Our concept of time is false – theoretical physicist
Aristotle said that time is the most unknown of all unknown things. We talk about its mystery with Professor Carlo Rovelli, theoretical physicist and best-selling author.
Sophie Shevardnadze: Professor Carlo Rovelli, theoretical physicist and bestselling author, it's great to have you with us today. Welcome to our show.
Carlo Rovelli: Thank you very much for having me.
SS: All right, so let's start with something blunt. You say there is no time in an objective sense of the term (correct me if I'm wrong) and that time exists only in the eye of the beholder and depends on what that eye is, where the beholder is and what they're looking at. But we do feel and experience the passing of time or the sense of something being after something. And here I am, like, I don't know, grabbing a phone, for instance, and putting it down. It was in my hand. Now it isn't. Isn't that time?
CR: Yes, that is time. And, of course, there is time for us, we wake up in the morning, we go to sleep, time passes, we get older, that's real. When I say there’s no time, what I mean is that there’s no time in nature by itself on the fundamental level. Let me make an example. If I say the sun does not move, it’s still, does not move, it does not mean that there is no sunset anymore, we still see the sun going up and going down and going up and going down. But we understand that it's not really the sun, which is moving. It's a complicated story between us and the sun, we are sitting on a big rock which is spinning. So from our perspective, we see the sun moving. So the movement of the sun that we see is real, but it's not part of just the sun alone. And the same is about time. Time is a more complicated story than what we usually think. That's what physics has shown that time is different than what we usually think. So the time of our everyday life is not really nature. The kind of nature is more complicated. I'm going to give you an example. Suppose you have a clock, or a watch, it gives time, and we all know that the property of clocks is that they measure time so there's the same time for all of them. If I look at them, and they have the same time and I look again, they're the same in time. Well, actually, if we measure precisely, that's not true. If we take two good clocks, not this one, this is my grandfather’s watch, it’s not particularly good. If you take a good one, we make one a little bit higher and wait a little bit, and then come back. And we look at them, they don't indicate the same time because there's more time up here than down there. And this can be measured today in a laboratory. So time is different than what we think. The time of our usual imagination is false, is an approximation. It's like the setting of the sun.
SS: Right, we're actually going to come back to this experiment a bit later. But before we get deep into the more scientific aspect of what your notion of time is, I just want to, you know, get it right for us, plain humans, because for me, the biggest proof that time exists, well, at least the way we feel it, is us and our bodies ageing and us eventually dying. I mean, we're part of nature, right? We're part of the bigger picture as well, I believe that like I'm part of something huge, and that huge is part of something that is me. So would you deny that time in that sense exists also for me, even though I'm part of that huge nature thing that you were talking about? Time is very precise for me because –
CR: No, I don’t deny at all. I think you said it very correctly. Time exists for us. We age. It's more than that, we remember the past and we think about the future and we know we're mortal so we have all this story in our head about our life, the past, the present, the future. This is all real but regards us. That's not the way a star lives. A star does not remember the past, does not expect the future. So we are part of nature and we experience things which depend on our own natural properties and not universal. We make another example: that's the up direction and that's the down direction. Is that false? No, of course, it's true. I mean, that's up and that's down and things fall down. But I'm in Canada, and you're in Georgia, when I point up and if you point up, you're pointing in a different direction. So which one is the true ‘up’? You know, it looks the same. But in reality, we are pointing in different directions, because the Earth is round, of course, so we point in different directions. So which one is the true ‘up’? Well, there’s no true ‘up’. It’s ‘up’ for me and ‘up’ for you. And if we, if your brother were floating as an astronaut outside the Earth, ‘up’ for him doesn't mean anything because for him all directions are the same, there is no ‘up’. So there’s no ‘up’ in the universe, the universe doesn't have an up direction and a down direction, but there’s an ‘up’ for me, which is one direction, and there’s an ‘up’ for you, which is another direction. And the same is time. There's no global time in the universe, the universe doesn't have a time, but we, creatures that are in an environment and there is heat, and there is a long story around us, so we experience a local thing, which is our own time. And if you look very, very carefully, we see the differences even from a time at a different altitude.
