The Passage of Time and the Meaning of Life | Sean Carroll

hello i'm alexander rose the executive director here at long now today i'm going to be joined by sean carroll from caltech and santa fe institute who's a theoretical physicist who's been really studying the nature of time itself and the human experience of time and really how we fit in it and here at long now we've been doing talks that kind of relate to time in many ways but this is the first time that we're doing it on time itself i'm very excited to have sean here welcome sean carroll thanks very much sandra it's a great pleasure to be talking about time the nature of time what it means for all of us at the long now foundation obviously folks along now have been thinking very carefully about time about the human aspects of time about how to measure it how to think about it and that's my jam that's what i'm very very interested in doing you know time is just so important to who we are as human beings right we feel like time is passing around us or we're flowing through time the metaphors they go back and forth i'm not sure whether we're moving through time or time is moving around us but either way time is just crucial to what it means to be a person we plan for the future we remember the past some of us are so fascinated by the nature of time that we become professional physicists and we learn about what time means from the physicist's point of view and we learn a lot like physicists understand a great deal about the nature of time but there's something interesting maybe a little scary that happens when you learn about time from the perspective of physicists you learn equations they tell you how physical systems evolve but this idea that time passes or the time flows or that somehow we move through time that central most obvious idea about time in our everyday lives disappears it's nowhere to be found in the fundamental laws of physics but it's clearly there in our everyday lives and we think that we're made of things that follow the fundamental laws of physics so what is going on that's what i want to talk about with you today how to reconcile how physicists think about time with how we experience it in our everyday lives and what that reconciliation teaches us for what time means and what it means to be a person so let me start by explaining what i mean when i say that this passage of time notion disappears when you learn about time from the physicist point of view what does a physicist do what's a typical physicist kind of problem to contemplate okay well think about the planets in the solar system moving around the sun this is the kind of thing that physicists are really good at paradigmatic kepler galileo newton those people you're given the state of some physical system by which in this case we mean the positions of all the planets where they're located in space and how they're moving right the velocity at just one moment of time you don't need to be given the history of the planets at one moment of time you're given where they are and how they're moving right now and then you have a recipe you have the laws of physics you have newton's laws of motion and newton's law of gravitation and from those you can predict the future from the present state of the system you can tell where it's going to be next you can predict eclipses in the future you can say where the planets are going to be you can make an almanac okay but there's something extra that is very very important you can also predict the past or retrodict if you prefer given the current state of the system the laws of physics tell you exactly what was going on in the entire past history of the system and real astronomers actually do this they extrapolate the current state of the solar system millions or billions of years into the past and the future so buried underneath that little discussion is an idea that we call conservation of information all of the information you need about the system right now its positions its velocities and so forth suffices to predict the entire future and past of the system if i told you exactly the state of the system a million years ago you could predict what it would be doing right now this is the disappearance of this idea that time is flowing time to a physicist ever since isaac newton it becomes kind of a label there's this moment there's the past moment there's the future moment there's nothing special about what we call now or the present moment the laws of physics describe every moment of time as being on an equal footing and the information contained in every moment of time persists into the past and the future that's very different from what we're used to in our everyday lives just as one simple example imagine you come across there's a glass of water on the table it's a slightly cool glass of water okay and you ask yourself well what was the state of that glass of water a few minutes ago maybe it was just a cool glass of water just sitting there but maybe it was a glass of room temperature water with some ice cubes in it and maybe those ice cubes melted and cooled off the water the point is just given the macroscopic observable information about the world you can't say where it came from unlike the perfect information we have for the solar system in the real world where we see the glass of water and we can see a little bit about it right it's temperature whether there's ice in it or not but we're not looking at every atom or molecule of water then suddenly information is no longer conserved there's information that used to be there in the system did it have an ice cube in it or not and that's disappeared over time we have an irreversible process so what we're seeing in the macroscopic world is what scientists and philosophers call the arrow of time the difference between the past and the future in particular the asymmetry that we have when we think about now versus the past versus the future and that asymmetry that arrow of time is not there in the fundamental laws of physics we have to dig in a little bit to understand where it comes from so let me just explain how that asymmetry between past and future shows up in our everyday lives one aspect is that there is an asymmetry of knowledge okay in a very real sense you can predict the future but you're never really sure what's going to happen right you know sometimes our predictions come out wrong whereas when it comes to the past we can have records we can have memories there are things that exist in the universe right now which we're very very confident