies nuclear power and nuclear weapons all of this is based on quantum mechanics it's a very well understood and very well-established theory and there's no question that quantum tunneling occurs and there's something called a scanning tunneling microscope even some of the scans you can get in a hospital rely on this so quantum tunneling is a real thing it's a problem in transistors if you shrink them down to small electrons start hopping out of the wires even though they wouldn't be able to classically because of quantum tunneling so it's it's a real thing now with that said a black hole is an exotic object that we can't do experiments on so you know I said a while ago that this prediction of Hawking is very plausible but it's still not something that's been confirmed experimentally because we don't have black holes to do experiments on but what Hawking predicted mathematically was that you have a black wall according to Einstein nothing can escape from it there's a kind of barrier that prevents anything from from getting out but if you include quantum mechanics in your calculations you discover that a particle or something it doesn't have to be a particle it can be some light or some radiation can escape from the black hole and will escape from the black hole at a certain rate in fact it will escape at the right rate that it looks like the black hole is a warm and I want to say hot because for it to be hot it would have to be very small but it's a kind of warm thermal object so it emits radiation in a particular way that's characteristic of objects with a temperature as it emits this radiation it's losing energy because it's emitting radiation to infinity it's losing energy black holes with less energy with less mass are smaller so the black hole gets smaller when in the midst of some radiation it actually gets hotter when it gets smaller according to Hawking so now it emits more radiation gets even smaller after a while it should just evaporate entirely however the time that this takes for a black hole that's the mass of the Sun for instance is enormous it's much longer than the age of the universe and that's because a black hole that big is very very cold so it's really really slow so to see this happen you actually would have to have a very tiny black hole with a mass that's much much smaller than the mass of the Sun if you could make one in a lab like I said it would be dangerous but you could see it happen but it's hard to make black holes in the lab because you have to produce something that's so dense that it forms our horizon and that's well we don't have any way to do that currently at least but according to Hawking this should happen black hole should evaporate okay so I won't say much more about that the reason that such an important discovery is that it creates a number of paradoxes it's a very confusing resolve for a number of reasons but well if people want to hear more about that you could ask questions in the in the Q&A session at the end but let me turn now to the arrow of time so this is a this is a puzzle this is something which goes back before even Einstein back to the late 19th century to Boltzmann and let me start actually by again discussing Hawking radiation in a certain in a certain way so let me give you an argument which which would suggest to you that it's not surprising that black holes can evaporate so here's the argument so imagine you have this is a kind of a thought experiment Einstein loved to do these right you imagine some situation maybe you can't do it really in the lab but you can imagine it and you can ask what would the laws of physics predict for it so imagine you have a really really really big box seals nothing can get in around it's a really giant box and it's it's out there and in the middle of empty space so there's no gravity apart from whatever is created by the by the particles inside the box and let's say you have a lot of particles inside the box but it's so big that they're very diffuse so they're not close together there's no high density and it's very unlikely by the way I mean this is not a bad model for what the early universe is like it's full of particles it's pretty unlikely that those particles will form a black hole and the reason is just that the box is so large that the typical density is very small particles are very far apart for apart from each other and each particle has only a tiny gravitational field to form a black hole they all have to clump together at one point very densely okay so that's unlikely very unlikely but it could happen there's nothing that forbids it from happening except that it's just not very likely and in fact if you were to wait long enough if you were just to sit there and twiddle your thumbs and watch this thing eventually just by random chance the particles would find themselves close enough together that the density would be high enough that they would form a black hole and remember according to Einstein once you form a black hole that's it nothing can get out of it so you have this black hole and it has a certain mass and that's it so let's imagine that it happens either by chance or you arranged the motion of these particles so they're all going towards each other so after a while they all find themselves at the center or something so let's imagine it does happen now there's a really curious feature of the laws of physics which is called time reversal invariance and what it means is that the laws of physics that govern everything the microscopic laws of physics that govern interactions between particles the forces between them and so forth those laws look exactly the same if time is running forward towards the future as if time is running backwards towards the past so to put this in concrete terms imagine I made a video a movie of something happening to particles bouncing off each other maybe think of two pool balls bouncing off each other and now let's say I play that movie backwards or I don't tell you playing it forwards or backwards if all you see is this collision you're not going to be able to tell whether time is running forward or time is running backward and that's true for individual particles bouncing off each other it's true for pool balls as long as I don't go long enough that you see them slowed down from friction but of course it's not true for most of the events in the world like if you see somebody diving into a swimming pool and you see a big splash of water come up and now I run that movie backwards that's a person coming up out of the water feet first a bunch of droplets of water converging forming a