IRA FLATOW: This is Science Friday. I’m Ira Flatow. How do you think about time? Well, if you’re like most people, you experience it as Newton described it as something that passes independent of other events, something that’s the same for everyone. It moves in a straight line. Or maybe you’ve come around to Einstein’s view of time, as a substance in which we are immersed. But my next guest thinks about time even differently than that. He’s working on a way to quantify gravity, which time doesn’t need to exist. And he joins me now to talk about it. Carlo Rovelli is a physicist and author of numerous books, including his most recent, The Order of Time. And I’ll tell you, welcome back to Science Friday, Carlo.

CARLO ROVELLI: Thank you very much, Ira. It’s a pleasure being back here.

IRA FLATOW: I really enjoyed reading this, because it just made my hair hurt in a good way.

CARLO ROVELLI: Thank you.

IRA FLATOW: Let me take a stab at some of the things here and see if I’m getting this right. It’s your belief, as a physicist trying to understand the universe from a quantum point of view, that time, as we normally think about it, is not a useful concept. We, in the course of our daily lives, think time exists. We are always running out of time. But you say that as a concept in physics, time, like that little t we use in calculating rate, times time, equals distance, really does not exist. Would that be correct?

CARLO ROVELLI: Yes, that would be correct. Of course, it keeps existing in our daily life, right? We have 20 minutes, or whatever, to talk, and this is not going to change. But it’s not a good notion for thinking at nature at a fundamental level That’s the point. In fact, it’s something that happened in a number of steps, because we have already have learned 100 years ago, I would say, that our common notion of time is not good for thinking about the world. Right ? Time passes a different speed for different people. We have learned that from Einstein, and recently, we have clocks to actually measure it. We see that time passes faster in the mountain and slower near the sea.

We have learned the notion of present, present all over the universe, is not well defined, is not objective. It doesn’t really make sense if we look far away from us. So time has lost pieces of the pieces, and my job is to study quantum gravity, and the question is, what is a correct notion of time that remain valid in this large– in this theory that is supposed to include everything we know about the universe.

And I think that the answer is forget time.

IRA FLATOW: Forget time.

CARLO ROVELLI: Try to write equations without any time valuable.

IRA FLATOW: We don’t need it.

CARLO ROVELLI: We don’t need it. And it’s far less dramatic and revolutionary than what it sounds. Because if you think for a moment, we really need time. I mean, I could say we started at, what is it, 3:20, and we’re going to finish at 3:40. And I woke up this morning at 6 o’clock. But I could also say, I woke up this morning when the sun was there, and we started when, I don’t know, the hand of the clock here had this particular position, and when that particular person moved there. So I could talk, mentioning only how things change one respect to the other and never referring everything to this time.

We never see time. Right? We see clock changing. We see sun moving. We see things moving. And I think that if we want to study the universe at its basic level, where all things move, but in a disordered way, not together in a single time variable, it’s better just to forget time and describe how the variables changed with respect to one another.

IRA FLATOW: Let me ask our listeners to chime in, if they have time. How do you understand time? We’re taking your thoughts and your questions about the physics of time and whether it exists at your number, and our number 844-724-8255. Also you can tweet us @scifri.

But there are so many questions. So you’re saying that we don’t– when physicists like you think about understanding the universe, they can write all their equations– and you’re trying to do it in the quantum world. You can write all your equations that seem to work, and you don’t need to put that t, that [INAUDIBLE] of time, into the equation.

CARLO ROVELLI: That’s correct. In fact, what happened is physicists stumbled upon that, already back in the late ’60s. Two American physicists, Bryce DeWitt and John Wheeler, wrote the first equation that was supposed to bring together quantum mechanics and general relativity. Today we call it the Wheeler-DeWitt equation. That equation had no t, had no t variable, had no time in it. And this sparkled an enormous discussion that has continued for decades.

What does it mean to– after all, all the equations of physics before this one, the fundamental equation of physics, our evolution in time. Newton equation, Maxwell equation, Einstein equations, were written as evolution in time. Well, I think that things have cleared up. And it’s not that we have a complete clarity about this story. Because we don’t yet have a theory of quantum gravity on which everybody agrees and that has been supported by experiment. So we have tentative theories of quantum gravity. But what we know, and I think and there is consensus about that, is that something dramatic happens to our notions of space and time when we bring quantum mechanics in. And a number of people, me including, think that the easiest way to think about it to just forget time.