tell us something specific about an event that happened in the past whether it's a photograph or a book or a fossil a bone underneath the earth these are artifacts right now that represent something definite about the past we don't have photographs or artifacts that tell us about the future in the same way but there's another asymmetry not just knowledge between past and future there's an asymmetry of influence between the past and future we have in our minds the idea the past is settled right it happened there's nothing you can do about it whereas toward the future we can make choices we can do things now that affect the future if you lift your left hand versus lifting your right hand that might have an impact on the future but nobody thinks that by doing something right now you can influence the past where did these asymmetries come from why is it that we can know more about the past but we can influence the future in some way again it's nowhere in newton's laws of motion okay nor in any update of newton's laws of motion general relativity electromagnetism schrodinger's equation in quantum mechanics all of these attempts at finding a fundamental laws of physics have the feature that they treat the past present and future equally now i'm not going to hold you in suspense too much i actually know the answer to this question why is the past present and future so different to us the answer is something called entropy and the second law of thermodynamics i'm sure you've heard about this before entropy is basically roughly not exactly but close enough the disorderliness of a system the randomness the disorganization and there is a law of nature that is true in our macroscopic everyday world that says that entropy increases over time so a typical example might be an egg you can have an unbroken egg it's easy enough to break the egg to scramble the egg and in that process from unbroken to broken to scrambled the entropy of the egg increases you're mixing things up you're making them more random and that would just happen very easily and naturally in the world whereas if you saw a movie of scrambled eggs reforming into an unbroken egg you would know right away someone was playing the movie backwards in time there's a directionality to the evolution of these systems so the first law of thermodynamics just says energy is constant that's good the second law says that entropy increases over time in a closed isolated system or in the universe as a whole you can clean up your room don't get me wrong you can lower the entropy in a localized system by influencing it from the outside world but overall in the universe as a whole entropy increases over time so unlike newton's laws or other attempts at fundamental laws of physics the second law of thermodynamics does tell the difference between the past present and future it says that entropy will be higher in the future it was lower in the past good that's nice that's an explanation of something but we want to do better why is it the case why does entropy go up over time so this was roughly speaking solved in the 1870s by ludwig boltzmann an austrian physicist and some of his friends what they did was they said entropy is not just a rough idea it's a highly precise quantitative concept that we can attach equations to that's what physicists like to do with their lives attach equations to things you thought you understood so entropy in boltzmann formulation is just look when you look at a system a macroscopic thing a glass of water with an ice cube in it or cream and coffee or an egg you don't see every little bit of that system you don't see the molecules you don't see the atoms of which it is made so let's take cream and coffee as a classic example okay if the cream and coffee are separate from each other there's a number of ways you could imagine changing the positions of the individual molecules in the cream or molecules in the coffee so that macroscopically you wouldn't be able to tell the difference but if you started mixing them together if you started taking the cream and mixing into the coffee you would be able to tell the difference so there's a small number of rearrangements you could imagine whereas if it's all mixed up right if all the cream and all the coffee are mixed together then there are a large number of ways you can rearrange the atoms and molecules to make it look exactly the same so boltzmann says that's what entropy is entropy is a way of counting how many ways there are to rearrange the fundamental constituents of a system so that it macroscopically looks the same to us from that point of view it's not at all surprising that entropy goes up as a function of time it's simply because there are more ways to be high entropy than to be low entropy if you have a medium entropy system and just let it go let it do its thing let it evolve into the universe it will naturally increase in entropy since there are so many more configurations that look that way that part is easy the hard part is if you have a medium entropy system why was it lower entropy in the past if it started lower entropy you can explain why but you seem to need a new principle to explain that and indeed that is exactly true think of the arrow of time this way there's no arrow of space right if you were an astronaut floating out there in space there'd be no difference between up down left right forward backward but you know if you're here on earth there's an arrow of space you can tell the difference between up and down nobody thinks that arrow of space is deeply ingrained in the fundamental nature of reality it's just because you're in the vicinity of an influential object the earth the same story is true for the arrow of time the arrow of time according to modern physics is not built into the fundamental nature of reality we experience it because we live in the aftermath of an influential event the big bang 14 billion years ago for reasons that cosmologists do not understand the big bang was very low entropy the early universe was very orderly compared to what it could have been now why is that true good question very active research question at the frontiers of modern cosmology but for our purposes today we don't know why but it's true the big bang had a low entropy and once you say that it's clear that entropy is going to go up it's been going up for the last 14 billion years it's going to continue to go up billions and billions of years into the future so good we think we know what entropy is the number of ways you can rearrange a system we think it