The Physics and Philosophy of Time - with Carlo Rovelli

Summary

Highlights

• Carlo Rovelli emphasizes that our experience of 'now' differs across the universe. 🚀
• Time's nature is deeply connected to physical phenomena, like gravity, explained in relativity. 🌌
• Quantum mechanics reveal time as granular, challenging linear perception. 📏
• Entropy and thermodynamics play a crucial role in attributing order to the past and chaos to the future. 📚
• Time perception is also a function of the brain, impacted by memory and emotion. 🧩

Key Takeaways

• Time is not absolute; its flow changes with conditions like gravity and speed. ⏱️
• Our notion of 'now' is not universal but rather a local phenomenon. 🌍
• The past and future distinction is influenced by entropy, representing order and disorder in the universe. 🔄
• The emotional and cognitive perception of time is deeply embedded in human consciousness. 🧠
• The linear perception of time is a construct that doesn't hold up on a quantum level. ⚛️

Overview

Chapters

• 00:00 - 05:00: Introduction to the Concept of Time The chapter titled 'Introduction to the Concept of Time' explores the intriguing and complex idea of 'now'. It highlights the surprising conclusions of modern physics regarding the nature of time and the subjective experience of the present. The discussion raises questions about whether the perception of 'now' is the same for different individuals, suggesting a deeper inquiry into the personal and philosophical understanding of time.
• 05:00 - 10:00: The Structure of Time in Physics The chapter begins with the speaker expressing gratitude for the introduction received. They proceed to prepare for the presentation by organizing their materials in anticipation of the audience's arrival.
• 10:00 - 20:00: Understanding Time Through Entropy The chapter explores the concept of time, employing entropy as a central metaphor. Time is depicted as a continuous line extending from the past to the future. It starts from the 'Big Bang' as the farthest past event leading up to the present, and continues towards an unknown future. Key points include the differentiation between the past, present, and future, with the acknowledgment that while the past can be known, the future remains uncertain. The chapter aims to deepen understanding of time's progression and its representation through metaphors.
• 20:00 - 30:00: The Mistake of Absolute Time In this chapter titled 'The Mistake of Absolute Time,' the speaker begins by setting the agenda for the session, indicating that it will include time for questions and further discussion. The main message conveyed by the speaker is challenging the conventional notion of time as an absolute concept. The speaker aims to provide insights into the current understanding or viewpoint on time, suggesting that it is not as straightforward or rigid as traditionally perceived. This chapter serves as an introduction to more complex ideas about the concept of time, possibly hinting at a discussion involving theoretical or philosophical perspectives on temporal reality.
• 30:00 - 45:00: The Role of the Brain in Perceiving Time The chapter explores the concept of time and its perception from a scientific perspective. The speaker, who has been fascinated by the mysterious nature of time from a young age, shares their journey through studying physics. They express how their work in quantum gravity has deepened their understanding and challenge them to rethink time. The discussion highlights the complexity and evolving understanding of time in the realm of physics.
• 45:00 - 55:00: Conclusion: The Emotional Aspect of Time The final chapter delves into the emotional and philosophical aspects of time. It centers on the question "what is time?" and presents a synthesis of current knowledge, beliefs, and areas of confusion. The narrative takes the reader on a journey, exploring both known and unknown aspects of time, emphasizing a back-and-forth exploration to deepen understanding.