You see, let me put it this way. We think– you said at the beginning, quite correctly that, we think time a la Newton. Newton gave us this idea of a time that flows uniformly all over the universe and that passes even if nothing happens, like a big clock outside the universe that tick-tocks away the moments of time. But this was Newton’s idea. It’s not a completely natural idea for humanity.

Before that, before Newton, the way people thought about time is just the counting of things that happen. So before Newton, if you had asked people, what is time? They would have said, well, it’s the days. Day, night, day, night, you count the days, one, two, three, four, that’s time. Or you count the oscillation of a pendulum. Or you count how much sands fall down in an hourglass. So time was considered as just the counting of change.

And if you think time this way, which is a pre-Newtonian, the old way of thinking of time, which I think is the most general one, that makes sense, even if you don’t have a preferred time variable, because you can count time in terms of days, or in terms of months, or in terms of oscillation of a pendulum. And each one might be different than the other one. They don’t need to be in tune one with the other. So you can have as many time variables as you want, which is to say you don’t have a preferred time variable in the world.

IRA FLATOW: So time is basically a mental construct that we’ve created to advance our cultural, let’s say, so that we can all get to meetings at the same place, at the same time, or something we have put in our head, but nature really has no use for.

CARLO ROVELLI: Yeah, to some extent yes. I think that– one of the main message of my book is that time is a complicated thing. It’s not a simple thing. We tend to think about time as just a clear, compact concept, something that flows, is the same for everybody, that passes [INAUDIBLE] from the future, and we all know what it is. But then when we study it, we realize that it’s a multi-layered thing. And all its various properties come from approximation, from different levels of reality, so to say.

And in fact, a good deal of what we can call time is not really in the things in nature, but is the way our brain interact with nature. Time for us is a lot memories, expectations, feelings, even the emotion of the time that passes, you know, the anxiety of the future, the fear of losing things. Time for us is a complex superposition of layers that go from the emotional, to the cultural, all the way down to the physical. And physical there is little. There is approximation, which works well at our level. And if you want to discard all that and just look at nature at its most basic level, very little remains of what we usually call time.

IRA FLATOW: A lot of people want to talk about this. Let me start with a tweet. Jimmy Morgan says, “If we remove the notion of time, what happens to the idea that the fourth dimension is time?”

CARLO ROVELLI: Well, in fact, if you do quantum gravity, you have to remove not just time but also space.

[LAUGHTER]

So it’s more dramatic–

IRA FLATOW: New book!

CARLO ROVELLI: –than that. You see, Newton had this, not only this idea of time flowing, but also this idea of space as a container of the world. And Einstein understood that this container of the world is, in fact, gravity, is a gravitational field, something that can move and can flex as a sort of rubber sheet, right, on which matter moves, but is itself a sort of material thing. And if you look at the quantum aspect of this object, which is space– or more precisely, space-time– like all quantum objects is granular, is made by little chunks, individual chunks, grains, so we have grains of space, grains of time, grains so to say, which move probabilistically because quantum theory tell us that things are probabilistic in the universe.

So space breaks up into–

IRA FLATOW: Tiny little pieces of granular–

CARLO ROVELLI: Which are not in space, careful, but they make up space, so to say.

IRA FLATOW: Woah, but so does that mean that there is no yesterday or tomorrow? There’s just– I mean, you talk about philosophers talking about there’s only present, because that’s all we can feel. But in time, one fascinating concept that you did bring up is entropy. That is one way of detecting some sort of movement, or a change, right?

CARLO ROVELLI: Yeah.

IRA FLATOW: Talk about that.

CARLO ROVELLI: Yeah, that’s another extraordinary, surprising discovery that came out of physics. In fact, it started already in the 19th century. And it was clarified step by step. Namely, we have the sense that the past and the future are completely different. Right? The past is fixed. It is known, has happened already, is unchangeable. While the future is open. It has yet to come. It can go in different directions. So past and future are completely different. We have memories of the past. We don’t have memories of the future. We have pictures of the past. We don’t have photography of the future.

