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

[Music] foreign [Music] 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 at long 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 that 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 it 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 is 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 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 and 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 if 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 one twentieth 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 that 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 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 mackyver'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 optical 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 gowdy 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 uh 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 thank you so much sean that was fantastic i i especially love it when we have somebody speaking that makes the ten thousand year reference of long now seem like a blink of an eye so i'd love to welcome you for our q a hi xander thanks very much for having me here thank you for joining us from la i'm here at the interval in san francisco um soon to be open finally to uh to people walking and i'm looking forward to it uh later this month um but i think what i'd love to kind of talk about a little bit is this um is this notion of you know you mentioned the universe as we know it is um is the you know the milk as it starts to swirl in the coffee that we're we're at this interesting part of the universe and i'm just wondering where a where do we think we are in that process did the milk just hit the coffee um is are we almost to a static uh you know homogeneic uh universe um and and our is there some notion that we kind of try and make ourselves feel better about being in the most interesting time in the world yeah you know i think we shouldn't get too greedy here um there's a lot we don't understand about this whole process and what's really really important is that there are things going on on very different time scales right the evolution of the universe as a whole is measured in billions or tens of billions of years human beings consider ourselves lucky if we live for 100 years so we don't know whether or not complexity is on the upswing in the universe as a whole or on the downswing there's certain things that we can quantify you know mo like 95 of the stars that will ever exist in the universe already have been formed the universe is more or less done making stars but it takes time after you make a star and make some planets to make a biosphere and things like that you know life came onto the surface of the earth relatively quickly once the earth was formed but it took a while for that life to become eukaryotic and multicellular and so forth so i honestly don't know i don't even know if that's a very answerable question but i think we should enjoy the complexity we have try to increase it if we can but not fret too much about whether or not we're at the absolute maximum nice and i think you started you started to answer this question but i uh maybe if we put it back in terms of the egg it will make more sense to me at least which is that so that there's an egg that gets cracked and goes into higher entropy state but there's also the part of the act that we didn't talk about which is the formation of the egg and the chicken and i think you talked about this as you know as humans are formed into life there's and is that a is that a process that is going towards higher entropy or lower entropy when the creation of something like the egg before it gets cracked you know if you include a large enough system the only processes that we have are ones going to higher entropy there's no processes that go to lower entropy you can have an entropy decrease like i said in the talk you can uh clean your room that's okay that's the localized champagne yeah you can put it in the refrigerator it will cool off its entropy will go down that's all allowed but it's only because it's increasing the entropy somewhere else the reason why you can't cool your living room by putting a refrigerator in there and opening the front door is because there's more heat coming out of the back than there is cool going out the front so overall if you include enough stuff entropy always goes up and that's why i don't like to think about life or other complicated things as fighting against the increase of entropy because that's a losing battle we're not going to win that one what we can do is take advantage of the evolution that is caused by entropy increasing and make that into something interesting and important and meaningful well that that example the refrigerator is really helpful it's like why you can't put a fan on a sailboat and get anywhere that's right right so that that that actually makes a lot of sense so uh and that you can do things in a localized system but you're you're basically robbing that from somewhere else in a larger system um yeah when we started uh building this ten thousand year clock project one of the first uh material scientists that i spoke to um gave me the best picture in my head of how we should think about uh materials that are gonna last over ten thousand years he said everything is burning just at different rates and that fundamentally like everything is just oxidizing um and you can choose kind of which which materials that you want to use and how fast they you want them to oxidize but it was it was kind of ringing to me as you as as you spoke is uh that's our that's our entropic process that we get to fight when building something like a cloud well and they weren't probably even worrying about the fact that eventually we'll fall into a black hole and be radiated