What Is Time? | Professor Sean Carroll explains the theories of Presentism and Eternalism

As we ponder the mysteries of

time

, this is the only lecture where we will allow ourselves to get a little philosophical, and by this I don't mean that we sit back and reflect on how amazing everything is and reflect on the majesty of

time

and space. What I mean is that we are going to think about the problem from a philosophical perspective from the rigorous academic discipline of philosophy. Now, of course, there is a long-standing relationship between physics and philosophy or between science and philosophy in general. there is, shall we say, a generally friendly rivalry, the two disciplines are a little different, they have absolutely different goals, but their topics often overlap and the study of time is one in which the philosophical perspective is absolutely useful even for physicists like me ,

what

philosophers try to do. to do is to really understand the inner logical workings of something.

Physicists like me are often very happy to get a theory that works, a theory that makes sense, sometimes it is too much to ask for and you might think that this is not possible, but we have it all the time quantum mechanics is a great example of a theory that doesn't quite make sense to us, but works for a physicist, that's fine for a philosopher, they would like to do it a little better, the same goes for time. We have an understanding of how time works physically in certain well-defined circumstances, but there are still philosophical questions, so in this lecture we are going to lay out

what

some of those questions are and that will help us understand the scientific Aspect that we will discover later. as we move forward in the rest of the conferences.

What do we mean when we say time, especially as opposed to space? What do we mean when we say the entire universe? When we think about the universe we generally think. Of space we think not only of space as outer space like planets, stars and galaxies, but of the space around us, the location of things in the world, the world, the space that defines everything around us and the things in the world, that's what we consider the universe, but we think that universe happens over and over again, so automatically from the beginning we treat time and space differently, we treat space as somehow more important or relevant to what the universe is, while time is just a label that tells us what moment in the universe we are talking about already in lesson one we discussed this analogy of thinking of the universe as a reel of film or a strip of film, a series of frames, a series of moments and what I am trying to say here is that both each frame and the entire series of frames, the entire reel of film defines what we consider to be the universe, it is a four-dimensional thing. dimensions, both space and time, and I must warn that when we use that analogy, it is very tempting to wonder if it is like a film strip moments of time are discrete as far as we know nothing in physics tells us that time is discrete as far as we can tell time is perfectly fluid and continuous people sometimes wonder if a future reconciliation between quantum mechanics, the theory that happens on very small scales, and relativity, Einstein's theory of gravity, maybe one day that will tell us that time really is discrete, after all, quantum mechanics comes from the word quantum, which means it comes in discrete packets, so maybe when you quantize space and time themselves, they will become discrete packets. but as far as we know that's not true, your best understanding right now of time and space is that time is absolutely fluid and continuous, that doesn't mean we won't understand it better in the future, but right now I'm not trying. use the film strip analogy to say that time comes in discrete packets, simply that the universe is both each frame of the film and the series of all the frames together, so the way that film strip works of the universe involves a fairly subtle notion: continuity of the universe from moment to moment in time We said before that the universe does not reorganize itself unlike space, so let's interrogate that concept a little more closely in space, what happens in this point here seems more or less completely disconnected from what is happening at this point here now it is true that in the real world things do not change completely from one point to another in space here is the air in the room here is the air in the room too but I could put, you know, my hands very close to the floor and then you would have air here and the floor right next to it, there is nothing that prevents me, in principle, from having an absolutely empty space at this point and some incredibly object dense or incredibly hot or incredibly interesting right next to space.

The things around us and the things within space have no rules about what comes after everything else, on the other hand, time has rules about what comes moment after moment, that's how the laws of physics work. , that's how we think about laws. of physics in the modern world and this is by no means the only possible way the laws of physics could work. This is a really interesting deep feature of the laws of real-world physics, which is that the way they work is, tell me what the world is doing right now or at some other time, but You tell me everything that happens in the world at any given moment and the laws of physics will tell you what happens next and then from that next moment, the laws of physics will tell you what happens the next moment, so the laws of physics start from a moment, they start from a state of the universe at a certain moment in time and they tell you using the patterns using physics, the equations that are the laws of physics, what happens in every moment after. that's completely different than space, obviously in space the laws of physics can't tell you just because something is happening in this place, what will happen in the next place, even as far as time goes, it could have been different, remember that Aristotle discovered the laws. of physics that worked in terms of the future, things have goals and to understand what will happen next you need to contemplate what the future conditions might be.