perfectly smooth surface just as the person gets ejected from the water it sounds impossible right but it's not it doesn't violate it in any laws of physics well except one that we'll get to but it doesn't violate this fundamental fact that the laws of physics are time-reversal agree so coming back to the black hole if something can happen one way in time there is nothing that forbids it from happening the other way in time however it may be very unlikely to happen one way or the other a person undying out of a swimming pool is very unlikely you've never seen it and you never will because it's fantastically unlikely that everything would be carefully pre-arranged to make it happen but remember I said that to make the black hole in the first place was actually the unlikely thing it was hard to get those particles to all find themselves in the same place at the same time so that they're so dense that they form a black hole so that was the thing that was unlikely and if you have this symmetry in the laws of physics it should be possible to go the other way that the black hole turns into this gas of particles which are flying around that's evaporation okay so this is a kind of general argument that tells you that black holes must be able to evaporate if the laws of physics are going to be reversible and we believe they are at least once you put in quantum mechanics so okay so that's just an argument that says well maybe you shouldn't have been so surprised that there's Hawking radiation and black holes can evaporate but it brings up this other question that I mentioned why do things only seem to happen one way in time once you get away from pool balls bouncing off each other or particles interacting why doesn't anybody ever undyed of a swimming pool why was it so hard to put Humpty Dumpty back together again you can break an egg good luck putting it back together so where is that coming from why do we remember the past and have very little idea what's going to happen five minutes from now and certainly five years from now right so why is that the future in the past are very very different why do we feel like we're progressing through time in the present well the sort of quick answer to that as far as we know is the second law of thermodynamics which says that entropy increases or more precisely it says entropy does not decrease with time now I just told you the laws of physics are the same going forward in time going back in time so to say entropy increases well that means it does something different going towards the future than going towards the fast or then going towards the past going towards the future would be increasing that would mean going towards the past it would be decreasing so that's not time reversal invariant so what am I talking about well the things that are type the laws that are time reversal invariant are the microscopic laws of physics the laws that govern forces and interactions of particles and so forth and we believe that the laws of thermodynamics like the second law are not fundamental laws at all they're just consequences of these microscopic rules okay so thermodynamics is the is the science of things like temperature pressure how the air is distributed in the room on average and what we believe is that those principles that govern how the air is distributed in the room and what its temperature is and so on follow from these fundamental laws but they're not fundamental in themselves so the second law of thermodynamics it should be something we can explain from the fundamental micro physical laws and yet those are time reversal invariant those are the same going forward and back and the second law so how can that happen well okay suppose the entropy happens to be low now it just happens to be you just start that way then what do you expect is going to happen well I should tell you a little bit about what entropy is entropy is essentially a measure of how many possible States there are consistent with whatever macroscopic properties the system has for example I might tell you how many molecules of air there are in this room and how much energy they have in total and how big the room is and that's it and then entropy is the number of possible configurations of the air molecules that's consistent with all that so I'm not allowed to add and subtract air molecules because I told you how many there are I'm not allowed to put them outside the room they're in this volume I'm not allowed to change the total energy but I can give one molecule more energy than another I can move this one over there how many ways are there to do that well obviously there's a lot because there's a huge number of air molecules in here I could put them all up in that corner I can put them up in that corner I could spread them uniformly I can change their energies relative to each other so there's a huge number of states of configurations which are all consistent with how many there are how much energy they have and how big of a room they're in and that number is the entropy so what's a low entropy state well a low entropy state would be insisting that all the air molecules are in that corner ok hard to breathe but let's say they're all in that corner that's low entropy because there's very little volume up there so there's a lot fewer states like that then there are where they could be anywhere if I put all the air up there what do you think is going to happen it's immediately going to spread out and fill the room and become very uniform and the reason that happens is just because there are many more ways for it to do that it's a statistical thing really now another example I have two little kids I spend a lot of time trying to decrease the entropy locally cleaning things up if you clean up a toddler's room and then the Tyler comes home that's what happens very rapidly and the reason is just that while kids and these microscopic laws of physics they're not really random but they're just doing whatever they want to do and because there's so many more ways for it to look like this then there are for it to look like oh it's next Lane oh come on this right this is a very special arrangement there's not very many ways for it to look like that there's really just one way but when you start throwing your socks in all directions or just mixing them up there's many many more ways so really what we think is that the second law of thermodynamics it's not that there's a fundamental asymmetry it's not that time is different running forward and back it's just that if for whatever reason you live in a universe with low entropy that's initially