away into intergalactic space but that's a better ten billion yeah i mean everything is burning everything is running down the most solid thing in the world is not 100 static that's uh that's physics for you and the the ultimate state we think of the universe is going to be nothing but empty space if you're not empty space you're not done evolving yet and so i mean i think you started a little bit with this but what do you what do you find is generally the kind of the largest misconception that people have about time i mean you talked about this arrow of time versus this um this notion of entropy and non-entropy is that is that really what where people seem to kind of have a misconception a common sense of time well that thing that i talked about where once you start studying time from the physics point of view a new mystery arises why is there a direction why is the past any different from the future why is there an arrow of time many many people including a lot of professionals who should know better really have a difficult time intuiting the fact that if you're trying to explain why that's true explain why the past is different from the future you cannot do it by using assumptions that are themselves asymmetric with respect to the past and the future that's just cheating that's explaining something in terms of just another assumption that is just as bad uh it's very very difficult to put ourselves in the mindset of imagining that there isn't an intrinsic arrow of time to the nature of stuff because everything around us has such a manifest unavoidable arrow of time it's hard for us to get around it so thinking in that sense is is a real trick when it comes to trying to think of how to explain the universe as a whole it's another effect of that localizing effect where on a local level time does feel like very much it's an arrow well as i said in the talk you know i referred to the fact that we can explain the arrow of time in our observable universe by assuming that the early universe near the big bang was very very low entropy we don't know why that's an open question that people argue about and there are plenty of papers in the physics literature that say oh i can explain this let me assume the following very simple initial condition for the universe near the big bang and it's simple therefore it's probably right but they don't assume the same condition for the final part of the universe for the end of the universe they put it on one side and not the other and then they pet themselves on the back for uh you know showing that there's something asymmetric about it it's just really difficult the flow of time is so embedded in who we are that it's it's hard to imagine not assuming it from the very start and how is it that we know um that the moment of the big bang was so much lower entropy than now what's what's our what's our evidence of this well it's sort of a two-part little dance that we have to go through that's a very interesting uh difficult question one is we can just look at it right we have data we have the images of the cosmic microwave background the leftover radiation from the big bang so we know what the conditions or we think we know what the conditions were like near the big bang okay and we can calculate their entropy we have formulas for doing this and the reason why well sorry that was a bit of an exaggeration we have formulas that work in certain circumstances we don't have the once and for all formula for the entropy of the universe as a whole but what we can do is to say what's the entropy of a black hole okay a single black hole you know our galaxy the milky way galaxy has a super massive black hole in the middle that is millions of times the mass of the sun you can calculate the entropy of that stephen hawking told us how to do that the entropy of that one black hole in the middle of our galaxy is bigger than the entropy of the whole universe near the big bang so we know that however you want to talk about it the entropy of the universe near the big bang was much lower than it was today it would have been higher if there had been black holes around but there weren't and that when you say a black hole has more entropy than it than another one are you saying that it's it's it's more the milk has swirled more into that coffee like you're seeing a more homo homeostatic kind of version of it you know i can't tell you the answer for that one that's above my one of right this is one of the reasons why this whole subject is so fascinating you know in the 1970s stephen hawking and some friends of his noticed that there was an analogy between the behavior of black holes and the laws of thermodynamics and they just said oh look that's a fun analogy and then jacob beckenstein who's a graduate student at the time said you know what i don't think it's an analogy i think it's actually an equivalence black holes really are thermodynamic systems with entropy and so forth and hawking got annoyed by this he said that's nonsense if black holes had an entropy they would have to radiate they would have a temperature they would give off radiation and everyone knows that black holes don't radiate so he sat down to prove it and he ended up proving they do radiate he invented hawking radiation which is one of the greatest discoveries of the second half of the 20th century in physics but we still don't know what that entropy represents in a very real sense so we have from hawking and beckenstein are formulas that we can plug in we can tell you very accurately how much entropy is in a black hole just as we can tell you the entropy of a cup of coffee but for the cup of coffee we can say oh because there's molecules there's cream molecules and there's coffee molecules and if you say well what is it that are the equivalents of the molecules in the case of the black hole we just don't know right now