But then you study physics, and it has been a surprise that the difference between past and future is much weaker than in our experience. In fact, all the basic equations of physics are invariant, if we replace past with future. The only one, the only fundamental equation that distinguishes the past from the future, is, as you said, is one that has to do with entropy, is the so-called second law of thermodynamics, which says that entropy grows over the future.

IRA FLATOW: Now, entropy is– what you’re saying, if I try to explain a little better for me, is that entropy starts low–

CARLO ROVELLI: It starts low–

IRA FLATOW: –and the original universe was very low and then gets more disordered. It’s starts out very ordered and then gets disordered, like my room does if I don’t clean it up.

CARLO ROVELLI: Exactly, so the entropy is a measure of disorder. It’s just a technical way for saying how much disorder there is. And it grows. And it’s not hard to understand why it grows. Because it’s like your room. If you let it– if you let a little child run in it, it doesn’t put the things in order.

IRA FLATOW: I’m the child.

CARLO ROVELLI: It disorder everything.

IRA FLATOW: So there’s one way to know which order things are moving, by changing– by deciphering what the entropy is from one stage to another.

CARLO ROVELLI: Yeah, but what is surprising is that the only difference between the past and the future is this increasing disorder. There’s nothing else. And that came as a shock for physicists. Because you know, the reason we have memories of the past and not the future, the reason we remember the past and not the future, the reason we think about cause in the past and effect in the future, all that is nothing else that an effect of the growing disorder.

And the open question for which we don’t have an answer– in my book, I try to suggest one possible answer. But it is an hypothesis. I don’t know if it is true. The question– the open question is, why was the universe ordered in the past?

IRA FLATOW: OK, hang on, Carlo. I have to give a break. I want you to get time to tell an answer. I’m Ira Flatow. This is Science Friday from WNYC Studios, talking with Carlo Rovelli about his terrific new book, The Order of Time. Do you want to– I have so many dog eared pages here. But go ahead. Finish your explanation.

CARLO ROVELLI: Yeah, the open question is, why was entropy low in the past? Namely, why was the universe ordered in the past? And we don’t know. It’s one of the open mysteries in time. In other words, we don’t know the real reason for which the future is different from the past. It’s one of the things that physicists are discussing.

I suggest in the book that it’s not really the universe which is ordered. It is our way of looking at it. You see, we see the sky rotating around us, right? But is not the sky that rotates. It is us rotating. Many things about the world we have understood by looking at ourselves and understanding that they are not properties of the world, but are dependent on perspective. And perhaps even this, even the difference between past and future is a perspectival thing. This is something that depends on the way we look at it.

IRA FLATOW: You say in your book that we have we don’t have a grammar to talk about the world without time.

CARLO ROVELLI: Yes, well now, that’s a great difficulty of talking about time. Because we are stuck in our language. We have verbs, which have past, and future, and present. And we know for sure that that’s not the way the universe is organized. There are things which are neither past, nor future, nor present. Or the things that are past for me but future for somebody who is in my future. And we don’t have a language, we don’t have a grammar, for that.

We’re little bit– you know, 2,000 years ago, people understood that the Earth is a sphere, is a rock, is a bowl. And they were all confused, because they wrote things like, for the people down, up is down, and down is up, which make no sense at all. They were trying to rearrange the meaning of up and down. And now we understand that in Sydney, up is different than up in Washington. I think we have to do something similar about time. We have to rethink what we mean by past, present, and future.

IRA FLATOW: And do you think that the public will ever understand?

CARLO ROVELLI: Yeah, look, the moment in which– I hope it will come– we start to travel outside the Earth, and we have people on Jupiter, or Mars, or perhaps even to other stars. We will realize that to ask what is happening now, in the present, on a distant star makes no sense. We will be used to the idea that people age differently if they move away from one another. So somebody is the same age, you meet again as a different age. So we’ll have to readapt our grammar for talking about time.

IRA FLATOW: We’re going to take a break, and Carlo is going to come back and talk more about the– it’s a great book– The Order of Time, by Carlo Rovelli. And a lot of phone calls coming in now, 844-724-8255. We’ll get to those, some of those, and the tweets. So stay with us. We’ll be right back after this break.

This is Science Friday. I’m Ira Flatow, talking with Carlo Rovelli, author of The Order of Time, great book, great new book, Carlo. Thanks for writing this and giving us something to think about. In the few minutes we have left to talk with you, let’s go to the phones. Let’s go to Lebanon, Pennsylvania. Dillon, hi, welcome to Science Friday.