The Physics and Philosophy of Time - with Carlo Rovelli Transcription

• 00:00 - 00:30 it's shocking in my opinion is one of the most shocking thing in its fact the most astonishing conclusion of modern physics which is we now are here what now means what is the meaning of now well I know what is meaning of now for me huh you know what is the meaning of now for you is it the same now
• 00:30 - 01:00 thank you thank you for this very kind of introduction the first thing I have to do is to pull up this which I've prepared before you came in let's see if it yes sorry which I'm gonna use and
• 01:00 - 01:30 that would be my high high technological equipment to talk about time that would be my metaphor of time right time is a time is a long line there is a present there's a the past of our past more for our past this is 10 minutes ago this is yesterday it is last year down there is Big Bang this is tomorrow and the future we don't know so this is this is a metaphor of time we're sort of here we
• 01:30 - 02:00 have one hour whatever with questions and and so on so we're gonna cover that and I'm gonna tell you that time is not like that the message and so what I'm gonna what I want to do is to give you a sense of where I think we are about
• 02:00 - 02:30 understanding of this strange mysterious things which is time it will be sort of a journey for me time has been a an open question from my early years and that'll say adolescent from all the way through my studying of physics and then into my work as a physicist you know sort of as obsession because quantum gravity requires us to rethink what is time I've been going around around around this
• 02:30 - 03:00 this question what is time so in the book and in what I want to tell you tonight is it's a bit of a synthesis so that where we are what we know what we think is true including the parts that we don't yet know we are confused we are confused about and I will do it is a taking you to a sort of journey journey which is back and forth so I'll start from this idea of time this long
• 03:00 - 03:30 line that passes we're here actually we moved a little bit already with more years and time passes and I'll show what quite rapidly tell you what's wrong with this idea why this is a wrong idea about time and in which sense it is wrong and this would be a step sort of down into more and more general views of nature where time has less and less property
• 03:30 - 04:00 and loses its pieces so to say all the way to what I think is the current understanding of the structure of time which is that there is very little time in nature and that would be the journey the one-way journey and then there is a journey back which is how from this nature at the level of quantum gravity which is sort of the bottom level of our understanding of the physical world in
• 04:00 - 04:30 sense of general Eiling generality flows how from there we get back to this time to the time of our our experience so it will be a long one one way and return trip where I want you to lose all the ideas of time to then sort of understand back where where our Comment I come from right so this is this is the time of our experience first of all it's a long line
• 04:30 - 05:00 namely what would I mean about a long line it's a it's a see what we mean it's a sequence of moments instant now before later which is ordered like the English when they wait for the bus so maybe I'm trying to Italian eyes you but I don't want you to crowd it for getting the next bus but all this
• 05:00 - 05:30 instant are for milonga along the line they're there the one next to another one in a one-dimensional thing this line is it has a direction right the past is completely different for the future this is one of the most obvious thing and it was trivial thing to say but let me say it precisely because then I'm gonna say it's not true the past is fix it it's given we remember it we know it we have
• 05:30 - 06:00 traces of it we have books talks about the past we have a department of in our university that study the past is history we don't have many departments that make a historical history about the future there are traces in in in nature if you look at the moon there are craters that are formed by stones fell into me in the past so the pasture is all the spaces where memories in our mind the future
• 06:00 - 06:30 there's nothing like that there's no memory no traces still has to come right so that's part of this idea that we have about time um time has a symmetric property maybe we can talk about duration of time this chart I'm giving this presentation for the last 45 minutes certain fix the amount of time that we measure with clocks and we know what it means this
• 06:30 - 07:00 interval being equal to this integral being equal to this integral in clocks measure that in clocks are devices that are meant to tell us where we are along this line what time is it where is where am I is it already tomorrow it's not yet in clocks are designed to work all together that's the purpose of a clock so if you take two o'clock some of them apart wait a little bit you bring them back together they still have the same time if they had the same time if not
• 07:00 - 07:30 means one crock is broke one clock is broken it's not going well thank you thoughts um this is a retirement a little bit past this is a time of our experience now what's wrong with this image first of all there's nothing wrong obviously as long as we carry on our daily life it's a very good construction conceptual idea about time that works very well in
• 07:30 - 08:00 our in our daily life but it does not work it stopped working when we look a little bit more far ahead and the more we look far ahead the more this number of properties that I described about time go away so the main message somehow
• 08:00 - 08:30 the fool talk I'm giving is a time it's not a single thing it's this multi-layered concept that has all this property that I told you is one dimensional as a metric is oriented there is the same for every body's measured by clock and so on and so forth this which in our intuition comes as a package is time it's properly thought as a combination of properties that we can dismount one by one time and sometimes