interesting and i wonder if you know part of the issue is that we don't have good visualization tools for you know that we keep showing time on a timeline um for instance but have you come up are there good visualization tools that would kind of break this paradigm better or show it better for people you know it's a good question uh i don't have any good uh answers as to what a good visualization tool would be what i find very interesting is that there is no universality in how people visualize time so if you become a professional physicist and you specialize in relativity or cosmology right space-time and the expansion of the universe all that thing and you want to draw a picture of the universe what we call a space-time diagram for you time runs upward from the bottom to the top in every space-time diagram ever written if you're a particle physicist and you have two particles bumping into each other and scattering off and you want to draw a picture of that a fineman diagram to you time runs from left to right if you're a computer scientist talking about how algorithms go to you time runs from top to bottom so there are different ways that we represent time in space and now analogically and uh there's a whole another you know hour-long talk to be given about how that gets represented in language different spatial metaphors different temporal metaphors and so whatever our metaphors are it's hard for us to break free of them you might be right maybe there's a better one that would help us think about this yeah when i first started on the project of the clock i realized some of the best representations i saw were actually spiral because you have this notion where you kind of it's harder to know things really far in the past and you know a lot more about your present actually one of the long now research fellows has done an interesting computer version of this that i'd love to show you and actually it probably could get posted in the chat and we're going to be moving to some questions from the chat uh we have uh kevin kelly who's logged in from his vacation uh to uh to sort questions um and one of his questions actually he's he has he has several of them here um but this one i really like which is are there any alternative theories about time that you don't believe but can't dismiss oh yeah i mean there's always plenty of alternative theories right like any good scientist knows that there's uh lots of alternative theories out there you know i made a lot of statements in the talk which i think are true and i would say that if i if i just said the statements flatly that not only do i think that they are true but most people think that they're true okay so for example the statement that the laws of physics at the fundamental level have no intrinsic time direction to them and like i said that's true for newton's equations for schrodinger's equation for einstein's equations for maxwell's equations what have you but there are people out there who have theories you know alternative theories where time intrinsically has a direction okay i kind of think that's a waste of time because they still need to understand why the early universe has low entropy and once you understand that time effectively has a direction in our macroscopic world so i i don't think it's a useful game to play to try to put time directedness into the fundamental equations of physics but i could be wrong about that so i can't dismiss it right and you mentioned how time and linguistics are are linked and i mean i assume that also you know one of the things that we do some language projects here at long now and it's it's often said that you know if you lose a language you lose a world view as well and have you found that there's other cultures either indigenous or modern that kind of inherently understand time the more the way physics physicists do ah so i don't know i what what i what i completely agree with is the idea that different cultures do understand time very differently um and some of them might become closer to physics than to sort of what we might call the modern physics attitude toward it but i'm just not educated enough on uh what those are you know and and if there are any you know i don't want to give either our culture or any other cultures too much credit because you know when it comes to things like the large scale history of the universe well there's two options the universe is eternal or it's finite in time and so some cultures had stories where it was finite it had a beginning other stories where it was infinite and one of them's going to be right but there's only two choices neither one of them were on the basis of data from telescopes right because even right now we do not know whether the universe truly had a beginning or whether the big bang was just a phase so i think that there's lots of different metaphors lots of different stories lots of different cultural ways of thinking about time but even modern physics doesn't have the right picture yet so i'm not going to give credit to anybody quite yet right yeah we had a speaker early on who uh dan everett is but studies the parraha in central or south america and they have a very very clearly a very different conception of time i'm not sure if it's more aligned with physics or not uh but much less about past and future um we have a great question here from uh raphael from the facebook feed says hi i'm a teenager it's very common for people my age to go through existential crisis because of how massively large the universe is and how little significance our individual lives have is feeling my newt the right approach or for visualizing ourselves or how do you deal with this conception of time in your own life yeah you know raphael you're not wrong to think that you're very very tiny compared to the universe on the other hand you're very very big compared to an electron uh