ordered it will become less and less ordered in other words its entropy will increase okay so the explanation for this second law of thermodynamics is just that the universe in the past was more ordered and had lower entropy than the universe does now but this whoops this really begs the question yeah let's say this first so the entropy will continue to increase until or unless it reaches the state with the maximum possible entropy and that by the way is called thermal equilibrium when we talked about heat death that's an example everything comes into equilibrium you don't have hot stars and cold empty space and warm planets everything comes to the same temperature you don't have variations in density or pressure or anything else you don't have any motion of any sort in equilibrium equilibrium is incredibly boring it's just everything is constant nothing changes there's no life nothing can happen there that's the future if we can describe the universe as a thermodynamic system because this entropy must increase until it reaches this maximum that's what will happen to it now as I said we can understand this if we can understand this the second law of thermodynamics why the entropy is increasing if the early universe had low entropy and I believe although it's hard to prove this that that explanation as far as it goes it suffices to explain why eggs are easy to break and hurt to unbreak why you remember the past and not the future everything that we think distinguishes future from past I think it can all be explained if the early universe had low entropy but there's still a question hanging in the air why was the answer below in the early universe wait where did that come from if there are many more states with high entropy then why was the early universe in this very special state with low entropy no one knows the answer to this it's still one of the central mysteries of physics but Stephen Hawking made an important contribution to this area of inquiry so this is someone named Jim Hartle he's a professor University of California in Santa Barbara and a longtime collaborator of Steven Hawkings back in the 80s the early 80s Jim and Steven proposed a mechanism for how the universe could be created from nothing the a kind of quantum fluctuation so quantum mechanics allows a lot of things to happen it allows for tunneling and it's fundamentally random so stuff happens in quantum mechanics without any obvious cause one of the things that they posited could occur is that an entire universe could be produced randomly by a quantum fluctuation and the reason this is possible at all is that it turns out that in general relativity the total energy of a universe that has a kind of spherical shape is zero now how could it possibly be zero it's got stuff in it well gravitational energy is inherently negative if you think about if I drop this it's gonna pick up kinetic energy it's gonna move faster and faster as it falls right so it's gaining energy but energy is conserved so there must be an increasing negative amount of energy as it goes down and that's the gravitational potential energy it's called in Newtonian physics so in that sense gravitational energy is negative and it turns out then that a closed universe a universe that doesn't have an edge has exactly zero energy so things that have exactly zero energy can be produced out of nothing because there's nothing to prevent them from being produced energy is conserved you can't produce something with nonzero energy out of nothing but you could produce zero-energy universe out of nothing and they were able to write down a mathematical solution for this which can be represented like that what it describes is a closed universe that appears and then starts to grow starts to expand and become large it when it appears it's very tiny but it grows and it becomes very big and according to this proposal when the universe appears it can have very low entropy so that's what we wanted we wanted to know why the early universe would have very low entropy but this is certainly not a complete answer to this question for a number of reasons first of all we don't have a theory in which we can create a universe from nothing there is no such theory we don't have one we don't know what it would look like if universes can be created they can probably be destroyed by whatever is the time reverse of that mechanism we have theories where particles can be created and destroyed not where universes can be and so Harlan Hawking lacking such a theory had to reason essentially by mathematical analogy so it's better than verbal analogy but it's still an analogy it's not something you can really predict precisely and even if we did have a universe in what theory in which universes can be created and destroyed if it's laws were time reversal invariant it couldn't explain the second law of thermodynamics they couldn't explain why the early universe would have low entropy it would be much more likely in such a theory for it to have higher entropy so this idea doesn't really answer this question but still it's a piece and it's a very interesting and important piece it's one of the few ideas that might have a chance of explaining this ok so I just want to close with a quote so I'll read you a quote from a book by Freeman Dyson and then I'll play a quote from from Stephen Hawking so I'm reading this book by Dyson for Freeman Dyson is a very famous physicist he never won the Nobel Prize but he probably should have he's still alive he's 94 years old and he's been at the Institute for Advanced Study in Princeton almost continuously since 1948 and he wrote a book recently it's called maker of patterns which is an autobiography by letters so he wrote a letter to his parents in England every week for many decades and I happened to be reading this book while I was preparing this talk and I came across this letter this is a quote from a letter in which in which he describes this visit by Stephen Hawking so this is back in 1970 when Hawking was 28 or so and just in case anyone has trouble reading it I'll read it I was taking care of Stephen Hawking a young English astrophysicist who came here for a six-day visit here means in Princeton Stephen is a brilliant young man who is now dying in the advanced stages of a paralytic nerve disease in the last few years he has produced a succession of brilliant papers on general relativity so this was before Hawking radiation these are the papers on singularity theorems in conversation he is one of the quickest and most penetrating minds I have come across he is confined to a wheelchair these