We could contemplate different laws of physics that involve the past in some important way that knowing the universe at this time was It was not enough to know how it got there, but in fact, the laws of physics that we use, whether they be Galileo, Newton, Einstein, Schrödinger or the superstring theory that we talked about today, they all have this characteristic that what happens at a given moment is enough to predict what will happen at the next moment and after that you can continue predicting all the way into the future. For our brains to really understand this, think about the difference between time and space.

Later we will talk about relativity and how they relate to each other, but for now let's accept the ordinary notion that time and space are completely different. We move through space however we want, we can choose to go somewhere. another location in space we cannot choose to go to another location in time we inevitably move through time at the rate of one second per second time is relentless while space is up to us how to move in it that gives us a certain perspective about what the world is we think of reality as a moment in time we think that a distant place 20 feet away or 20 miles away or 20 million miles away might be inaccessible to us right now, but that's not It means that whether it is not real we think of different places in space as absolutely real, whether we are there or not, but now think about time, think about the past and the future, are they real?

And this is a question that as soon as you start thinking about it, you start to think about it. I suspect the answer is simply that this is not a good question, however, it is important to understand why we think at different times differently than we do at different times. different spaces. We think the universe now exists, but we think the past is over. We don't think of the past as real in the same way that the present is and we certainly don't think of the future as real, so the important thing is not to discover what is real and what is not real, the important thing is to ask why we treat the past and future so differently, why do we think about them so differently when we think about different parts of space, so to answer that question we should think about how we use these concepts, how we actually go.

About describing space and time Let's imagine that you want to meet someone for coffee and you say let's be at a certain café at 6 p.m. m. What you are really doing is giving them coordinates in the universe - what a physicist would call an event - you need to specify the space where they will meet and also the time at which you will meet them. Fortunately, we live in a world where we are basically glued to the surface of the earth, so giving someone a coordinate in space is giving them a direction. or two numbers, let's say what street you are on and what number you are on that street, but if you found them anywhere in space, you must give three numbers.

Space is three-dimensional. You have length, width and height. Time is another dimension. In the universe we can join the three dimensions of space with one dimension of time to make something called four-dimensional spacetime. That space-time is the collection of all the different elements of the universe. Now you might think that the notion of space-time is something that Einstein gave us, but it is equally important to think that Newton, who preceded Einstein, could have talked about space-time. He had space. He had time. Newton knew this as well as Einstein. To locate someone and meet them somewhere. the universe you needed to give four numbers three locations in space and a number to know what time you would meet them, the difference is that for Newton space and time were so different that there was no temptation to put them together, you could have done so.

It wouldn't have been against the rules, but there was no benefit to doing it either. Einstein, as we will see, showed that what counts as space versus what counts as time will be different for different people, so you have to think of them as space-time once. you start to think about the universe that way, you find a possible different way of thinking about the world our everyday way of thinking about the world philosophers would call

presentism

presentism

is the idea that what exists, what is real, is the three dimensional universe at some point in time and everything in that universe the past and the future are not real the present moment is real the past is memory the future is prediction but physics suggests a different point of view physics says that if you knew the universe exactly correctly could now predict what the future would be and could also reconstruct what the past was.

The laws of physics connect the present moment with future moments and with past moments so from that perspective we started to think that the The past is the present and the future may all be equally real. This is a point of view called

eternalism

as opposed to presentism which says that the present is real and the past is a memory of the future. It is a prediction. Eternalism says that all Moments in the history of the universe are equally real. It is true that there is nothing special about the present moment except that you are experiencing it right now.