and on the third hand who cares who cares how big you are who cares how long you're going to last uh i do think it's important to appreciate that we're finite we're ephemeral as i said in the talk we're not going to last forever and therefore whatever it is that you find meaningful and important and motivating in your life uh it can't be that you're gonna leave an impression on the universe that lasts literally forever because there are no such impressions and i actually find this liberating rather than leading to existential despair it takes the burden away from me to try to do something to the universe that will actually leave an imprint that lasts forever the imprints that we leave on the universe are finite but that doesn't make them any less important for that don't think about the universe think about the people who you can affect including yourself uh the impact that you can have here and in the near future that's hard enough and it also matters more than enough i would say nice i like that actually uh rafael also had raphael or rafael as a second question um as as does kevin kelly kevin kelly is a little bit shorter version of it but they're both about dark matter and dark energy and so kevin is asking if we know that there's dark matter and there's dark energy is there dark time no the equivalent no all right because let me amplify you know time is not a substance time is not something you can have a cup of here's a cup of time right dark matter is a substance it's a set of particles you can move it around you can say oh there's more dark matter over here than over there time is a label in modern physics right it's a it's a way that we think about where you are in the universe in space-time what exists is the universe time is a language that we use to talk about the universe so there's no sort of equivalent different version of time that would be darker or something like that now again having said that there will always be people who disagree you know that in the universe in which we live there's three dimensions of space there's one dimension of time so it's very very natural to say well why aren't there two dimensions of time or three or something like that and people have tried to do that none of these attempts has really led to increasing our understanding of the universe in fact they tend to make things much much worse so at least as far as we currently understand it the best way of thinking about the universe is for time to be unique for there to be one direction of time one dimension of space time that we call the temporal one again as a good scientist you should always be open to changing your mind in the future if good information comes along and that actually leads into a great question from uh steven from the facebook feed as well who's asking about some of the recent quantum experiments and how they can entangle the future in the past in some ways there's is that changing some of notions of causality and and entropy that maybe there's connections beyond between and around the present from the future impasse so here i will give you uh my personal slightly grumpy view about these things because now we've moved on from time which is easy to quantum mechanics which is hard and what i mean by hard is that the greatest theoretical physicists in the world don't agree on how quantum mechanics works what it says etc i have a point of view on this i just wrote a book called something deeply hidden where i explain to you what my point of view is but for the purposes of this particular question here is my point of view no there's no thing as entanglement at different moments of time not really you can choose to talk about the quantum evolution of the universe in such a way that you can kind of make sense of such a thing but you don't have to when you talk about different uh parts of the universe at one moment of time there's an objective fact of the matter are they entangled with each other or are they not and everyone agrees on that and you can talk about it and use it etc in the conventional way of talking about quantum mechanics that's all there is the state of the universe exists at each moment of time just like in newtonian physics right in newtonian physics as we talked about if you have the positions like the solar system positions and velocities of everything at one moment of time you can predict the past present and future same thing in quantum mechanics if you know the quantum state at one moment of time you can predict what's going to happen so i don't personally like to talk about entanglement between different moments of time you can but it seems to be just an attempt to make things seem even more confusing than they really are right well yeah and you're taking away so many good plot lines for science fiction movies when you see this uh yeah my favorite uh description of the quantum mechanics is from richard feynman who said no one understands quantum mechanics as one of the inventors of the field and uh kevin kelly is also asking uh how has your own life changed since since learning this is this aside from this is something that you teach but does this change the way you actually think about your the way your life works well um in a in a fairly direct way it has because you know i uh since i was 10 years old i wanted to be a theoretical physicist and i read about black holes and the big bang and particles and i i did it i eventually you know got to the point where i was doing that professionally um when i was in college as an undergraduate i also discovered philosophy and i fell in love with that also but it seemed to be like a very different kind of thing like the kind of philosophy that i studied was actually political and moral philosophy and there was some philosophy of science in there but that wasn't what my favorite kind of philosophy was it wasn't until you know i was