• 08:30 - 09:00 this is is layered or is like a mekinese likely engine of your scooter you can take away one piece the carburetor you can take away the piston you can take away this you can take away that and what remains is not the scooter what remains is nothing at all and that's going to happen we think when you take away this take away that taker with that what remains is nothing at all because time is a combination of these properties so let's see what properties one by one the first one maybe I
• 09:00 - 09:30 mentioned three or four of these let's say for to dismantle this idea of time some of this you may know depending of how much physics you know or Shifa you've heard these stories already some you probably haven't known some are very solid knowledge some are things we suspect them I would say the first three a very solid knowledge but the implications of taking all them together
• 09:30 - 10:00 are strong so first one clocks measure time fine and clocks go all together at the same spin speed oh that's wrong why is wrong because it's a fact that if I take two clocks and if I take two good clocks and I move them apart remove one or bring one higher and keep one lower and I wait a little bit and I move them together if these are good enough clocks
• 10:00 - 10:30 they will indicate a different time it's a fact okay it was clearly understood by a nice nine hundred years ago so in a sense we've been knowing this for hundred years but it's recent that we actually have good rocks not these that we can measure this and today with atomic clocks with clocks that measure easily one part time with an error one
• 10:30 - 11:00 in 10 to the 18 17 18 even 19 this is very rapid advancing the technology of clocks so with these clocks you can take two clocks at a different of altitude of 40 centimeters 50 centimeters and measure that so the clock up here goes faster than the clock down here there's more time here there's more time for thinking for for growing older for everything so
• 11:00 - 11:30 there is more time in your your head is older than your feet your head as unless you've spent all your time upside down in which case is the other way around okay why well why is it it's it's a funny question yes one could say why well because we know from general relativity that the switch is a big rock big mass slows down time so math slows
• 11:30 - 12:00 down next to it but why I think the right question is why not why is still this hard for us to digest because we're used to time being the same for everybody we used is to person have the same age and they separate one who goes to America the other goes to China and they come back after a while they still have the same age that's our experience but that's not right because they've aged it
• 12:00 - 12:30 differently depending on how they moved the paintings have lived ups more up or more down and and so on in our experience these differences are small and smaller than our perception of time or the the precision of our watches that we we carry so we don't we don't we don't notice that but if we lived a little bit in in the future when I hope there will
• 12:30 - 13:00 be starships going around a fast speed and coming down we will have to get confront that so the reason for which we think the time is the same for everybody is because we used it we got used to each by our approximation it's like you know if we if you grew up in in the Netherlands which is completely flat you think that the earth is just all of the same level and somebody says you know there are some places of the earth with
• 13:00 - 13:30 us goes down who say no yeah it's called mountains Wow right you have never seen mountains then so you're surprised we live in a region in which this difference of time is small so we're not used to this if we had a big massive black hole not far from here will be used to the fact that when you go to a black hole you come back everybody else as much has grown older than you good which means already that this line is
• 13:30 - 14:00 just wrong because the the time between here and here two events is not fixed it depends whether you were higher or whether were lower for instance so it's not there isn't a single time in the universe already there are many times in fact in ice ice and generativity they don't have a single time we have one time probably for every line good that's point one now point to that follow quite
• 14:00 - 14:30 rapidly from that which in fact something that Einstein realized even before generative is special activity but it has long take long for for becoming absorbed because it is shocking in my opinion is one of the most shocking thing in fact the most astonishing conclusion of modern physics which is we now are here what now means
• 14:30 - 15:00 what is the meaning of now well I know what is meaning of now for me you know what is the meaning of now for you is it the same now obviously yes because I look at you you look at me and I say now you say yes now I hear yes now so we're in the same now right then wait a little bit look at we are new now so a little bit more moved here but we are together always together the same now the think
• 15:00 - 15:30 of a moment when we look her out if I look at you do I see you now well not really because time lights take time to come from you to me right so I see you sort of a little bit in the past now time takes a few nanoseconds to come from you to me so I see you as you were a few nanoseconds ago nanoseconds are irrelevant but if you were on Jupiter I
• 15:30 - 16:00 would see you two hours ago and if you were on a star I will see you four years ago close the star and if you were on different galaxies and drum and I would see millions of years ago so I couldn't say now is you see I see you and if you look at me in the moment I see you you you are gonna see me in the past in the future for me so between the UIC and the you that see me in the moment I see you
• 16:00 - 16:30 there's an interval between the two which is a few nanosecond here but few hours in Jupiter and millions of year if you were in a different galaxy so if the people in a different galaxy want to know what we are doing now they don't know because they're looking somehow there is all this time for so one say well alright okay this is because it's hard to check but we can devise things to say what it means to be