makes sense that entropy increases over time what we're left to do is understand what this has to do with our experience of time in the everyday world how does that help explain this asymmetry we have about the past versus the future this feeling that we have that we're flowing through time well let's just take one example here's my favorite example of an asymmetry this is an asymmetry of prediction versus records you're walking down the street you look down the sidewalk you see a broken egg can we ask ourselves because we're in a philosophical mood what is the future of that egg likely to hold you don't know right there's many different possible things that could happen to the egg you could just sit there it could be cleaned up by somebody there could be a rainstorm washing it away there's many possible futures open but if you ask yourself what does the past of that egg probably involve with very high confidence you will believe that there used to be an unbroken egg and it got dropped and it broke so right away just seeing this thing in the world right now this artifact broken egg on the sidewalk you infer a difference between the past history of that egg and the future history of that egg why well it's not because of the fundamental laws of physics given the configuration today egg on the ground messy etc there are many many arrangements of the individual molecules that could account for that and accordingly there are many many possible futures as we said many things could happen to the egg if all you knew were the fundamental laws of physics there is a precisely equal number of past things that the egg could have gone through the conservation of information convinces us that the number of past possible histories of that egg is exactly equal to the number of future possible histories of that egg but there is one big difference that we know the universe started with a big bang and that big bang had a very low entropy this is what philosophers call the past hypothesis of low entropy in the universe so we know two things we know the current state of the egg and we know the initial condition that we started with very low entropy we that's where the imbalance comes from that's where we know a little bit something more about the past than about the future and we can use that information to say given the existence of a broken egg on the sidewalk today and the existence of a very very low entropy big bang we can infer that there used to be an unbroken egg as you go through your life as you are remembering things and predicting things about the future you are constantly relying on the fact that the entropy of the universe was very low near the big bang you might not have known that you are relying on that fact but it's crucially important to how we live our lives that's the good news we can say something about where this asymmetry comes from this asymmetry of both knowledge and influence toward the past and the future but it raises kind of another worry that we didn't have before which is the following you know if all that happens over the history of the universe is that disorder increases how did i get here i think of myself as relatively orderly right the organs of my body the neurons in my brain they're organized in kind of a precise way how could you tell me that something as exquisitely organized as a living being just came to exist out of increasing entropy over time and of course you know because i'm asking you this question that i know what the answer is to this at least i know a little bit this is another active research area but let's go back to our favorite example of the cream mixing into coffee it begins in a very low entropy state the cream and the coffee mix together it ends up in a high entropy state but that early low entropy state is very simple right all the creams on the top all the coffee's on the bottom that final high entropy state is also very simple everything's mixed together it's in between it's that intermediate state on the journey from low entropy to high entropy where things look complicated where the cream and the coffee are mixing together in unpredictable ways forming little swirls little fractal patterns that would require a lot of information to specify so that's the secret and we have a lot to learn still to go about this but basically complexity naturally arises on the journey from low entropy to high entropy this is a we think a fundamental feature of how the universe works that there's something about the journey from a simple beginning with very very low entropy to a simple ending with very very high entropy that can give rise to complexity so entropy just goes up but complexity first goes up and then goes down it's a temporary phenomenon which might make you pleased to understand what's going on maybe a little worried about the ephemeral nature of things like that in the universe and indeed this is not just a story about cream and coffee this is a story we can tell about the universe as a whole the big bang was about 14 billion years ago and right after the big bang the universe was very low entropy also very simple it was hot dense smooth rapidly expanding that's all you need to know over time what happens is the universe expands and cools and structures begin to form gravity pulls things together and we can see this we can take an image of the background radiation from the big bang from a few hundred thousand years after the beginning you can see the first faint traces of structures forming today 14 billion years later our universe is rich with structures there are planets stars galaxies people clusters voids it's a very lumpy interesting exciting kind of universe but that's temporary it's not going to last forever stars use up their fuel they burn out they go dark stars fall into black holes even black holes don't last forever in the 1970s stephen hawking taught us black holes radiate out into the universe and eventually evaporate away completely we think roughly speaking that 10 to the 100 years from now which is a long time even by long now standards all the black holes in the universe will have evaporated away into almost nothing we think that the universe is expanding and it's going to keep expanding forever we don't know that for sure remember predicting the future is hard but that's the best current theory we have so the very far future of the universe is very high entropy but once again very very simple we for what it's worth live in the exciting part of the history of the universe and we are part of that excitement and maybe