You might think there's a hard way to think about the world, and it's hard because it's not our usual way of living life. Sometimes the point of view of

eternalism

. It's called the block universe perspective because you try to imagine stepping out of the universe and seeing the entire set of four dimensions as a block of space and time. This is sometimes called the view from nowhere, when the view is not from any moment in time, but outside of the whole thing, kurt vonnegut once wrote a novel called the sirens of titan where he had alien characters called truth Midorians and the Trophy Midorians had a special characteristic that they lived outside of time, they experienced the block universe just as you and I would experience three-dimensional space, they could choose, you know, go to the year 1600 or go to the year 2500 after the breakfast that day, now that you think about it carefully, which I know is not the work of a satirical science fiction novel, but if you thought about this very carefully you would realize that the Tralfamadorians don't make much sense, we can talk about the universe four-dimensional, but if you imagine an alien species that might decide to go visit the year 1600, then they clearly have a personal notion of time.

They didn't visit the year 1600 yesterday, they will visit it tomorrow, so you really are adding a whole new time dimension to the universe. It's something you can imagine doing, but it's not the universe we live in for us. The task is to step out of our everyday experience and imagine looking at thisfour-dimensional block universe to treat the past, present and future on equal footing. This seems to be the correct way to look at things as suggested by the current laws of physics. I must be honest in saying that it is a controversial point of view. Many philosophical positions are controversial.

There are certainly philosophers who do not believe in eternalism. To me, it seems to be the logical consequence of the way physicists see the universe, but we have to keep in mind that there are other ways of doing it. What does it mean if it is true that the past, present and future are equally real, we certainly think about them. very differently we treat the future very differently than the past another way of thinking about our kind of traditional, everyday way of thinking about the universe is the growing block universe model, so I said presentism deals with the moment present as real, the past and the future are not real, so a slight twist on this idea is to treat the present and The past is real but the future does not yet fit very well with our notion that we can make decisions about the future, we cannot choose what to have done yesterday, but we can choose what to do tomorrow so that the past is fixed, it is in the books.

It's real, we may not be living there, but it happened, it's decided, while the future is at stake, so maybe we should treat them very differently, this seems natural to us as human beings, but it's not reflected in the laws fundamentals of physics. it is not something we can gain from our better understanding of the universe. I think the best way to say it is to understand that the reason we treat the past and the future so differently is because of the arrow of time, in other words, it is not time itself that treats the past, the present and the future differently, it is the arrow of time that, ultimately, as we will see, depends on the things of the universe, on our macroscopic matter in the configurations in which it is found in the entropy of the things of the universe. that started low and is growing and will continue to grow towards the future is the arrow of time that gives us the impression that time passes, time flows, that we advance through different moments so from that perspective we understand that it is not that the past is more real than the future is that we know more about the past, our memories access moments from the past and when I say memories I don't just mean the actual memories in our brains, I mean any record, any fossil, anything we can look at. we believe it gives us reliable knowledge about the past we live in a world where memories exist where there are history books there are no history books written about the future that is the difference between them that can ultimately be traced back to the arrow of time so It's not that The past and the future don't exist, it's that we have different access to them than we had in the past and the memories of that past are still reflected in the present, but that is ultimately due to the growth of entropy, which We will discuss in great detail. but it has to do with this question of what is real and what is not real, the answer is that all moments of time seem to be real, but some of us understand them better than others, that is something that we are going to develop when we come to reflection about the arrow of time and how we think and how our brain processes information, this is all a bit philosophical as promised, so let's be more concrete about how it plays out in the everyday world.

We mentioned that one way to think about time is that time is what clocks measure. We live in a world full of clocks and one important thing about clocks is that time doesn't just pass. There's just a before and a. Now and then, but we can measure it, we can say that not only has time passed, but a certain amount of time has passed. A second is less than a minute, which is less than a day. There is an amount of time that we can quantify using clocks, so we can measure it. What a clock is is something that does the same thing over and over again in a repeatable way and then you might ask, isn't this all circular?