days while Stephen was here I was in a state of acute depression thinking about him except for the hours when I was actually with him as soon as you were with him you cannot feel miserable he radiates such a feeling of strength and good humor I was running after him to escape my misery so here again is Hawking in this zero-gravity airplane and you can see he's having a pretty good time I'm not sure I think maybe 10 years ago but I'm not sure probably you can find it with with Google if you just google images of him it's it's everywhere so I'm gonna play a quote this is from an end of a talk that he gave a TED talk that he gave I couldn't figure out how to embed it so we have to go so thank you for listening we're gonna take a break now for ten minutes and then we'll have a Q&A session for about half an hour and we need to finish right around five o'clock okay so we'll have a Q&A session now and again please wait for the mic okay I have a question and then the person was sitting next to me he's not a physicist at all and was a little confused about it asked me to ask you a question she's sort of recovering upstairs okay okay my question is of course why is there something rather than nothing and her question was what does nothing look like since she's an odd physicist so there's a book by Lawrence Krauss called something very similar to that universe from nothing yeah and and I mean what he's describing in that book is essentially this idea I think the way he describes it is a little bit too strong he makes it sound like we know the answer and this is settled or in any case there's a reasonable theory that really does this like I said this idea is essentially reasoning by analogy and the analogy is to the production of particles in vacuum or in the presence of something like an electric field where you can produce an electron-positron pair out of nothing but the electric field I can just spontaneously appear to really have a theory of production of something from nothing you need a theory of nothing what does that mean so at least there have to be some laws of physics or some sort of mathematical equations it's not clear that that's really nothing and even then we like I said we don't actually have a theory like that we don't have a theory even at the level of mathematical equations that that describes this production of universes from some sort of vacuum that doesn't have any universes in it that doesn't really exist so I think a lot of people suspect that it might exist when you when you add quantum mechanics to Einstein's theory and since there is a theory of space and time space and time are dynamical they can expand they can contract there can be waves and usually when you add quantum mechanics to a theory like that there is a state which contains nothing in the case of electromagnetic forces it means that it contains no photons no light no electric field and there is such a state it's called the vacuum state so if we had a quantum theory of Einstein's theory of gravity the vacuum state of that theory would be this thing we're discussing it would be there's nothing but we don't actually have a quantum theory like that that we understand so yeah we're a long way from knowing the answer did I answer both questions is yes specifically with regard to the shape here why does it have what appears to be in an inflexion configuration rather than being strictly conical yes so there's a little bit tricky to explain but I'll try what this is representing is two different shapes glued to each other so this one is a hemisphere and this one is half of a hyperboloid and and the inflection point you're talking about is is right here where you glue them together the reason for doing that is that again in this analogy that essentially was being made here so the closest analogy is if you have an electric field you can imagine a capacitor that's two plates of metal and one has positive charge and one has negative charge in between is an electric field that points from one plane to the other there's a quantum process where an electron that's a particle with negative charge and a positron that's an anti-electron that has positive charge can spontaneously appear in between and then one flies to one plate the other flies to the other plate and partially cancels this charge that was there and reduces the field and there's a mathematical description of that process which would look a lot like this it would look like the edge of that so there'd be an electron here a positron here and they then fly apart so the vertical direction would be time the horizontal direction would be position and this motion is an accelerated motion where the electron or the positron is flying off under the existence under this field this part of it is the quantum tunneling part and the reason that has a different curvature and therefore this inflection point is because quantum tunneling is a strange process it's not classical physics it's sort of like an event that takes place in time that's the best I can do this is a technical questions and there's not a simple explanation but that that is the answer at some level you mentioned the black hole by itself in space you couldn't see and my question is it relates that to dark energy and dark matter and trying to figure out what dark energy dark matter is I'd like your viewpoint on why isn't all this dark energy dark matter just a bunch of black holes we can't see yeah that's it that's a great question so first of all dark energy and dark matter are different and for anybody who doesn't know what they are when we talk about this expansion of the universe I was saying okay if there's some initial impulse that makes it expand that part we don't understand but once it's expanding it keeps expanding unless there's enough gravity and it to pull it and turn it back around right but while it's growing you would expect the expansion rate to slow down because all the gravity in it all them galaxies and so on are pulling on each other and slowing down the expansion just like when you throw a rock up in the air it might escape but it'll slow down as it goes out instead the universe is actually expanding faster and faster with time its expansion is accelerating and whatever is responsible for that is what's called dark energy all right so that's dark energy dark matter cannot be the same thing if it's matter well it really cannot be the same thing dark matter is some form of mass which is presence we think in most galaxies and which is not visible it doesn't emit light but the reason we think it's there is that you can determine how