this is a story you're willing to believe on general grounds complexity comes and goes etc but the leap from there to the specific complexity of a living organism might be a little bit hard to swallow right you in fact you're taught sometimes i wish you weren't but it's true you're taught that living organisms are somehow fighting against entropy they're resisting entropy right i think the opposite of that is true my friend michael russell who's one of the world's leading researchers in the beginning of life the origin of life here on earth once told me the purpose of life is to hydrogenate carbon dioxide now that might not be the purpose of your life but what he meant was why did life start in the first place in his theory and no one agrees okay there's very large number of different theories that are competing right now but he has one of the interesting ones russell says there was an overabundance of carbon dioxide in the early earth and that was a low entropy way for those carbon atoms to be it would be much higher entropy for those carbon atoms to get rid of their oxygen and take on hydrogen convert into methane okay hydrogenate the carbon dioxide the problem was there's no simple way to get from there to here from carbon dioxide to methane no simple reaction makes that happen what does make it happen is a complex network of reactions under exactly the right circumstances so russell used this idea to make a prediction that underneath the oceans there should be warm hydrothermal vents of a very very specific chemistry and after he made that prediction it was shown to be true we found things like the lost city formation at the bottom of the atlantic ocean a warm hydrothermal vent which have exactly this complex chemical structure and russell's idea is that this network of chemical reactions can get caught up in a vessel a cell membrane break away and form a precursor to life here on earth now again we don't know it's one theory but the point i'm trying to make is the existence of life is not something that is a struggle against the growth of entropy it is something that is made possible by the growth of entropy and once you have that once you have these little complex networks that are using up the entropy around them they can last for a long time i want to emphasize this because this goes into not just the origin of life but what you are doing right now the nature of us as bodies and beings here in the world roughly speaking you can think about things that persist okay configurations of matter that last for a long time there's two ways to be a persistent object here in the world one way is to be made of individual pieces individual atoms and molecules that are themselves more or less stationary we might call this microstasis the chair underneath you right now is made of a whole series of molecules that are held together in a rigid pattern they're not doing anything they're not going anywhere there's a solidity to it which maintains over time but there's another paradigm which we might call homeostasis where you have patterns that persist over time even that the individual pieces that make up those patterns are moving that sounds a lot more like life to me right it's not just life think about the great red spot on jupiter this is a storm okay this is a storm of swirling gases in the atmosphere of jupiter that has persisted for hundreds of years it's not that there's one lump there that is absolutely stationary the pattern maintains over time that's what you are that's what i am we are patterns that get maintained over time because there's a lot of activity beneath the surface you can try your best to be completely stationary right you can still your breathing lower your heart rate enter a meditative state but whether you notice it or not at the cellular level there's a lot of action going on a lot of atp molecules are being created and used atps are little batteries that fuel your system you can stay as still as you want but if you plop your body out there in the desert away from fuel away from water and food and so forth you will not last for very long what we really do is we maintain the persistence of the patterns we call our body by taking in fuel from the outside and using that for maintenance for homeostasis why is that possible because entropy is increasing all of those processes that keep us maintained require an increase in entropy of the universe here on earth mostly that comes from the sun right we think to ourselves what does the sun do for us as in terms of life here on earth the sun gives us energy you might be tempted to say and that's true but it's not the whole story the earth radiates into space just as much energy as it gets from the sun the total amount of energy here on earth is not really changing that much the difference is for every one photon of light we get from the sun the earth radiates 20 photons into outer space we get visible light from the sun we radiate infrared light out into space so 20 photons of infrared with 1 20th of the energy each is what we do to the energy we get from the sun so in other words the sun gives us energy in a low entropy form one photon we increase its entropy we do photosynthesis we eat our food we think we listen to talks we make great speeches then we radiate out into the universe the same energy we got but with a much higher entropy that's what enables all of the marvelous activity that brings us to life here on earth and that process is to come back to where we started what explains the idea that we feel that time is passing around us that imbalance between past present and future caused by the fact that entropy is increasing so if you think about it you think about yourself right now at the present moment you are in some configuration right your body is doing some particular thing and a day ago or a second ago it was also in some particular configuration a second from now or a day from now it will be in some particular configuration but you don't know right you don't have exact knowledge you have vague knowledge of what your body's doing right now you don't even have perfect knowledge of that you kind of have a memory of where it was and you can predict where it will be and let's think about that process not the scale of days or years but of seconds or milliseconds okay the point is we are constantly carrying with us in our minds an image of our body and not just an image of our body right now we have a pretty good idea of what our bodies are doing right now even though it's imperfect right we don't have