Aren't we defining time as what clocks measure and defining clocks as things that do the same thing over and over again as time passes? so I want to take the opportunity to say that it is not a circular definition, there really is some substance in the statement that time is what clocks measure, a clock is something that does the same thing over and over again, but the existence of things that do the same thing over and over again, the same thing over and over again in a predictable way is not taken for granted we could have lived in a universe where everything that was repeated did so in an unpredictable way the important characteristic of clocks is that there is more than one clock in our universe we are lucky enough to live in a universe full of clocks I mean of course that by clocks something with which we can measure the passage of time the first clocks were not on our wrists or on our computers they were in the sky the earth rotates around its axis it also rotates around the sun these are two different things that the earth does and they do it predictably they are comparable to each other in general terms the earth rotates 365 and a quarter times each time it rotates around the sun and the beautiful The important thing about this is that it is predictable, it is not a different number of days each year, it is the same number year after year, that is what makes the movement of the Earth give us reliable clocks and it is not the unique, of course, that we have. the moon we have the other planets we have the stars in the sky the sky gives us repetitive phenomena that we can use to measure the passage of time so you might ask what are the other good clocks in our universe besides the ones we have in the sky certainly not they are our memories you know that we experience the passage of time but we all know that that experience is not very reliable sometimes we think that a lot of time has passed other times we are not so sure we think that very little time has passed What we are looking for in a good watch is something that can be trusted and that will do the same thing over and over again.

Obviously, the rotation of the Earth's revolution is an obvious choice. Another is the swinging of a pendulum back and forth. That is why today we put pendulums in our grandfather clocks, in our wrist watches. The best watches will be in the form of quartz crystals. In fact, you can make a given quartz crystal in the shape of a tuning fork that ensures it vibrates back and forth exactly. 32,768 times per second the reason that number is chosen is because it is 2 to the power of 15. So you can take 32,768 and divide it by 2 15 times, you can divide the time periods as finely as you want.

It is reliable and predictable no matter what the humidity is what the temperature that quartz crystal will tell you the time with very high precision this search for reliable clocks is not always easy let's go back to the idea of ​​the pendulum swinging back and forth turns out to be It's true that the swing of a pendulum makes a pretty good clock, but it's certainly not obvious; you'd think that if the pendulum was moving just a little bit it might be faster than if it were moving a lot. This was actually proven not to be the case in 1583, by none other than Galileo.

He was a boy. He went to church like all the young boys did in Pisa in Italy in that time period, but occasionally he got bored. sitting in the church and since he was a Galileo he did little mental science experiments so Galileo noticed that there was a chandelier hanging from the ceiling of the cathedral of Pisa which is still there you can visit it today and the chandelier was swinging back and forth and Sometimes it only swayed a little, sometimes a little bit of air had bothered it and it swayed a little more and Galileo got the idea in his head that it seemed that the time it took for the chandelier to come and go was the same regardless. what was the amplitude of the swing the time was approximately the same the question is how would he verify this idea he was an empiricist he wants to do an experiment to determine if this idea is on the right track and again because he is a Galilean he realized he was able to compare the swing of the chandelier with the beats of his heart Galilean calculated that he could count his pulse while watching the chandelier come and go and what he realized was that his guess was correct that it didn't matter the amount of swinging it was the frequency the time it took for the pendulum coming and going the chandelier that hung from the ceiling was going to be the same this is the basis we have for pendulum clocks even today it is not the most reliable way of telling time we have but by the 16th century it was a very, very good step forward, we still use that way of measuring time today, you can imagine, as we said, a different world, a world in which there were no regularities, that there were no such good clocks anywhere in the universe .

They were things that happened over and over again but they happened at unpredictable rates compared to each other. This would be a very strange world to live in. It would be completely crazy from our point of view. It is a world in which the passage of time could not be measured. There would be no clocks. reliable there would still be time there could still be the idea that there was a moment before another moment but you could never tell how much time had passed from one moment to the next so in a way we are very lucky at least scientists are very lucky that we live In a world where time can be measured, a world where clocks exist and we can build them and they will tell us how much time has passed, the reason clocks are so important when thinking about time is because they give us an operational way When thinking about time, you can easily get confused when thinking about the fundamental nature of time and clocks help us because they refer to things that really happen, not just abstract concepts, so you ask how are the past and the future equally real?