much mass there is in a galaxy by observing how it rotates how things orbit around it it's like if the Sun was somehow black but you could see the planets moving around in orbits you would know there was a Sun there had to be an object there with the mass of the Sun because that's what's making the planets move in orbits it's just like that you see a galaxy and you see it rotating you see the stars and they're orbiting around its center and you can figure out how much mass must be in it and it's much more than the the mass that's in gas or stars so whatever that stuff is we call dark matter now could it be a bunch of black holes actually maybe probably not because of various constraints so one of them is this lensing effect that if dark matter were black holes these black holes but sometimes just happen to drift in front of distant galaxies there would then be a lensing effect and people have searched for that and not seen it another reason is that in the earlier universe those black holes would not have been in vacuum they would have been surrounded by plasma cuz the universe was full of plasma they would have sucked it in produced light from the accretion and we don't see that either so there are a number of constraints like this none of them are totally solid and if there's a range of masses of these black holes it might be possible it's certainly something people are thinking about very actively actually especially not at LIGO detected all of these all of these mergers yeah excuse me I appreciate the clarity of the answers and particularly the last one that was a good question I'd like to go from your discussion of entropy to the inverse which in Claude Shannon's presentation of information theory that would be information I'm familiar with that as a computer geek but I don't understand the use of that term information with Stephen Hawking's work I know this is a little bit outside of this but why are you worried so much about information loss women lose it so easily over a computer pipe right yes so information in the context of information loss and Hawking's work is is not quite in the sense of Shannon it's more let's call it a loss of productivity so even in quantum theories if you could know exactly what the state is of everything then you could evolve that state in time and determine precisely what the state is at a later time so for example if you know exactly what the quantum state is of two particles and you shoot them at each other you can predict what the result will be what the state will be at a later time with 100% confidence predicting the quantum state is different from knowing what you're going to measure when you measure it but you can predict the state and you can actually measure the state if you could do this experiment many times over and over again the same initial conditions so you can check your prediction what Hawking was worried about when you discovered Hawking radiation is that it seems that since black holes have no characteristics other than mass spin and charge they have no memory of the state that they formed out of so for instance if you formed a black hole out of encyclopedias where graduate students you would get the same black hole as long as you had the same total mass and then it would evaporate according to Hawking into thermal radiation which wouldn't remember those encyclopedias or graduate students that it was formed out of so yeah so there didn't seem to be a way to preserve the information about the initial state meaning that the quantum state didn't seem to evolve in a predictable way so that's what he was worried about I mean it is related to Shannon entropy in the sense that in quantum mechanics the analog of Shannon entropy is conserved it doesn't change in time but in this process in this Hawking evaporation process at least the way how can thought about it it would change with time it would violate a postulate of quantum mechanics so and which is that the state is predictable the state of later times is predictable I think there was a I think there was maybe a question here though that you've been waiting for a while yeah thanks very much two questions one what happens in the infinite density of a black hole to atomic and subatomic particles what what do they become and the second question is if if you could control the government's budget and if you had some truly brilliant graduate students which of the sort of unknown the sort of questions that are lingering in in theoretical physics what would you allocate that money to and your graduate students to what are the really big priority questions okay so the first ones easier so we start with that yeah so what happens to atomic and subatomic particles there's a theory of atomic nuclei which says they're made out of protons and neutrons and there's a theory of protons and neutrons that says they're made out of quarks quarks are something that we're pretty sure exists that you can smash together nuclei hard enough that the quarks briefly show themselves and you can measure them if you if you made a high enough density state quarks would no longer be bound as the nuclei they would just all be crammed together in a different state a little bit is known about that but not a whole lot it turns out although there's a good theory of quarks it's really hard to calculate with it's sort of like Einstein's theory of general relativity you need a powerful computer and even with that it's difficult so we know a bit about that state but not a lot if you smash those together even more as would happen inside a black hole we don't know there's probably some sub structure to quarks there's a theory called string theory which is an idea for what might be lie below that level it might be right it might not be so we don't really know the answer past a certain point it becomes just anybody not quite anybody's guess because there are a lot of constraints but but there's a lot of uncertainty it's like an onion right you keep peeling away layers but there's always more so so we can go down to some level in in length scale which is quarks below that we're not sure and when you squeeze everything together you get all the way down you get an infinite density state it seems okay as for what questions I would allocate money to first of all I think of science and scientists as like I don't know an anthill in the sense that ants sort of swarm around and they look for food right and and and if they find food they all swarm they're not all most of them go there and and sort of you did until it's gone there's always a few wandering off in various directions