SS: Can ask you something? So why does time if it doesn't exist at the microscopic state of things, have a direction? I mean, why is it never pointed backwards?
CR: This is a great question. It came as a surprise in the development of physics that time is different than our usual way of thinking. So this thing I said that time goes at a different speed - it was understood about 100 years ago, in fact, Einstein is the first one who understood that and he first got to this idea theoretically, sort of try to match various things we know about nature. But now we measure in the laboratory so it's not an idea by Einstein. It's something true that we can measure. But even before that, a great surprise was that the laws of physics do not distinguish the past from the future. And that's so strange, because for us, the past is completely different from the future, right? Past is fixed, we cannot change it. I was born on some day, and I cannot change it. There was the Russian Revolution at the beginning of the last century, and that's it. The future seems open. So they seem completely different stories. But physics does not distinguish the past from the future. So there was a lot of discussion of what this means. And it turned out that the difference between the past and the future only comes about when we don't look at all the details. That’s what we do all the time, because we don't see the individual atom moving. Then the big story, all things moving in one direction, and this is what the physicists call ‘the growth of entropy’. This is a name for this difference between the past and the future that we use in physics. It’s hard to conceive because we think in terms of past and future, but once again, it's because we are very big things compared to the atoms and the molecules.
SS: Did you see the new Nolan film? I mean, is it possible theoretically, whatever is going on there?
CR: Nolan took this idea –
SS: Of entropy?
CR: Yes, and he took the idea which is at the basis correct, which, in principle, it could be possible to move towards the past or towards the future by playing with entropy, by changing the arrangement of the molecules. In principle, it’s true. But then, of course, he made a movie, which is absurd because we cannot arrange the molecules in the way... So it's sort of Hollywood stuff, which is that a lot of fantasy in the movie, and I found it a little bit confusing and complicated. The beauty of the problem of time, that’s what fascinates also all of us about time, is that there's all this complexity, there is some aspect of time which is in the mechanics of things, in the thermodynamics of things, you have many, many atoms moving in our brain. And even in our psychology, I would say, because you started and I think that was very good, you said, time for us is our growing old. And you know, time for us is also what takes away everything we have, right? We know that we are going to lose everything we really are today, we're going to die, all the people we love are not going to be there and we are not going to be there. So there is a sort of anxiety. And at the same time, time is also what brings us things. So we have a very emotional connection with time, the way we are wired in our brain is that we are attached to go grasp something in the future and the fear of losing something in the past. And I think when we say ‘time’ that's what we mean to a large extent. So I’m a physicist, I do science, but the physicist makes a mistake if he thinks, “Okay, I don't want to think about psychology. I just look at the motion of things.” And then you don't see any more what time is for us. Time for us is a lot this emotional aspect of time.
SS: Professor, if time also is mostly a human perspective, because emotion is a very human thing, or even a delusion in some sense, how come physicists like yourself use it to find out what exactly happened during the Big Bang? I don't know, it always fascinated me when I read about how this or that appeared on a second or third nanosecond after the Big Bang. Is all that also a delusion?