• 16:30 - 17:00 at the same time for instance I could say well I look at you but I know you at a certain distance you're in Jupiter but I know it takes half an hour I'll take four hours light to come here so four hours in the future for when I look at you that's now okay except that you be maybe moving fast and four hours in the future could be in my own future maybe 10 years in my future so how come
• 17:00 - 17:30 the now be 10 years the future does make any sense so the more you play this game the more you realize that it's a question which is ill posed there is no meaning of now outside the approximation in which we can disregard the time light goes back and forth so what we mean by now is we both look at one another and we are in the same temporalities within
• 17:30 - 18:00 the resolution of time that we have which means that the notion of now makes sense on a radius on our honor on a bubble around us which has a length size L which is given by the speed of light time the minimum resolution of time that we can resolve the delta T we can resolve that's the meaning of now there is no meaning of now outside this bubble with our brain without using
• 18:00 - 18:30 sophisticated clock to resolve probably what a tenth of a second if you're a musician you will be better than that a tenth of a second light is much bigger than the earth it's a big bubble around the earth so on the earth we are all the same now but it's just this Proxima B which is a closest star has its own now there's no sense of putting them
• 18:30 - 19:00 together you can set signal so one to the other signal back but in between there is no way of saying there's no meaning of now and in fact people work in general activities they're additions work in general activity study solution of Einstein equations they know very well that this is four-dimensional solution there is no preferred now no objective now that you can pick up us from from physics of from anything why I
• 19:00 - 19:30 said why this astonishing because what is it real what does it mean to be real well we are told that what is real is real now right it's a Roman Empire real no it was real in the past not now ok but this now is only local it's something happening on a distant galaxies real well it may be in the future maybe in the past but if everything is real in between it's confusing because there are things which are the past on the future one another
• 19:30 - 20:00 what did this mean to be real after we have understood that the world works like that namely that there is not now in the universe there are philosophers who are discussing there that's why I talked the philosopher who are discussing that and and and you know pulling each other here is and fighting the the very meaning of being real of reality in the sense of what is real now has to be rethought after what we've
• 20:00 - 20:30 understood about about physics so the idea of one insert of time it's completely destroyed in our little bubble we can talk about instance of time that we share provided that we don't go too fast we don't we don't not precise but globally there is this series of events in the universe with some temporal relation between a couple
• 20:30 - 21:00 of them but no common now at all and no order no order in the mathematical sense two three and this is a little bit more complicated and less well understood the first well understood the first part less understood the second part the past
• 21:00 - 21:30 is different from the future right where there is a local past local future it's the past Atif of the future so let's forget that other galaxies select loop let's look around us and yesterday is completely different story than tomorrow yes it is fix that we know it tomorrow it's open we don't know it good so I'm a physicist I want to know where does this distinction come from in the laws of
• 21:30 - 22:00 physics and as you probably know the laws of mechanics don't have this distinction the laws of electrolyte Yeti's don't have this distinction Standard Model of particle physics does not have this distinction general relativity does not have this distinction right so all the basic equations of physics quantum field theory does not have this distinction all the fundamental equation of physics luckly distinction between past and future but for us the past is completely
• 22:00 - 22:30 different for the future we have books written in the past we don't have book through it in the future so where does this thing shall come from well there is a story about that which is there's one finds that sees clearly the distinction between the past and future with thermodynamics and in thermodynamics is one law which is a second thermodynamics the the second principle of thermodynamics that says that entropy
• 22:30 - 23:00 always grossed over the future so that clearly makes a distinction between the past and the future fantastic so we've found the basic the key fundamental grammar of the world the elemental machine that distinguish the past or the future that law was written by trousers 19th century but shortly later sometime later Boltzmann and his friends marks
• 23:00 - 23:30 will also but Boston was I think the person was greatest clarity understand that this law is statistical namely Boltzmann understand that is entropy this quantity that you can compute category and chemistry always compute the entropy in in a reaction energy heat you do this calculation you get the entropy before and to be look after and it always goes up Boltzmann understand that the entropy is just not a
• 23:30 - 24:00 fundamental quantity but a measure of how how mechanical things are disordered right you know that entropies disorder that's a great sort of understanding that we we got footballs but what mean disorder you take you take a box you have some green balls some red balls you put all the green on one side the red on the other side this is ordered you mix everything this is disordered and if an entropy