just in case they get lucky and find something and you never know what you're gonna find if you're an ant right you're crawling along on the ground you can't see very far a lot of those ants that wander far by nothing maybe they fall into a into a river or something and die right I mean you just never know I think it's important to have both of these things going on so there's some questions that are clearly important that you should put a lot of resources into and at the same time you shouldn't abandon these ants that are wandering off and and asking big questions and taking a big risk so you don't want to focus all your resources on the obvious things and you also don't want to ignore them and put resources into you know high risk high gain projects and exactly where this balance lies I'm not sure but I think there's a structure that does a reasonably good job of identifying it so there's structure of federal grants and so on as for which of these problems I think is most important well I briefly mentioned cosmic inflation so I think that's a very important question because we're pretty sure it happened so this is a phase in the very early universe which was responsible for all of the stars and galaxies that we measure today and there's pretty good evidence that it happened but we don't know what the physics was that's responsible for it this black hole information question which arises out of Hawking radiation that's been very fruitful thinking about that and it's far from well understood so there's a theoretical research program there which is really active and really interesting and in fact you mentioned Shannon entropy it's connecting information theory to fundamental physics in a really interesting way let's see what else this the story with black holes detecting merging black holes it's very very exciting because all of a sudden there's a new way to observe the universe if we build a few more like O's that are more sensitive we're gonna start seeing black hole mergers all over the all over the universe and it's yeah it's like having a whole new kind of instrument to it to observe the universe with it's a new kind of telescope in a sense yeah I mean pretty much everything that I mentioned is is interesting some things are a little bit more so that's a very active area I don't know singularity theorems and general relativity is not so active because it's pretty well understood at this point so the stuff that Hawking did in the early 70s late 60s but yeah the early universe understanding the universe better and understanding the current universe better through gravity waves though those are really exciting topics so right you mentioned in your talk there are two dominant theories the quantum mechanics and the theory of general relativity so quantum mechanics deals with very small things at the atomic level like photons and the atom the electron and general relativity deals with everything on a large scale the planets moving and so forth but we are looking at the galaxy and one of the tools that were determining this these ideas is at least with the expansion of the universe is the Doppler effect on light but light is inherently a quantum mechanical thing so how do I ask your physicists square the fact that these two laws don't agree with each other but they work very well at a small scale or a large scale but suddenly we have astrophysicists applying quantum mechanics at the large scale that is to explain large-scale phenomena where it suddenly using the Doppler effect which is about light and mechanics quantum mechanics right so your question is is predicated on the fact that it's really hard to find a quantum theory of general relativity let's find that hard to find a quantum theory of gravity this is true but the difficulty there it only comes in certain aspects of that so so what's hard is to understand what happens in extremely high energy or high density situations like the singularity of a black hole or a Big Crunch singularity and it's also hard to write down a framework which is clearly consistent and makes sense and quantize is gravity but something like the Doppler shift of a photon and the expanding universe that's not so difficult to to analyze but the techniques we have so for instance this gravitational time dilation that we discussed that's an intrinsically general relativistic phenomenon right the fact that clocks run faster higher up in a gravitational field then lower down and you can do that experiment you can measure that time delay with clocks you can also do it by beaming light up from say a lower elevation to a higher elevation and you'll see a red shift or a blue shift depending on which way you beam it and you can do it with individual photons that's not at all difficult and you'll see that each photon is blue shifted or red shifted just as the the beam of light was classically so there's no problem with with quantizing particles in a gravitational field background the thing that's hard is quantized in the gravitational field itself so if you tried to quantize gravity waves the particle would be called a graviton that we don't really know how to do although even there as long as the effect is weak as long as you're not very close to a black hole horizon or near a singularity of some sort we still think we know how to do it but for quantum particles moving on in gravitational fields there's there's no real problem I actually have two questions first of all I want to thank you for your use of analogies the whole way through it's very helpful so I'm gonna ask for two more analogies from Hawking's book one of my takeaways and I worked very hard just clawed to pieces it was one concept that there is no fixed physical point in the universe and there is no fixed time and space did I get that right well okay that that was compelling to me if I got it wrong just say so the the other question that I'd love for you to take a throw at is I can understand what a mile is I can understand what a second is and 186,000 miles per second it really doesn't seem very fast to me in this whole you know cosmic universe why is there a speed limit you know why can't anything go faster than light and if you figure that one out tell Hollywood okay yes so let's start with the first question so I think I understand what you're getting at and it does sound right so you say there's no fixed point in space meaning all points in space look more or less the same or are you talking about the relativity of motion the relativity motion yeah okay so so you know how okay you're on the train and there's another train next to