Carlo Rovelli: The nature of time

[Applause] thank you it's pretty scary it's a full house like that stand up and talk in front of all of you is scary and exciting so thank you very much for being here for for coming I'm gonna talk about time so I'm gonna try to tell you in the simplest way I can everything we have learned about the nature of time everything we all the many things we do not understand about the nature of time and a few ideas which might be right or might be wrong and I'm not sure about them so this is about time 45 minutes of trying to clarify where we are in our understanding of time and why why there is there something to say about time let me start from this if I just forget what I learned that a physicist and think what usually time ins for me I think for many of you as well time is like a long sequence of events a long line of things that happen in one of the other along which the sister of the universe unfolds so this is a this is a metaphor of time a long long line this is a I bought this morning and we are at some moment of time say here and so this is where we are and this is a future this is a past so this is few minutes in the future this is where I have to end this conversation this is when you were coming in this is one morning this morning when you woke up yesterday before 10 years ago the Big Bang maybe who knows there and the future who knows even that Rex it at some time or whatever it is or something about obviously the future it's very different from the past in some sense we move in fact we have just moved a little bit I'm just here now and obviously we are all in the same present so this is the present we're all together in the same present and right now the universe is in this presence of the right now of entire universes here more than her universe will be here that universe change like that and of course between two moments say now and tomorrow there is a certain amount of time 24 hours and that's 24 hours for everybody that's a time difference between this moment and this moment so this is time in our everyday life long line a sequence of moment of time that we can order which has a preferred direction past and future very different that we can measure with clocks with watches with clocks to measure it and we all agree about the return intervals between two different moments along this line good so almost everything had said is wrong it's really factually wrong like if I said the earth is flat it's not flat wrong earth is fear time does not work this way it works in a different way of course to some approximation if we forget something it works this way but if you look at closer if we had better clocks if you look into small if you look in the large time is not this line is not a unique sequence of moments there is no sense in which the universe is all in the present there is no sense in which between one moment and the other there is a time difference with the same for everybody and so forth and so on these are not speculative ideas of strange dreams of the physicist this is a fact facts that we measure in the laboratory we measure with instrument and can be can be checked so what I'm gonna do is first of all describe this fact to you and show why this picture of time is wrong and then I will tell you what do we think this time at this point so if you if you take away pieces of this picture what remains what is the remaining idea about time that we have and then the last part of this conversation I'll tell you how from this world without time we can come back and reconstruct this idea of time for us okay so first part is what's wrong with that and I will list the four things for fact that show this picture to be wrong three are very very solid every all my colleagues and I will agree on that all the physicists agree on that even the chemists agree on that and nowadays in the engineering it is the force is much less clear but it's pointing the same direction and I'll mention it let me start for the first one first one is very simple and many maybe you you know about this this is my watch the good watch and I I always need a second watch so I asked Valerie if I could borrowed her and she I bought her watch so these are two watches and they indicate exactly the same time for 32 or for 32 and of course you know that if I if I move one watch I put it higher I wait a little bit one two three and I bring it back I still indicate the same time they both have moved but this is because these are not very good watches if you do this with very good clocks and today we can do this with very good clocks because we are very good clocks you move one higher wait a little bit you bring it down and this is ahead this is behind I repeat here there is more time if you if you move if you think we're moving on this fine line if you move up and and and wait a little bit and then come down the clock up has measured more time than the clock down and it's not just a matter of clocks everything I mean if you're high you have more time to think flower will blossom or a a machine would do more cycles it's really more time more things happening up and down because the difference is small otherwise we would have noticed it clearly earlier we had to wait for for having good good good clocks for measuring it but quite remarkably this was understood before having good cross in fact this was understood hundred years ago by Ashton is one of the great intuition by by Asha in 1915 he completed this theory generativity what this makes it's very clear time clocks were not good enough for measuring gauge so at the time it was a theory a good speculation that seemed to put things together today is not a serious nothing to do with Einstein is a fact that we measure in the laboratory in fact it's it's so clear and definite that technology today needs this to be taken to account you probably all use one way of the other GPS navigator the thing in the car and the computer that on your telephone that tells you where you are it