• 24:00 - 24:30 counts exactly that how much order how much disorder if they're separated this low entropy do your little calculation or entropy for mixes high entropy which makes a lot of sense because when you shake things I get disordered so entropy goes up good now you look at these balls green and red and your friend is colorblind so he doesn't see any order because his collar bright it doesn't see
• 24:30 - 25:00 any order so for him there's no order for you to his order but actually is worth of that your friend a very keen eye that distinguished some wolf you were a little small and some little bit bit larger much better eyes at yours you don't see this difference so for him order means all the little balls on one side all the big balls on the other side so for him the entropy is a completely different story so what we mean by the order of the world see I
• 25:00 - 25:30 think you see clearly that order is in the eye of the person who looks it's not in the things themselves order counts once instead of recognizing the balls one by one if you if you number all the balls and all of you give names to all the balls whatever the way you put them they are ordered in that particular way [Music] is only by distinguishing two big bunches of balls that you can talk about
• 25:30 - 26:00 order or disorder and in thermodynamics when Boltzmann understands a statistical and depending on thermodynamics he thinks he understand is that the notion of entropy depend on the macroscopic variables that you use to describe the system on the coarse graining of the system which is a simplification of description the system which disappears if you look at the micro physics of the system all right so what does it mean it
• 26:00 - 26:30 means that this growing of entropy it's tied to the approximation we make in describing a system it's not fake is not illusionary but it depends on the way we've we've described the system or the macroscopic wave we have described the system which in turns depend of how interact with a system so it's not a mental thing is not idealistic thing so the subjective things the really concrete things depend
• 26:30 - 27:00 on the way you interact with a system then we interact with the system is your interaction with some variables of the system which are few out of the many and these few determine a coarse graining of the microscopic variables and define a name to be and then to be gross oh this is fine so in the future the MPT grows in the past the entropy with respect to our coarse graining was slow why was it universe all do it in the
• 27:00 - 27:30 past who order it I mean few if you go to your little children little child bedroom and they even is a complete mess and the morning is an order you know why is in order because somebody put it in order but why the Union who prepared the universe in order and the universe in order in the right way that looks in order for us mystery yeah what does it have to do with time
• 27:30 - 28:00 everything because the distinction between the past and the future is only that is this strange order in the past and I want you to reflect about that because I'm saying something very very strong this has been recognized by some people I came back in the philosophy of science Russell have clear ideas about that but few it hasn't permeated I think the the the global thinking about the
• 28:00 - 28:30 order of time everything in our experience which is ordered in time is because of entropy there is nothing else in the world that distinguish the past in the future except this entropy concretely any phenomenon that you can imagine where you distinguish future for past this an entropy if you if I throw this
• 28:30 - 29:00 it stops right if you film it and if you look at back back backwards this is starts by itself so you said no no no no that's wrong so this is a irreversible phenomenon okay there is entropy namely this heat what is going on is that is friction and the mechanical energy of the clock goes into thermal energy off the table that go is a little
• 29:00 - 29:30 bit heated up so all the molecules start of the table start agitating because of the friction and the order the motion of the clock is transforming a disordered motion of the of the molecules right every time there is this thing shoved in the past the future is because it's heat this entropy this temperature involved if there's no heat no temperature if this rolls this is move without heat it will go forever and if you feel and and and
• 29:30 - 30:00 project it backward it goes forever in the opposite direction you wouldn't distinctly the future or the past any mechanical thing you can think about pendulum without friction you take a movie you project the backward with a pendulum the solar system the earth of the moon goes around suppose you film it you project it backward who can say which isn't the right one it's exactly the same motion to distinguish in the past in the future you need entropy disorder which means
• 30:00 - 30:30 that the reason we have traces of the past has not in the future is nothing else that entropy okay if we have a texture say something it had to be this has had to be some enter production some heat some disorder at some point if a monk in the Middle Ages in a monastery
• 30:30 - 31:00 wrote down something and now we know something about the middle age because we have this text in a world without friction the ink would have not stayed on the paper would have chopped away so you need friction is a the ink glues to the paper because it produces a little bit of heat that this dissipate and so it's some disorder which can happen because it was a higher order in the
• 31:00 - 31:30 past so it is high order in the past that determines the existence of traces and we think in terms of cause and effect cause coming first and effect becoming after only because of the second dynamics because of entropy otherwise there could be no dis symmetry no physics says that the physical if this is connected to that this is connected to that both way we cannot say this causes this more than this causes that because the symmetric Russell used
• 31:30 - 32:00 to say that the notion of course would disappear is wrong it doesn't belong to physics it would be recognizes as something what is it totally useless and mistakenly mistakenly considered non damaging like the British monarchy and and and sooner or later we'll get rid of it he was wrong but we'll come back why why he was