you and you have this moment of disorientation where you can't tell if your train is moving or the other train is moving and it's it's really disorienting sometimes and then if it's your train is moving you realize you feel some bumps or something and you can tell but you know for that for that moment you may not you may not be able to tell so motion is relative in that sense even according to Galileo and then according to Newton and according Weinstein there's no absolute notion of rest right so we're very used to thinking of one because while we live in an atmosphere and being at rest with respect to the atmosphere which is usually more or less at rest with respect to the ground is different than not I mean you feel wind or you don't you're running or you're not but in outer space there's no there's only relative motion that matters and that's true on earth - I mean it's relative motion of you in the atmosphere so it's actually a surprisingly confusing concept just because we're so rooted in the environment we live in where it really does seem like there's a restaurant but yeah so so - trains you can't tell which one is moving if you're in a sealed lab and it's not bouncing around or if it's in outer space so it's just floating smoothly you'll never be able to determine how fast you're moving it just doesn't make sense it's just how fast you moving relative to something and to be able to determine that you have to be able to interact with that thing so so that's built into basically all theories of physics why there's no fixed time well I mean the universe itself defines a kind of clock and it's expanding and it's getting less dense maybe it'll turn around and we collapse but for now at least it's expanding but apart from that there's nothing different about now and tomorrow I mean there's nothing there's no clock built into the laws of physics just like there's no position or rest frame built into them sorry your other question was um speed limit yeah yes okay well first of all the speed limit is really in special relativity and in general relativity it's still there locally because according to general relativity locally inside a small sealed lab space looks pretty flat it looks like it would according to special relativity but it's possible that there could be a way around the speed limit and general relativity because of the curvature so there are there's a solution called a wormhole which most likely doesn't exist and cannot exist because most likely it cannot be formed the kind of stuff you need to form it probably doesn't exist but you could I mean you can find a solution to Einstein's equations if I can find them if you put the right the correct sort of stuff on the right-hand side of Einstein's equations anyway if you put the correction of stuff on the right-hand side you get a you get a you get a wormhole solution which would allow you to move from one place to another very rapidly there we go so yeah if you're willing to do if you can put anything you want on the right side you can get basically anything you want on the left side but there's there's something there's a special form of matter which again probably doesn't exist but if you put it over here you produce this mission that connects to distant regions so that's one way you might be able to beat the speed limit why the speed limit is there at all it's built into the geometry it's because speed is not exactly the way you think of it it's not something moving in time because you have to remember that time and space are linked in this intricate way so I don't have a very good analogy to explain that but as you get closer and closer to that speed your clock slowed down more and more compared to someone who's not moving and you can keep accelerating all you want but you'll never exceed that speed as measured by this other person who's just sitting there and not accelerating so to you it'll seem like you're accelerating forever but to them you'll just get closer and closer to the speed of light and never quite get there still not very much time will pass for you so you know you can sort of travel into the future in a way right you can you can go very close to the speed of light very little time will pass for you a lot of time will pass for someone who's not moving so it's not really like getting around the speed limit but you can do that and come back and find everybody you knew you know died a century ago or something right so so yeah I don't know it's it's it seems to be the way the universe is it's built into the geometry of it ickey mentioned black holes have mass spin and charge what about magnetic fields good yeah a spinning black hole with charge would have a magnetic field so if you take it just forget about black holes if you just have some charge and you move it around do you make a magnetic field a spinning black hole a spinning charge black hole does have a magnetic field too so yeah they have electromagnetic fields there's no as far as we know magnetic charges so those would be called magnetic monopoles so you don't have the kind of magnetic field which sort of points away from the black hole you have the kind of magnetic field then the magnet has which you know sort of loops around in and the field lines come back in but yeah a spinning charge black hole would would indeed have that just like a spinning charge ball would yeah thank you Sagittarius a start there is being observed soon is there you have any takes on what what is expected to come out of the the observations yeah so it's been observed indirectly in a number of ways already you can actually image the orbits of stars around it and because it's so heavy and the stars are so close to it the orbits only take a few years or maybe even a few months and so people have seen complete orbits multiple complete orbits of stars so by the way I should I should say for the rest of the audience Sagittarius a star is the name given to the black hole at the center of our galaxy of the Milky Way and yeah one of the ways we know it's there is by seeing stars orbiting something it's just like if you saw the planets of our solar system orbiting the Sun but the Sun was somehow invisible you would know it was there same thing here you see stars orbiting it and so it's clear that there's a very very heavy object there with mass of about a million solar masses a million times the mass of the Sun there is a proposal I think it's called the event horizon telescope or something to image it in radio with a resolution that's about its size if it's a black hole which it almost certainly is and well we more or