works with satellites it gets message for satellites on those satellites there are clocks so essentially the satellite broadcast the time and your your little device used the time to triangulate where it where you are exactly and when this was built the physicist told the engineering that were putting up the system careful because up there your clocks are gonna run faster than down here because up there there's more time that down down there the engineering who trusts the physicists of course yes of course we're gonna do that the entire project was controlled by the American army because the GPS was made belong still today to the American army and so it was supervised by the generals of the American army the generals looked at the address and what's the story of the time goes the different speeds I didn't believe it so the first satellites were put up there with a switch which could or could not take into account this fact the difference of speed of time and of course it did not work does not work at all if you don't God take this into account so it's a hard fact now our technology would not work we would crash the car against the wall driving with a navigator if if we didn't take into account this this means that the time between this point and this point is 24 hours here but 24 hours something if you're higher and more if you're more high you're a lessee for less now here on earth is small this effect is small we're talking about nanoseconds it's measurable fact now it's measurable at 30 40 centimeters altitude in the lab but this small you need bit clocks but if you go near a bigger mass what makes the difference is the masses mass as yours so there's the big mass and it slows down kind if you go to a bigger mass like Jupiter is stronger if you go near a big star is stronger you know black hole we know that the universe is full of black holes is even more strong so strong at very near to the black hole times essentially stops down goes very very very slowly this means that if we had if we had a starship we could move near a black hole stay there for half an hour and then come out outside hundreds of here could have gone so we would come away from the black hole into the far future easily if you have seen the movie interstellar the hero comes back on earth and finds his daughter much older than him and that's not an Olivet fantasy the the scientific advisor of the movie was keep torn the great relativistic got the Nobel Prize because of the gravitational wave detection and he checked all that so the the stories is realistic the only non realistic thing is that we don't have the money for building the starship and coming back so Chinese far more elastic than our intuition of this rigid passage altogether the time interval between one point another point depend where you are and that's the first fact that makes this intuition about time wrong second factor this is also something very clear very established very accepted but perhaps is the most striking my opinion is the most striking discovery in in in modern physics and maybe even in modern science together with darling for probably the the fact is simple but the consequences is astonishing the fact is that the the time difference between two clocks does not depend only on where they are but also how they move so if you again if I take two clocks I move one okay I come back this is this has measured less time than this one so this is ahead this is late so if I move fast I have less time so slow down don't move don't move much again it's a small effect you have to move very very fast to be important have to go the speed near speed of white light to be to be remarkable what is that but this is a consequence of that which is not immediate you have to work out you have to do drawings and convince yourself but it's a consequence of that that it is impossible to say clearly when is the same time in two different points see imagine that I stay here and my sister goes somewhere else and then we want to say I want to I want to think what is my sister doing now well half an hour has passed for me half an hour later from the separation what is my sister doing a half an hour later from the separation for her can be something else that's half an hour for me if my one my sister has moved fast and the other half moves slowly they meet and for for one is half an hour phone that is 45 minutes so when is it now you can say well I sent a signal to her I look at her I can look at you and say I see what you're doing now but wait a minute when I see that you light comes from you to me takes time so I don't see you now yes I see you in the past I'm seeing all of you not now in a little bit in the past you're not seeing me now you're seeing me in the past with respect to what what you see and I see you in the past so we're not in the same now well I mean like takes what four or five nanoseconds to go from here to there nanoseconds very very small but if you were on Jupiter it would take a good hour to go light to go there I don't know I mean for 15 minutes to Mars more to Jupiter dependent world they are in the sky so it would be impossible to I cannot see somebody in Mars in the moment in which this person see me because this is 15 minutes and if I try to come out with a story to say when is exactly the same moment it turns out because of these attendants of the time on the speed that there is absolutely no way of doing that the consequence is that literally it makes no sense to ask what is happening in the far distant universe now and I let you but think this in because this is for me there as I said the most astonishing discovery of in in modern physics the notion of now is very clear to us right we are now here but now here makes sense at a small distance now all of it you know it does not make any sense now is a local notion how now is a bubble around us how big well the pen or the solution of time we perceive time we can perceive a fraction of a second I don't know a tenth of a second that's