• 32:00 - 32:30 wrong the point is that the entire distinction between past and future is a microscopical effect due to the fact that under our perspective the past look more ordered so not only the instant of time is not aligned because every line in the universe has its own it's on its
• 32:30 - 33:00 own temper allottee not only makes no sense to take one to talk about the now of the universe but the distinction with one direction of time they are the direction of time is some strange microscopical effect which you don't see in the micro physics 3 oh boy time is passing fast us I should speed up I am gonna speed up because the fourth reason for which the our community of time is completely wrong is less solid than the first three
• 33:00 - 33:30 first two is science of unison very well the fourth reason is the science that I do so it's less solid why because I'm still doing it right it's not yet in the textbook i and all my friends we're working on and the point is that there is a connection which I just mentioned before between time and gravity gravity the mass effect the speed of time but we know that
• 33:30 - 34:00 gravity that the understanding way of gravity may disregard quantum mechanics so this is the proper quantum gravity this job of my life so I'm paid for to try to write the equation for quantum gravity there are a set of equation that we have written down for describing the quantum properties of gravity and these equations tell us something about this what the clock measure and tell us I
• 34:00 - 34:30 would say three things one is that a anything that works like a clock cannot measure time local time continuously because there's a minimal amount of time so there is something granular in the passage of time I understand that in the UK there is a current debate on children not being able to read analogous clocks so they only read the digital clocks so
• 34:30 - 35:00 they come out with the wrong idea that time is discrete that is digital right which is the right thing so very good I don't see why kids should be able to read digital clock I wasn't very good in reading digital clock when I was a kid because I didn't use clock so Chinese digital as a very short scale of course not seconds of fraction of a second of the Planck scale 10 to the minus 44 seconds very very small time intervals
• 35:00 - 35:30 but that's a minimal timing there's no smaller time interval than that not only that but the clocks itself because a quantum object everything's a quantum object can be in super positions of different position of the handle a different reading so you cannot say that between this event and this event is a certain amount of time there because as always in quantum mechanics there could be a probability distribution of time passing from one and the other so you see that everything you use to think
• 35:30 - 36:00 about time disappears when you go to the widest possible situation which is quantum gravity which you keep into account everything we know about the world world today in fact in the equations of look onto gravity the theory which I'm working brings together quantum mechanics general activity in the equations of that theory there is no time variable at all why because you don't need a time variable you have a lot of variables you have a lot of
• 36:00 - 36:30 clocks you have a lot of things that change at the basic level time is just any way of counting the change of something which by the way is the definition of time that Aristotle gave 25 whatever centuries ago time is a number of changing you look at something changing day night day night day night you count and this is time time is a counting of change then Newton came out
• 36:30 - 37:00 with this idea you completely knew that time is not just a changing of something but it's some uniformly flowing something that flows irrespectively of whether is matter changing or not is completely Newtonian idea is a new idea it is in your mind because you studied it at school because it's a permeated our society but it's not the old way of thinking about that the old way of thinking about time is that you see sink
• 37:00 - 37:30 changing you number them she's fine but Newton comes up we said well there is that way of seeing of times it's not other way of thinking of time my newtonian way which is the time just passes even if nothing changes then I still recognizes that this Newtonian time in this Newtonian space which also you to introduce city of empty space there's nothing else about space um are actually the gravitation field it's
• 37:30 - 38:00 nothing else the gravitational field so I think in a sense says oh yeah you're right Newton that there is something else beside the counting of things there's this absolute time maps of space but you're wrong in not seeing that these are dynamical object is just electromagnetic wave it's just a little magnetic field this is a gravitational field so it's a sort of something that fills up everything which is the Einstein four-dimensional a curved space-time and once you realize that this ice and four dimensional
• 38:00 - 38:30 curved space-time which is a gravitational field as quantum properties it can be in superposition is granular it has all the funny thing there does all the funny things up when two things do the Newtonian time is a completely exploded it depends on the interaction is granulars different point of point there is two T Leontine the basic idea that you can count event still there so in quantum gravity of events not order it in line that the British but all confused like
• 38:30 - 39:00 the Italians connected to one another you can follow this one this one this one and you can see okay doing this blah blah blah number of things happened so this moved a certain number of clicks and so you can have a local notion of sequence of event which is a minimal notion of time and that's the only thing that remains locally probabilistic discrete without a prefer a time variable anything at work as a variable and that's the no time the minimum time