less know what to expect it should have a chrétien an accretion disk around it and so you should see this disk spiraling but probably they'll learn something I'm not a very expert on this but I suspect I'll learn something about the environment and the interior of the like right near the center of the Milky Way I don't think that they'll find anything sort of earth-shattering I mean it won't change our conception of black holes but it will probably teach us more about what's going on in the very Centers of galaxies and maybe it'll help resolve this mystery of how these black holes got so heavy because there's not a good theory for how a black hole given the current age of the universe could have grown to that size that's actually a big hot topic right now that could have happened on uninflated theory of the universe can you describe some of the mathematical basis of that and is it related to quantum mechanics yeah right so inflation in the context of cosmology is the idea that the universe expanded very rapidly and more importantly it accelerated a long time ago so shortly after the Big Bang it underwent this period where its expansion rate was increasing exponentially or close to exponentially so that means in a certain amount of time it doubled in size and that same amount of time it doubled again doubled again doubled again and so on and there so that idea was proposed a long time ago almost 40 years ago now it seems to be true in the sense that it makes a set of predictions for what you're gonna see when you build when you look closely at a very ancient light from the early universe and those predictions have been confirmed over and over again but in all that time no one has come up with a very convincing explanation for what made that happen or they've come up with many explanations none of which is very convincing let me put it that way there's there's many many alternative theories of inflation it's hard to distinguish between them and it's hard to know fundamentally what was causing it as for how quantum mechanics comes in it's sort of like my answer to this other question that that you can study quantum mechanics in the background of a gravitational solution like an expanding universe and we know how to do that so we know how quantum mechanics what what role quantum mechanics would play during inflation what it does is it makes the density of the universe not exactly uniform inflation well economic inflation sort of equalizes all money right because it makes bank accounts disappear and it makes debts disappear so everybody has the same amount of money namely zero after a while the same thing happens in cosmic inflation it spreads everything out so after a while the density is the same everywhere that would be true without quantum mechanics but quantum mechanics causes fluctuations it means that the density is not exactly the same everywhere and those fluctuations in density are the seeds from which galaxies grow later so when inflation comes to an end if you have a region that's a little denser than what's around it it starts pulling stuff in by gravity it gets denser and denser and denser pulls more in and eventually it becomes a galaxies and stars light up and so forth so so quantum mechanics comes into it there it's actually an amazing story it means that when you look at the night sky you're seeing quantum fluctuations that were produced in the very early universe 14 billion years ago mapped out for you so yeah so quantum mechanics comes into it in that sense yes yeah there's there's if anything too many of them pretty much and then they try to test them and sometimes they fail and sometimes they don't but there are too many of them that have not failed so there's there's a lot that are still alive let me put it that way it's hard to constrain them so I think we can maybe have one more question or do we need to end now one more ok one more question and I think I think you had your hand up before so straying slightly from the topic of today Shelton who apparently had a number of telephone conversations with Stephen Hawkings several years ago abandoned string theory as the unified theory was it appropriate for him to do so I have to admit I've only ever seen that show a few times I'm pretty sure I know who it's based on it's a guy I was a postdoc with years ago I won't say who but but well string theory isn't is an interesting story it was proposed as a theory of the strong interactions that the force that binds nuclei together and it didn't work for that because it contained a particle like the graviton like the quantized version of gravity and so when people realize that they abandoned it as a theory of strong interactions and adopted it as a theory of everything as a theory of gravity plus it by the way contains lots of other forces and it's it's been described as a piece of 21st or 22nd century mathematics that fell into the 21st century it's it's really difficult it's very hard to solve any of its equations with a very limited understanding of what it is mathematically but it seems to have this self consistency it's it is a quantum theory of gravity it has lots of forces in it that look kind of like the ones in the real world but then it has more it has too much stuff in it and it's it's really hard to solve these equations so people lost some steam I mean I mean it was there was a period where it seemed like they were right on the verge of solving these problems and this was the end of the story and then they kind of ran out of steam a little bit because well things didn't pan out as easily as it seemed like they should so I don't know the situation now is that there are still a lot of people who work on it I'm working on it at this majus wrote a paper about a very spring theory has nothing to do with the real world it's really about string theory and a very mathematical regime but I also working on other things and I think most people who work on string theory these days are like that there I think about lots of problems and they're interested in it but they're also interested in other ideas so yeah it's not like you have to abandon it or not right it's not a monogamous relationship you can do whatever you want and you know you can spend part of your time working on that and fire you time working at something else and that's that's fine so okay thank you very much I think we have to end [Applause]

Channel: New York University

Views: 6,767

Rating: 4.8636365 out of 5

Keywords: nyu, new york university, NYU DC, DC Dialogues

Id: 5Zz1-ZktKYk

Length: 142min 8sec (8528 seconds)

Published: Tue Jun 12 2018