CR: Well, it also fascinates me. It's a big part of my job to try to figure out what happened near the Big Bang. And that's part of my daily work to contribute to the study of what happens. When we study the Big Bang, we say it was long ago. What does it mean? I mean, we say that the Big Bang was 13-14 billion years ago. Well, it means that if we take all the clocks, and we go back in time- They're different, but on average, if you go back 14 billion years, that's a Big Bang. And then we get confused precisely because of the Big Bang. It was so hot, so compressed that we're in a quantum situation, it’s very different physics than the one we're used to. We're deep in a quantum gravity regime, that even the physicists’ notion of time is not good anymore. We have to change it a little bit. We haven't figured out exactly what happened at the Big Bang. Perhaps there was no time at all there. It just started time. So physicists are trying to write equations that tell us what happens and compare with observations of the sky, and come up with a good story. And one difficulty is precisely what you said that the usual notion of time, either in the physical one, the one in the Maxwell equation, in the Newton equation, the one one studies at school, when one studies physics, you know, a pendulum moves in time, even that notion of time is not good anymore at the Big Bang. So that's, for me, the beauty of physics, it forces us to rethink our basic notions, sometimes in a completely clear way we have understood. It's Copernicus telling us, we're not still, we're moving, and so on and so forth. Sometimes, like, today we are at the border of what we know. And even the notions that we use are not clear. So not everything is clear about time. What exactly happened with time at the Big Bang is a topic of discussion.
SS: But can I ask you something, because we've said previously, that, you know, time never goes backwards, it always goes forward, right?... So if time is an arrow, which is moving forward, do we remain static in reality, because there is no time really, or are we moving along with it?
CR: No, I don't think we remain static. And it's not that easy. When I say or physicists say sometimes ‘there is no time’, people try to imagine, alright, so the universe is just a block, is down there and nothing happens. But that's a wrong imagination. Because if nothing happens, what you're really thinking is that there's a rock there and time passes, and the rock doesn't change. So nothing happens – it’s a way of thinking time passes, but nothing changes. And that the correct imagination is the opposite. The universe is changing. The universe is happening. It’s a lot of things happening in the universe. Just imagine many, many happenings but they are not ordered in a common time, they are sort of disordered. It's our way of organising them (a lot through memory and through anticipation) that we read the full of happening of the universe as a single sequence of things happening, that's a mistake. You see, when I say the sun doesn't move… We learn at school- I mean, the motion, we see the sun going up and down, up and down every day. And then we are kids, we go to school and the teacher or father, mother tell us, “You know, it's an illusion, the sun does move.” But this does not mean that the sun in the sky - boom! blocks and is not gonna move anywhere. The sun keeps going around, it just is not the sun moving. It's just a more complicated story.
SS: I wanted to also use the example of music if I may. It's a thing that doesn't really exist but only does because it is happening in time, right? Like instruments play together in a rhythm or a pulse that brings time alive. And we only feel something when music changes in time. And non-changing note becomes a drone and stops having meaning like, you know, my refrigerator making a noise. So if on an elementary physics level, time doesn't matter, or it doesn't exist, why does it matter on a non-elementary level, on a level of music, for instance? Does music essentially create time?
CR: This is a great question. And in fact, you know, in the history of thinking about time, music played a huge role. Suppose, you're hearing music, a song. And in some moment, right now, you're hearing one note, not the full song. And then in the next moment, you're hearing one note. So you're never hearing the song. You only hear one note at the time, in every moment you hear one note. So why do you react to the song? There's never a song up there in nature. At every moment, there is only one note at a time. I mean, this was before harmony. Most of music was just melodic. Okay, so what is the answer? The answer, if you think for a moment, is obvious. When you hear a note, your brain still remembers the previous notes. So what you're hearing, you think of the melody, just think of your preferred melody, in a moment, you hear a sound, but at the same time, you remember the previous one. So if you think at the time, the melody itself is really memory stuff, it’s not out there, it's in your memory that stays together. Out there, there's one note at a time. And this shows that a lot of our feeling and perception of temporality, it's connected to the fact that we remember. And in a sense, if you want, let me put it in some way, a stone cannot hear music. Because it doesn't have memory. To hear music, you need memory. And memory requires a brain or a computer that has memory, not brain because we're spiritual things different from the rest of nature. I think that we're just pieces of nature, like everything else, but we're very peculiar pieces of nature with a lot of wiring and neurons and stuff, and it’s this peculiarity that allows music to exist. In a very precise sense, music can only exist because we have this brain, which remembers and it's not only that, because when you hear music (I think everybody knows music) you hear a piece of a melody and your brain anticipates what would come next. And music is all a game, that the music is coming and in part, it comes what you expect. The next note - you know what it is. So you're happy that you get and in part it surprises you, because there comes something [else]. It's all this game of surprising and satisfaction, which makes music. So music is in our brain in some sense. This doesn't mean that there aren't, you know, sound waves moving around. That's all true, that's real. But what we call music, it's what we make with all that in our brain.