• 39:00 - 39:30 of quantum gravity I promised you a journey back so I have less time remaining because I was supposed to finish here and already here to take you back so I hope I convinced you well I don't hope I convince you I hope I gave you a sense of the very very weak temporal notion needed to do fundamental physics I've written a paper whose title
• 39:30 - 40:00 is forget time so write the fundamental equation of quantum gravity and if you don't see there which one's the time variable don't worry you don't need a time variable because the time variable is something microscopic not something down there down there if many variables change one or respect to another how do we go back from there from that basic reality we have to find the conditions under which we live which
• 40:00 - 40:30 allow us to talk about this long line oriented and common which is time and this is a number of steps there are many steps different levels different layers each one is interesting by itself but they're separated time doesn't come up off in one step comes up at various levels first of all the granularity of time is so small we don't have such precision so we don't see it
• 40:30 - 41:00 this flow we can see it continues the quantum mechanical superposition aspect of time we are big and heavy and much work at scale much larger than the Planck constant which is what determines the quantum mechanical effect so we don't see it so we actually see a gravitational field as a continuous things like I stand described but we live in a region in which gravity is very very weak compared to black hole
• 41:00 - 41:30 though is really strong and is very very weak essentially this nice thing curved space time is flat and this is flat Einstein space time are assembled very much Newtonian space in Newtonian time not exactly because link Oskie space-time but very much but we don't move fast to respect one respect to one another so we can disregard the time back and forth of the light and we are
• 41:30 - 42:00 so so we can assume that light instead of going a finite speed goes at infinite speed because now experience life is sent essentially infinite velocity don't resolve the light travel at a time travel of light so if time is a at infinite speed then we have these surfaces of simultaneity we can think that we're in this in our bubble and so we can talk about one single time right
• 42:00 - 42:30 so now we have a one-dimensional line which is still not oriented past and future so what is it oriented it past if you well let's go slowly first of all remember it's entropy entropy depend on the way we look at the world or the way we interact with respect to the world so I think and take this as an hypothesis not as something demonstrated some of my
• 42:30 - 43:00 colleagues disagree that what makes entropy low down there in the past saying the early universe entropy was very very low he's not the fact that the universe was arranged in a very ordered way the universe was arranged whatever it was a you arranged is the fact that we look at the universe we interact with the rest in a way which is very particular because we are particular we meaning we as a physical system
• 43:00 - 43:30 interacting with the race though such that under this perspective the past was special imagine you take a cart you mix them you look at them and you remember the order okay that order disappears when you mix again so you see a increasing of disorder but that order was special because you decided to a special you looked at them and learned it so in some sense we are special
• 43:30 - 44:00 subset of the universe that interact with the rest of the universe in such a way then the past looks ordered to us what is special is not the universe is a subset of the universe to which we belong this is called area the Pacific title origin of entropy and I think it could be an ingredient for understanding why the past even for the future there's nothing special in the early universe is special the microscopy the coarse graining that we do are we done not yet
• 44:00 - 44:30 not yet and there is a last step and to me this has been the greatest surprise and here I'm stepping a little bit out of my own comfortable zone but it's what I've been learning and learning and um there is something about time which is still missing and there are some philosopher even some physicists who says we have got the entire story wrong because time flows
• 44:30 - 45:00 train passes time sort of passes at a certain speed and we don't see that in all this physical picture so maybe there is some fundamental fundamental fundamental law of time that says after all there is this passing of time I think they're right they're wrong and and and many other colleagues of mine including many Philosopher's increasingly getting convinced that what we refer to when we refer to this flowing of time this passing of time
• 45:00 - 45:30 this clear feeling that we have about the flowing of time is not in the quantum gravity is not in generative it is not in quantum mechanics is not in thermodynamics is in the specific way our brain works and there is an enormous amount of working today in the neurosciences in the function our brain with a title of a book by a neuroscientist called dilben Amano
• 45:30 - 46:00 a recent book in America if the title is your brain is a time machine what does it mean the way the brain works is to exploit this entropy gradient and the fact there traces of the past to build memories and in terms of this compute the future and anticipate the future that's basic working of the plane according to one of the importance of how the brain works so the brain is the time machine brings us continuously sort of grasping to some events in this
• 46:00 - 46:30 confused set of events around ask we are connected by traces to some event tour the past we are connected by anticipation on the other direction so in our mind there is this opening space okay you think that this is a metaphor of what this is metaphor our memories of some of the past and in fact that's why is much more confused over the future because we don't have