small this time we come to so in a tenth of a second lies right goes quite far away well that's our now beyond that there is no way to a court time in a objective way so time is not a single line every bubble has its own time every object moving has its own time it's not a single line it's a complicated structure of things that move differently and each one has its own time of course they communicate I can send signals to Mars Mars can sell signal to me but we cannot think that we're all together in a single time it just doesn't work to third one um the third one is in a sense is the most disturbing it came as a surprise slowly in physics one of the things I said about time is a past is very different for the future it's totally obvious to us right I remember what I did yesterday I don't remember what I did tomorrow I have a picture of myself as boy I don't have a pictures of myself as a young old man well yeah I don't have pictures of older than now men of myself so it's no doubt we have traces of the past we have memories of the past with information about the past the future isn't and who knows what's going to happen in the future good so this is so clear that it was a big surprise when slowly it became clear that the laws of physics have a hard time of distinguish the past from the future when Newton wrote the basic law of physics the laws of physics are the same for the past or the future so whatever can happen in one direction can happen also the opposite direction if you take a movie of something satisfying the law of Newton and you show the moving Reverse that's still possible then cane you know the electromagnetism of Maxwell came unto mechanics quantum field theory general relativity Einstein theory these are all theories which did not distinguish the past from the future and that's I mean doesn't make any sense because this theories describe a reward well and in our world there is past before the future where does the difference come from well there is one piece of physics where there is an equation that distinguishes the past from the future and that's the science of heat of temperature so there is one equation in the one fundamental equation of physics which make a difference between the past on the future it is an equation about heat about temperatures the famous second law of thermodynamics entropy I want to talk of and took a little bit later entropy will always grow doesn't decrease and it is a fact I don't know why is not more talked at school this fact but it is a fact that every time in a phenomenon there is a distinction between the past and the future there is some heat it's some temperature there is no distinction between the past and the future unless there is heat and temperature let me give you an example if I have a pendulum and re I study a pendulum I'll make a disaster I know I'll make a pendulum here please don't break so I I study this I make a movie of this okay I surely mu keep moving backward and you don't see which one is the right one it moves the planet if you look at backward you is the same you make a move you the solar system the moon the star you show the movie backwards is still reasonable but if you wait a little bit after a while it stopped it slows down now if you make a movie and you show the movie backward you say no no no something is wrong because this doesn't start by itself but when it slows down and stops is because there's a friction and friction generates heat right so it's a heat that distinguish the past from the future if I throw something and it stops right if I throw this it stops if I take a movie and it see it moving by itself say no no that's wrong so something that stops distinguish the past for the future and again there's friction there is heat so the distinction in the past in the future has to do with it and temperature and what is temperature what is heat is the molecules moving very fast so what happened when this slows down and stopped it happens that the energy of its motion goes to the energy of the molecules so the single energy of one moving object is passed to the energy of many many many molecules it's like spreading something around and the ordered motion of all these molecules together becomes a disordered motion of all the molecules heated them and that's the only source of difference between the past and the future things get disordered everything that distinguished the past of the future including us becoming it's always for the physical point of view something will is order dis Greg a ting and getting in disorder that's a physical meaning of the second law that then i'ma thermodynamics that's the only thing that this thing is the past for the future which is very good because we have understood something but it's also very baffling because this a knows difference between past and future memories traces as only to do with the fact that things were ordered in the past and who ordered them why they were ordered what does it mean we don't know that's one of the most debated issues by physicists by philosophers of science I'm going to a conference in New York in a month just to debate these things what is the meaning of this initial order because the universe started up in a special thing what does it mean but clearly this distinction between the past and the future is much less fundamental than what looks like it's not something about the basic working of the walk of the world it's something about the way things are ordered so you see this clearly line asked future equal time for every body fundamental distinction with the past in the future it's just block blown up fourth and last this solution of the notion of time it's my own work is quantum gravity I worked in Kanto gravity we know that the structure of time is governed by gravity and I gave you one example the earth is a big mass slows down time the gravity of the earth key effect of the gravity of the earth is slow down time every mass of the universe' slows down time around it fact in general