They may or may not be interesting questions, but you can ask what a clock would do. That's an interesting question that gives you an answer one way or another, so consider, for example, your favorite thought experiment: What if time just existed? stop, what would happen if time stopped passing everywhere in the universe? or alternatively, but what would happen if time slowed down everywhere in the universe? What would that mean? I'll give you time to think about it. The answer is: it would mean absolutely nothing if time stopped. everywhere for everything in the universe there would be no way to know, you would stop the processes in your brain, you would stop your pulse, you would stop, your breathing would stop, so you wouldn't experience any time at all because time has stopped, but at the same time At the same time all the clocks would stop the earth would stop spinning the pendulum would stop swinging back and forth so when you compare the amount of time experienced by any object in the universe with the amount of time experienced by any other object in the universe universe time stops everywhere It leaves no trace while I am lecturing you right now.

You could imagine that time has stopped and you would never know it, but then you realize that if time has stopped you can't even say, "Well, time has stopped for a century." It doesn't mean anything because time has stopped, so when you get lost in these questions about the meaning of time, what's real and what's not real, it's always helpful to go back to what clocks do. There is a story that illustrates this very well. novel written by nicholson baker called the fermata is about a man who can stop time he only has a magical ability it is not explained it is not difficult science fiction or anything like that here there is only one person who can decide to stop time whenever he wants but obviously, he doesn't stop time everywhere in the universe, he himself keeps time going if you read the novel, which is a good read, it's interesting and thought-provoking, the man himself is not an admirable character, he mainly uses his ability to make people look stupid. in different ways he is not saving the world, but we as scientists can ask ourselves, we are philosophers, what it would mean for someone to be able to stop time.

Well, how would that actually work? Forgetting if it could be done Forgetting the technological problem of how you would build a time-stopping machine, what would really happen if you could stop time for the rest of the universe while still functioning just fine yourself? You would quickly realize that even if you grant the possibility of such a thing, it doesn't actually work like that, it says that it would work in the novel on the one hand if you stop time for the rest of the world let's say everything that is more than a meter away from you.

You time stops suddenly you can't see anything more than a meter away from you no light is coming to you from any of those things that are outside your time stop zone because time has stopped, you can't breathe anymore If the air in the room has stopped moving, if you started moving through the air and these other air molecules are absolutely stationary, they would be a brick wall for you. You couldn't even walk in a room like that, so when you actually You sit down and think about what it means to stop time or evenTime moves at different rates for different people, it's actually a very, very slippery notion.

We will see later, when we talk about relativity, that there is a well-defined scientific sense in which different people can measure time by moving at different speeds, but the only way they can do so is by being in different places in the universe or moving through the universe. universe at different speeds. different old-fashioned speed rates, if you're trapped near a black hole or if you're in a spaceship that's moving very, very close to the speed of light, then the amount of time that passes on your wristwatch It will be different from the amount of time experienced by someone who stayed at home, but if you are looking at your wristwatch, it will appear to move at the absolutely conventional pace because you and your watch, whatever watch you bring with you, all travel together, all They experience the same. passage of time I think that somehow in the modern world time has a bad reputation we feel like slaves to the passage of time there is a clock there is a calendar we have to do things there are deadlines and they are met in our own way, but if you think about it , all of these aspects of time are social, the reason we are pressed for time is because we are trying to coordinate with the outside world, we have a watch, someone else has a watch, we agree to meet for coffee in the same place at the same time.

At the same time, time only puts pressure on us when we want to match our actions in the world with the actions of another person. I would say it's not fundamentally a bad thing. We shouldn't blame the weather for this. We must be grateful. lives in a world that allows us to coordinate and be social creatures