DILLON: Hi, Ira, thanks for taking my call.

IRA FLATOW: Go ahead.

DILLON: My question is for your guest, Mr. Rovelli. If you’re taking the concept of time out of measuring gravity, which, if we’re talking about standard physics, you use time 9.8 meters per second to measure the rate at which something is falling, how are you quantifying gravity without considering time or space?

CARLO ROVELLI: There is a very close connection between time and gravity. They go together. That’s a great discovery of Einstein. The speed at which our clocks move is determined by gravity. So that’s a masterpiece of Einstein. A hundred years ago, he wrote this theory, and he told us how to do that. And in fact, the bottom line is that we should never talk about time by itself. We should talk about the time measured by a clock. And the theory tells how clocks move one respect to the other, at which speed they go, one with respect to the other. So in a sense, we never measure time. We measure clock time. And we measure how the clocks move in different manners, and how the clocks move differently from one another.

IRA FLATOW: Because the clock is moving, and its movement is influenced by gravity.

CARLO ROVELLI: A clock– well–

IRA FLATOW: Because that’s what–

CARLO ROVELLI: –the hand of the clock is moving and give us a reading, and the speed at which the hand goes depend on gravity, very much. So literally, you take one clock, a little bit higher, a little bit lower, and it goes a different speed.

IRA FLATOW: So when you are in a massive gravity, like a black hole, then would you say time stops?

CARLO ROVELLI: Slows down dramatically to the point of stop. So what we– if we could– it’s just a matter of money. If we could go near a black hole, and wait a little bit, and come back, we would come back in the far, distant future.

IRA FLATOW: OK, let’s go to the phones to [? Emisha ?] in San Francisco. Hi, welcome.

[? EMISHA: ?] Hi, thank you. I loved Mr. Rovelli book, Seven Brief Lessons in Physics.

CARLO ROVELLI: Thank you.

[? EMISHA: ?] Oh, I loved it. And one of the most moving parts for me was where you described all of what we take as reality as really being, as I understood it, sort of a complex mesh of energy. And we view reality as being concrete, but really all we’re viewing is this mess that is ephemeral. And I’m wondering whether part of what you are saying is that time itself is woven into this mess that you described in that book.

CARLO ROVELLI: Yes, very much so. The– both in that book and in this new book, The Order of Time, I do various things. I try to describe what we’ve learned so far convincingly about the world, but also the kind of exploration that is going on now, and the way we think we’re understanding the world. And this book is centered on the fact that, as far as I understand, the best way of thinking of the world is not something there that is changing in time. Time itself emerges from this sort of very microscopical network of events happening. And time is a multi-layer of things that emerge step by step from there up.

IRA FLATOW: Do you think in your work to unify gravity with quantum– because that’s really what you’re trying to do, is it not? How long is that going to take? Because we’ve been talking about it for 100 years.

CARLO ROVELLI: Well, yes, I mean, the first to talk about that was Einstein himself. I mean, a hundred years ago he said, look, we have a problem. We have to find a quantum theory of gravity. But for most of these 100 years, almost nobody was working on that. It is only the last decades that have been a lot of work on that. The answer to your question is that we don’t know. We have tentative theories of gravity. It’s not that we are in the total dark. And we have some empirical information. We don’t know which theory is right, and we are not sure when we will be able to find empirical information to confirm that. I think that’s part of the excitement. Right?

[INTERPOSING VOICES]

CARLO ROVELLI: I’m sorry?

IRA FLATOW: Is it frustrating as well as exciting?

CARLO ROVELLI: Oh yes, well, scientific research is horrendously frustrating. There’s a lot of things that don’t work, right. Most of the things don’t work. You throw away, and you try something else.

IRA FLATOW: Well, what people should not throw away and should try is your new book, The Order of Time. Carlo Rovelli, it’s a great book. You know, it makes you think a lot. Thank you for taking time to be with us today.

CARLO ROVELLI: Thank you very much, Ira.

IRA FLATOW: Carlo Rovelli is an Italian theoretical physicist and head of the Quantum Gravity Group at the Centre de Physique Theorique, Aix-Marseille Universite. There you go.

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