activity things fall just because time is go slower down is we understand falling in terms of this long dial time good that we know well what we do not know well and that's the field in which I'm working quantum gravity it's a quantum properties of gravity quantum mechanics is this funny strange theory that Bath's described very very well the working of small things and when we apply it to gravity itself we're still in the dark but what seems to be obvious is that in this small the strange quantum properties that small objects have also effect time which means there was in this morning in the news one little experiment at Allen Australia was published in these days so I got the news perhaps some of you so saw them in which they they made some something happening some event a and then some event B but because of quantum mechanics each one fluctuates so the actual situation is like this happening first in this second and also vice versa because time goes a little bit in this direction a little bit in this direction so quantum mechanics implies sort of small fluctuations of everything you know particles like a wave is not precisely defined and time itself is not precisely defined into small it fluctuates the idea of a single time variables in terms of everything passes it just doesn't work anymore so in my work I try to write down equations that describe the world keeping to account everything we have learned generativity quantum mechanics thermodynamics and what we do are me and a lot of my colleagues is try to write this world and just forget time completely time is not a good notion when you go to the fundamental grammar of the world it's an approximate notion it's a local notion is a notion that has to do with our daily life it's good at our scale is not good everywhere many things like that in physics think about up and down that's up that's down totally I'm just totally clear and controversial but many of you remember physics of high school in the Newton equations there is no up and down in the universe there's no up and down when the astronauts go out we're flying to the moon the astronauts were in their capsule right the world floating around like that and if one says to the other where is where did I put the gravy says in they draw out which one the one up wouldn't mean you I up it's no up here so up and down make no sense in the universe it makes sense here why because there's a big mass the earth and down means toward them yes so we understand up and down perfectly well and of course in Sidney up is the other way around we expect to us up is that way in Sidney so time is something similar is not something Universal common to the entire universe something a local that something produces it at the fundamental level and these are the equations that I use in my work we write this equation and there's no time there's no T naught T variable this takes me to the second part how do we think about the world without time so how do I do physics how do I my colleagues do physics with equation where there's no time variable and let me let me try to take away a confusion first of all no time does not mean that everything is iced and not moving blocked frozen tell it we around a frozen something which is frozen if something does not move move wild time passes right it's the other way around so in the fundamental description of the world things happen everything happen but it's not all of it in time so try to visualize a lot of little events disconnected to one another or loosely connected with one another but they don't sing together like a Orchestra with a single director that beats a time for everybody that's the world at the fundamental level of course we can have a clock but we can have two clocks which be different times or three clocks or four clocks every thing that moves can count as a clock we can use it as a local clock for locally and in some sense asking how other things change with respect to that one so every single event or sequence of event can be used with time but there is no preferred time all over the universe that's the physics in which I work control gravity no time and in particular since there is not a unique time since there isn't a preferred time identification of events at different times stone is not that for don't work which means and I've already gone there in a sense and that's a key point that at that level I think we should think the world not as a set of objects what is an object an object pen is something that now is a pen I was still a pen now is still pen okay if I wait ten minutes is still a pen so it's something which leaves through time objects are things that / mean some time they remain themselves through time but from the point of view physicists nothing really / means so time okay this is some molecules that for a while got together and then they will dis Greg eight so this is really an aggregation of molecules is a process is a set a sequence of events a set of events which means it's much better to think of the world in terms of events happenings not in terms of things and I think this is a deep lesson from modern physics in my opinion that we think better at the world in terms of happenings rather than in terms of things you know a war is happening a sunset is a happening is a process a I am a process I keep changing where processes the human being if we try to block entities think in terms of entities which get confused because entities dissolved and come together from other entities but these other entities themselves come together from other entities if we think in terms of events and set of events that make processes won't make more sense so absence of time of fundamental level does not mean frozen the other way around it means happening a lot of disordered happening so that's the world I think that is emerging from contemporary physics not a single time no time at all events relation between events an equation to describe how these events happen with us - what another good um