SS: So interesting.
CR: Thank you.
SS: So the experiment you just showed me with your watches, –
SS: I mean, the whole thing really just blows my mind. Because I mean, I see time as something universal, right? Like a minute is the same here in Georgia, as it is, I don't know, somewhere in New Delhi, nine hours on the plane to New York would be the same nine hours here on Earth. If there is this difference in the speed of time, why don't we feel it? I mean, how far above do I have to climb to feel the difference? And will my subjective perception, you talk about memory, but I feel like perception is also a big thing that actually distinguishes time, will my subjective perception of time even allow me to feel it even if I'm high enough?
CR: Yes. Let me answer both questions. Firstly, the second one, then the first one. Would your subjective perception match that? Yes, absolutely. There are some movies (‘Interstellar’, for instance), in which the hero goes near very heavy things and so time slows down a lot for him. So when he comes back, for him, a few days have passed, and when he comes back - years have passed. And in fact, at the end of the movie, he comes back to Earth, and finds his little daughter, who has become an old lady. So the main character is still a man in his middle age and his little daughter now has become an old lady. So the subjective perception of time has been completely different for the two. So absolutely, we are certain of that, this is solid physics, this we understand. Now you ask, well, wait a moment, why don't we perceive that? I mean, as you say, a minute for you is a minute for me... Well, the reason is, because, as long as we stay on Earth, so Georgia, Canada, Delhi, or we go to the mountains, Everest mountain is the highest, these differences are very small, extremely small. And in fact, when I teach physics, I give my students sometimes the exercise: “Compute what is the difference of time. Your brother goes up to live on a mountain, the highest village you can imagine, and then he comes down after 50 years, and what is the difference of time?” So they compute it and the difference is just a fraction of a second, so too small for us to get it. But if you have instruments, you can measure it, and we do measure it. And the spectacular thing is even today, we can measure it, not just the mountains, but even 20 centimetres higher, 20 centimetres lower, we can see that this clock goes faster, this clock was slower. GPS, the little thing we're using while driving the car that locates us, it works with some satellite. And when the satellites were put up there, at the beginning it was the Americans in the 70s, and then it was done, you know, by the Russians, the Chinese, the Indians, now we have many systems like that... When Americans put the first satellites up there, the physicists told the engineers, “Careful, because up there times goes faster.” And since there's a clock in the satellite, because essentially what happens is, you know, there's a satellite with a clock and it beams down the time and then your little machine uses that to locate yourself. And the physicists tell the engineers, “Careful, because time there goes faster.” The engineers adapted the system, but that was in the hands of the military. And some general in the American military said, “Oh, come on, this is silly – time goes faster, I don't believe that.” So at the beginning, the first satellites, the experimental ones we put, that had a switch, they could keep this into account or not. And it doesn't work if you don't keep this into account. So technologically today, when you drive the car, and you have the GPS system that tells you which way you're going, the system works in a way that keeps into account the fact that up there time goes faster. So it's part of our technology today.
SS: Really fascinating. Professor, I wish I had another three hours to talk to you about the notion of time, but it's been really interesting to at least tap a little bit into the notion and perception of time and what is it, does it exist, isn’t it an illusion. Thank you very much for this wonderful insight. And I wish you all the best of luck, and I hope we get to do this again soon.
CR: Thank you. I hope so as well. And thank you for your questions, which were very much on the point. And this is a fascinating topic. I hope to talk to you again.
SS: Definitely. Thanks a lot. Be safe.