• 46:30 - 47:00 of the future when we think about time very often we're not thinking about the physical part of the physical future but we are considering the memories we have about physical person and and the anta patient we have about the future um philosopher has been saying that for a very long time who sir has been saying that it goes back to st. Augustine famous pages about the nature of time in his book the confessions he says when I
• 47:00 - 47:30 listen to music I get a meaning from a musical phrase but I never listen to the phrase I listen to one note at a time right if I listen to one note how do I know about the previous notes okay get the meaning well of course I know because I remember them but if I remember them and who sir is very clear in that the meaning comes from the notes
• 47:30 - 48:00 beings playing now and the memories of the previous one so it's all in the present so to say and it can be only the present together because as memory but it's more than that because since the brain is designed by evolution to use memory to anticipate for a purpose because it it designed to try to get somewhere that that's how living evolution design our our our
• 48:00 - 48:30 behavior then all this is strongly emotionally charged the passage of time is not for us a rational thing to contemplate it's something we live in - we are the past this passage of time where this constant computing of time you can think about reality without space we can think about reality without things you can say it's very hard to think yourself in a reality without time you wouldn't know how to start thinking
• 48:30 - 49:00 but the confusion is is this because rarity by itself cannot be thought without time no is because our thinking cannot be talked about time we cannot think we are a time machine not the universe use a time machine you use just a bunch of thing of events vertically connected to one another which in some approximation are nicely ordered but what we think when we think about time is the personal of the memory and in the anticipation give us the sense that we
• 49:00 - 49:30 are in this space right all of us what is the real time real time is now ten minutes ago my life but then I've been to school so I know about all the kings of England so this long line right the line of the kings of England in Queens and and so I have some extended memory which is a cultural memory I've studied physics and cosmology so I have this memory longer it's all today it's all my knowledge days all here in the book I talk about
• 49:30 - 50:00 post post as the reason has written this fantastic novel in search of lost time it's all about time he says that explicitly section of the meaning of time and the novel is full of stories character things but then novel is presented not as what is happening in the world or what has happened in the world but as the memory of the main character whose name is probably Marcel
• 50:00 - 50:30 so all the 3,000 pages of Proust novel are about what's in this you know five inches between the years of Marcel it's all here and that's the greatest intuition of truth to understand what is time for us we have to look at the specific of our brain functioning more than the temporal structural universe
• 50:30 - 51:00 itself of course these religions were not saying that temple structures unit still is irrelevant but I'm saying that by looking only at the temporal structure of the units you always get the feeling that there's something missing with respect to the time of our experience and so you do metaphysical speculation about the primary key of time to completely wrong because of course there's something because doesn't pertain to the fundamental strike for us it pertain to the way our brain machine works and last
• 51:00 - 51:30 point it's truly emotionally colored time is not emotionally neutral to us precisely because we are our brain is a machine designed to tell the story about the past and do something in the future we are full of motivation hunger if thirst we have ambition with curiosity with
• 51:30 - 52:00 love with hate that's what we are before being rational beings and this drive is the drive of the brain that you know control our hostesses and and and and and struggle to make us survive and do better because evolution wants us to be like that it's already entering time it's all in time so time is emotionally charged for us time is what brings us things we want is the opening of the
• 52:00 - 52:30 future okay in time is what is motional charge for us because it makes us lose thing constantly all the time in in in in Buddhism there is the speed psychological side I think the main truth of Buddhism IRS's life is sufferance but the second is that the life is sufferance because we have difficulty of dealing with impermanence with time so time is a source of our suffering we suffer because we lose thing or because we have lost things or
• 52:30 - 53:00 because we shall we think we will lose things so it's we're gonna die so this is a quintessential source of anxiety for for human so x is strongly colored emotionally and why I'm saying this because I think if we think that this emotional side of time is a sort of fog that does not allow us to see the real nature of time we are confusing yourself because this
• 53:00 - 53:30 emotional aspect of time is precisely what is deeply time for us time for us is this emotional connection to the event of the world that go away that pass that flow that we lose and this is a looot of our strong sense of passing time and feeling feeling time is ingrained working of our of our brain
• 53:30 - 54:00 this is vaguely connected to this event it works because it's entropy of course because it our scale we can orient Orion things like like like this but you see understanding time means bringing out the pieces of the of the of this layer the complex object time is not a single thing is all my grass is not a single thing is all these layers that come from approximation from this regarding aspect
• 54:00 - 54:30 of nature from thermodynamics from our particular relation to the world n from our function if our brain including this emotional level out there the more we go general in the picture of the universe the more time loses species there is a very weak form of temporality and in closest to us I think the strong emotional connection with time the
• 54:30 - 55:00 motion of time is what time is for us you