I've done half of my way I've taken this image of time I've killed it destroyed all the way through until this sort of timeless world with justice events without a preferred time variable but I'm not done because now I have to come back I have to tell you where does this idea of time come from where does a line come from then where does the past and the future come from where is the idea of this flowing of time come from and that I think is the most beautiful part of the story where is our sense of time in this union where there isn't a preferred time there is no time variable at all and I think that the answer is not a simple answer is a step-by-step answer so from the equations of quantum gravity where there's no time at all you can see that there are approximations special situation regimes physical situations in which something resembling this line is approximated think about up and down they stop not up and down in the universe but if you have a mass if your things moving on the surface of the mass with respect to them with respect to a point you can talk about up and down so the second part of the story is start from a world without time and try to understand where our notion of time come from and the outer assessories is multi step first of all let's let me list some of this step first of all one to gravity is my field describe the world into very very small but we don't see all this very small we see it as large average the quantum gravitational wall is evolved into very small and we don't see there so so our vision of the world is very blurred is very emphasized if you if the astronauts when they go to the moon they looked at the earth they see a a blue shining little ball nice and and and and uniform with its little colour simple ok they have no idea that they don't see that in this little ball you know though it's all of us London and tall people talking and all this all this and and when we see it just as a as a drop of water we just see one little shining drop of water we don't see the media millions of zillions of monocles that jump around and in fact we think each molecules the zillions of little quantum space dancing around so the universe is incredibly complex and we've access only to teeny teeny imprecise foggy version of it and it is in this fog this imprecise version that we start saying something that resemble our usual space in our usual time and something that happens in this foggy this blurred version of reality is that we lose control of a lot of reality and remember what happened with him with the molecules with a friction we call heat the motion of the molecules which we don't see anymore with the energy disappearing into heat so the notion of heat exists because we're this blurred version so in the thing is in our blurred version of the we wouldn't talk about heat and temperature if he saw the single molecule moving just move there is nothing hot in things that move okay so just energy of the molecules so this means that when we talked about thermodynamics we're really talk about heat and temperature we're really talking about our blurred version of the world not because we are not natural object we we we as natural object we interact with a glass of water but only a few variables not all the variables and in this in the simplification we average over things we have this blurred version and it is this blurred version that distinguishes the ordered motion from the unknown ordered motion so you see the distance between difference between the past and the future has to do I think with the way we interact with the world is not just in the world is in the perspective between us in the world and physics have science in general very often has understood as solved strange question deep questions by realizing that we make a Mis do we make a mistake when we think that something is there when it's really in the relation between here and there think about one of the most beautiful shows in nature which is the the sky that turns around us right we see the Sun going up and going down the moon gonna happen going down if you go out in a in the inner night in a clear night not in London you see all the stars that move during the night they make a piece of a circle during the night it longer probably don't see any star don't know but you just woke up in the countryside you have this wonderful show and then you ask why is the universe turning around us and we have understood it has taken only 2,000 years but we have understood why the universe is turning around us it isn't turning we are turning okay there's we sit on a rock and this rock happened to be spinning and that's why we see everything turning because we are okay this does not mean that is false or this illusory or a spiritual or user they said though it meant it's perspectival so we understand the turning of the sky by looking at ourself and seeing the relation to other motion and the rest of the universe so we understand in terms of the relation between us physical objects and the star and the moon us and the Sun as physical objects and the same has something like that has to do with time the passage of time I believe has a specific type of aspect with with it it has more to do with the way we interact with the rest of the world tan by the ball by itself but because that is false not false Eliska that the Sun rises and said it's just we understand it not just us looking at the Sun but looking at how we move respect to the Sun and let me make the two last steps which perhaps are the two most beautiful for us time is flowing is passing what does it mean I insisted a lot in saying that the difference between the past and the future is that we have memories of the past we know about the past of course this is possible because of thermodynamics because of physics but if you think for a moment for us the time is not really the clock that goes around is exactly the memories of the past when you think about the past you're thinking about what you remember about