perfect knowledge of anything we also carry around where our body was we carry the memory of what we were doing just a tiny fraction of a second ago and we project into the future our our brains are constantly doing this they're making predictions about where we will be where our bodily configuration will be a fraction of the second in the future and they're constantly comparing its prediction as we made it to what we're perceiving as where we are right now and to what we think we were doing in the past it is that constant process of comparison past present and future on millisecond time scales that's what gives us the feeling that time is passing the feeling that we are moving through time because there's an imbalance we know a little bit more about the past we make predictions about the future we constantly take in more data more information and we update and every single update gives us the impression that we are moving through time that's not just us of course that's not just human beings every animal species does something like this but we human beings do a little bit more than the average animal species we think about the future in different ways okay every animal species needs to react to its present circumstance but you know that human beings can react to a lot more than a fraction of a second right we can think about the future we can contemplate it we can plan we can imagine different possible futures that is a very precious gift that took a long time for evolution to give us malcolm mciver who is a neuroscientist at northwestern university has a theory about this his theory is that it comes back down to the first fish that climbed onto land okay you may have heard of the fossil tiktaalik which is probably one of the first fish that climbed onto land and mciver's idea is that when you're a fish you can't see that far in front of you even if you're in very clear water the attenuation length of light in water is only a few meters you see things in front of you but not very far away so all of the evolutionary pressure when you live in the water is to see something and react to it right away is it friend is it faux is it food whereas when you climb onto land now you can see forever roughly speaking right on a clear day there's no obstacle to how far you can see you can see things so far away it will take you time to interact with them in some way and therefore a new possibility opens up for evolution to choose to develop in your brain which is the possibility to see something and rather than just instantly react to it think about the possible things you could do hypothesize different scenarios imagine them this idea is the birth of imagination and in fact you can see how evolution did this evolution uses things over and over again and neuroscientists have shown that the part of your brain that lights up when you're imagining a scene in the future is the same part of your brain that lights up when you're remembering a scene from the past it's this ability to imagine different future hypothetical scenarios that really has reached its apotheosis in human beings we are better than this at this than any other species of animals and this is what really makes us particularly human you could talk about this in many different ways here's an example la sagrada familia this is a cathedral in barcelona and it was started constructed back in 1883 under the guidance of antoni gaudi he took over from other people who started it but gowdy designed what he thought the cathedral should look like it's an absolute masterwork very very complicated a million different moving parts and different kinds of styles and so forth and gowdy passed away in 1926 and the cathedral was nowhere near finished when he passed away and he knew perfectly well that it would not be finished when he passed away but he still wanted to do it even though he knew it wouldn't be finished when he died what is going on what's going on is gowdy could use his brain to imagine what it would be like in that future time when the cathedral was done and it made him happy then the point is not that gowdy thought that he would be a ghostly persistence over time that would be looking down on the cathedral and admiring it he gained pleasure right at that moment from the prospect of the future and that's something that we humans have the ability to do the conditions of ourselves right now depend out on our visions of the past and the future as well as our conditions here in the present so it all goes back to the cream mixing into the coffee okay entropy increasing over time that's the story of the passage of time in the universe but it's that middle phase where the cream is mixing into the coffee where those little swirls develop those swirls are us that's what you and i are we are temporary little bits of complex structure in the universe that are part of the overall increase of entropy over time that means we are ephemeral we're not going to last forever that's the bad news okay we're not going to last for 10 to the 10 to the 10 years we have a lifespan we have an expiration date but it also means we are interesting we are the interesting part of the universe part of this complexity is our ability to think about and model ourselves and the rest of the universe to do what psychologists call mental time travel to imagine ourselves not just in different places but at different times it's that ability that imagination that flow through time that makes us what we are as human beings [Music]

Channel: Long Now Foundation

Views: 70,795

Keywords: Evolution, Science, Futures, Past, Time, Physics, Universe, Astronomy, Cosmology, Big Bang, Black Holes, Entropy, Asymmetry, Complexity, Thermodynamics, Life, Cathedral, Galileo, Newton, Einstein, Hawking

Id: EntaPoSEXog

Length: 33min 46sec (2026 seconds)

Published: Mon May 17 2021

Reddit Comments

[u/kalakau]

Really love the content Sean's been putting out. It feels genuine and like a labor of passion while still being informative and interesting. Saving this one for later

[u/Nukidin]

Sean Carroll has taught me a lot and his way of thinking about things is super nice to listen to. Great content

[u/eejolley]

Outstanding. Thank you Sean.

[u/Hodentrommler]

Sean Caroll is a beautiful human, geniunely trying to teach, educate and capture people in the universe of science. Takes criticism, talks about various topics, pours his hear into his work. I can only refommend his "mind scape" podcast

[u/PhilosophicalPorygon]

I love listening to Dr. Carroll. Such a great communicator!