a professor already and i thought a little bit about cosmology a lot about cosmology actually but i was thinking about the early universe and i got connected with philosophers who weren't doing philosophy of science in the sense they were studying how science works they were doing philosophy of science in the sense they were studying how the universe works but in a more philosophical way and what i realized was they were making more sense when it came to talking about the universe and time than my physicist friends were and so i became more philosophical myself and that has continued on and i started thinking about quantum mechanics and how that works how time works how space time itself works how probability works and these days what i do professionally in my research career is not even recognizable as cosmology anymore so in the down to earth sense that it has changed my intellectual direction uh very very much thinking thinking about time was my entry into thinking about the universe in a more philosophical way nice well and we have a question from uh toss rock from youtube it's actually asking about you know how you got involved in science and then going on to doing things like working in the movie industry and advising on science fiction movies as we were just discussing but i think maybe another way to to think about this is you know if if somebody was starting a science career now in in this space um what would you tell them what would you tell them to learn what's the interesting space for a young scientist to get into so that eventually they get to be as cool as you advising marvel look honestly there's the two secrets to becoming a science advisor for hollywood are number one live in los angeles that's where it's located uh number two have a at least medium-sized public profile be out there on social media or writing books or doing youtube etc and they will find you there's no systematic way you don't apply for a job uh it's not a job you don't get paid it's just kind of fun to talk to directors and screenwriters about their hollywood movies i got a sweatshirt once an ironman sweatshirt that was kind of fun um but the other version of that question is you know how do you get into doing interesting science forgetting about for the moment uh movie consulting or anything like that because there is a fundamental flaw in the system where if you're like me uh so i said i got interested in physics when i was 10 years old so that's the middle 1970s and what are you going to do you're going to be reading about what is the most interesting physics going on at that point in time so i was reading about quarks and black holes and the big bang and all that is great it's still great now but what you do for a living is not going to be what's interesting when you first stumble across the field because there's decades in between somehow right so it's very hard to sort of plan ahead for what will be the most interesting stuff when you become a grown-up scientist so my advice is you know to think broadly you know to be open to all the different interesting things going on um be ready to say well you know what i hadn't even thought of this whole field that seems really cool let me at least take it seriously you know learn everything you can and put off to the last minute choosing once and for all what has to be your specialty nice so we're going to be wrapping up here shortly but i think one of the things that you said that i thought was really interesting is that this idea that while we have a lot more information about the past we obviously have much more influence on the future and i think it's an interesting thing that in our time actually people feel as though they're having less and less influence on their future but i think it's a nice message this idea that that's actually the one thing we do have influence over um and so that physics backs that up i think is is really great um and you mentioned one of your recent books um what are you what are you working on now as slash next i know that this topic was kind of a kind of dipping into your past a bit and which i thank you for but um what what is your next current and next uh work well the reason why i thought i was very excited about doing this talk is because even though i wrote a book on the arrow of time over 10 years ago now called from eternity to here i'm actually again doing research on it so i'm doing research on this this question of why is it that you can have effects and causes but the effects always come after the causes in other words why is the arrow of causality aligned with the arrow of time you know i i talk about the arrow of time a lot of my friends in physics or philosophy will talk about multiple different arrows of time the psychological arrow the influence that we talked about the thermodynamic arrow so one ongoing project is relating them all to each other and i'm someone who believes that the increase of entropy is the reason why all these other arrows exist but it's easy to say that out loud it's hard to prove it with equations so i'm i'm working on that as a as a research project using ideas from causal networks and bayesian thinking as well as thermodynamics and entropy and philosophy of time cool well thank you so much for joining us this was so great to have you and to actually get a kind of a very fundamental sense of time and us in it so thank you so much my pleasure xander thanks very much for having me on thank you and i'd like to thank you our along now audience um we actually have several talks coming up in the future that cross over uh fundamentals in science such as color and weather and how trees communicate so please join us for the weeks to come you

Channel: Long Now Foundation

Views: 278,355

Rating: 4.8215008 out of 5

Keywords:

Id: 7BH6XCRZad8

Length: 62min 5sec (3725 seconds)

Published: Tue May 04 2021