the past or what you have read about the past or a movie you've seen about the past or a book of physics or history that you've read about the past so for us when we talk about time we are really talking about that we remember the past and we can anticipate the future and this I think is a very deep intuition that goes back to philosophers too if that goes back to st. Augustine who wrote a very famous text on the nature of time he was saying talk about music so when I listen to music as a musical phrase it's beautiful smart values what I marry listening to because in every single moment I hear only one note is a melody a chant I hear one note how do I know about the others it's only one single sound which is there what about what happened first and then st. Augustine realizes that the music's music exists because each single moment we have somehow retained in our mind the rest and in fact we even anticipate what's coming out so we grasp something about the past and something about the future together but that's has nothing to do with physics that's our brain that does it and the people who study the brain in neuroscience are telling us I'm a physicist I'm not a neuroscientist but I'm crazy about reading neuroscience books because his fantastic was happening in neuroscience we're telling us that the brain does exactly that the brain is like there's all sort of things to say it's a machine that the most important things it does is keep learning it absorbing memories it keeping be connected to something outside and keeping this such that in every single moment the brain has memory of the past and try to compute the future on the basis of this memory of the past so the brain is this machine connected to the outside which constantly work between something here and something there and that's the time for us the time for us is memory expectation and the artists have often insisted on that this is Marvel's book by Proust the the the the the French titles are like a chef to talk Purdue which the sack translation is looking for lost time searching for us for for lost time the exact translation of the title and the book is a huge novel all sort of things happen but it's not presented as a story of happening is presented as everything that the main character his name is probably Marcel remembers so its thousand pages of description of what is between the years or Marcel what is inside here that's is where time is time is here is not out there is our memory our expectation and so in this really construct enough time to go to understand what time really is you shouldn't ask physicists only of course physicists have their say but you should also ask people to determine AMEX psychologists and neuroscientists to understand that's where the answer should be it's like you know when we understood colors colors have Latin Lewis Maxwell equations have something to do with max education light but the reason we see frickin three colors and combination of three colors is because our cells the in the eye receptors of three kinds it's nothing to do with physics has to do with us so the nature of time has to do with the physics but as much more to do with in the way our brain works and the last step I want to get to and it's gonna be my last point and perhaps is the most important of all for understanding the nature of time is this time touches us is not a neutral notion for us why because you know time passes we live in time we are a time in some sense we can't even imagine ourself with some time we are this memory and this anticipation if you if I destroy it completely my memory wouldn't remember even me three seconds ago I wouldn't be myself myself I'm this narration of myself but this narration of ourselves the story of ourselves it's also not only our identity but also the source of our good and bad emotions about ourselves there is a key core teaching in Buddhism which always I found telling which is that life is sufferance but the reason for which life is sufferance is because we cannot accept impermanence which meaning the things passing things that go away we suffer because we lose things because we we think we don't have we might have we think we have we will not have we will lose we're gonna die that's time for us is what takes away everything to us so it's hard to think about time without is think tively immediately be connected at what time makes to us not only it makes us but it destroys us at the same time we are in time and at the same time were destroyed by time so there is this emotional component in our sense of time but I think that this emotional component is not really some foggy emotions that blinds us with respect to the clarity of doing science about time it's the other way around this emotion when a component is to a large extent precisely what we are talking about when we are talking about time because the sense of flowing of time which is so hard to find into physics it's nothing flowing in a physical equation is so hard to find into thermodynamics even neuroscience it's completely clear when we think about our emotional relation to time its passage we are this thing that keep remember you keep anticipating so our own perception of what time is this time I have to finish I'm no time is passing it's over fifty minutes are gone that's emotional time is exactly what time is for us so we should not forget it to try to understand time only a physical equation if you do that when you sense something and we feel that something is missing and what is missing is precisely our emotion of time it's deep emotion of time which makes us and destroys us so to a large extent I think the emotion of time is not the thought that makes hard for us on the same time but it is precisely what time is for ourselves and I'll stop here thank you [Applause]

Channel: New Scientist

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Keywords: Carlo, Rovelli, Time, physics

Id: NrjFE_Rd2OQ

Length: 50min 44sec (3044 seconds)

Published: Fri Apr 10 2020