Why physics and philosophy need each other

[MUSIC PLAYING] IRA FLATOW: Hi, it’s Ira. And you’re listening to Science Friday. Before there was what we call science, there was a term called Natural Philosophy, dating back to Aristotle, that explained basically the same principles we call science today– nature, physics, chemistry, biology, astronomy.

And I’m thinking about this now because my next guest, physicist and author Carlo Rovelli, has a new book called On the Equality of All Things– Lessons on Physics and Philosophy. And there’s that connection once again. And while it’s a small book, there’s a lot in there, from quantum physics to the nature of time. And I’m glad to have Carlo with me again. Welcome back.

CARLO ROVELLI: Thank you very much. It’s wonderful being here.

IRA FLATOW: You say the Scientific Revolution we are undergoing is momentous, yet its full philosophical significance has yet to be properly realized and understood. Tell us a little more about that thought.

CARLO ROVELLI: Yes, I think that science and philosophy have always talked to one another. Major scientists were influenced by philosophers. And philosophy was influenced by science, in a larger sense. Our philosophical view of the world was changed by Copernicus, was changed by Darwin, when we understood we were cousins with all the other animals, was changed by classical mechanics, by electromagnetism, and now, I think, is being changed a lot by especially the physics discovery of the 20th century, relativity and even more quantum mechanics. We need a rethinking of philosophical basis at the light of quantum mechanics. And that’s what my book is about.

IRA FLATOW: What do you mean by that? You say that dialogue between philosophy and science is fruitful and, indeed, necessary.

CARLO ROVELLI: Oh, yes. It’s necessary. It’s easy to see this historically. All major figures in science have got a lot of inspiration from philosophy. Schrödinger was reading philosophy. Heisenberg was. Einstein read Kant when he was 15, actually, on space and time and clearly influences science on space and time, but also vice versa. If you think of the greatest philosophers ever, like Kant, Kant started off by thinking about the Newtonian revolution– what could it mean– and tried to make sense of it.

So it’s a dialogue. It’s a constant dialogue. Of course, this dialogue is not particularly relevant when science is just solving minor problems. If you want to understand iron atom better than another kind of element, atom, you don’t need philosophy. But when you are rethinking basic ideas about the universe, like has happened with quantum mechanics and relativity, then there are philosophical issues coming in. Quantum mechanics is extraordinary, not just because it gives us all the beautiful technology that we have today, but also because it’s telling us that the way we were thinking about reality before doesn’t work.

IRA FLATOW: Well, but a lot of quantum mechanics and a lot of the new kinds of thinking involves ideas that we laypeople really can’t comprehend. I mean, are we too caught up in our limited human experience of the world to be able to grasp the way things truly are, as quantum mechanics people want us to?

CARLO ROVELLI: Quantum mechanics, it’s a bit complicated, but not more complicated than classical mechanics. And it is not harder to understand than the fact that the earth is spinning, or is going around the sun, for a man of a contemporary of Copernicus. It requires a rethinking of something intuitively clear, but not right. And this is the kind of difficulty in quantum mechanics.

Quantum mechanics, nowadays, is taught in high school, the basis and calculations. Every student in physics and mathematics learn how to do calculations. So there’s nothing really mysterious about it, once you start understanding it. What is really mysterious is that it’s telling us that the world is not exactly the way we thought before.

IRA FLATOW: And how can philosophers help us bridge that understanding?

CARLO ROVELLI: Philosophy has always been a wide exploration of ways of thinking. Philosophy is not just one. It’s multiple. There are many philosophers, many ideas, and many centuries and many countries and many continents, and explores ways of thinking. And sometimes they’re able to point out assumptions which are not obvious. Think of what Einstein did. Einstein, when he understood relativity, what he did was not math, special relativity. The math was already done by others. Not even a measurement– he never went in a laboratory. He just realized that we had a wrong idea in our mind.

The wrong idea of our mind is when you think about two events, if you snap your both hands, you get two snaps. And you ask whether they are simultaneous or not. It’s natural for us to think that this is an absolute fact, whether they happen at the same time or not. And Einstein realized that doesn’t mean anything, I mean, because simultaneity depends of who is looking at it. It’s relative. That’s the core of thinking, understood. How did he understand that? He learned from the philosophers the idea of being careful about your intuition about time. Perhaps, time is not the way it looks usually to us.

You see– and let me put it this way– we grew up on the Earth. And so we get used to the way things are on the Earth. But then we are like a little kid who grows up in his own village and thinks that the entire planet is like its own village, right?

IRA FLATOW: Right.

CARLO ROVELLI: Thinks that everybody speaks that language. It’s those things. Then the kid goes out and says, oh, boy, there are different languages, different habits, different ideas. So it grows up. That’s growing up. And so science is a little bit is humanity growing up. Here on Earth, where Earth is relatively small, so we never see the kind of alteration of time and space that relativity describes– but just because Earth is small.

Look, if you have a friend everywhere else in the galaxy, or in another galaxy– imagine a friend in another star– you can have a conversation with this friend. But when you send a message, time the message goes there and comes back, even if the closest star, it’s a question of years. Earth is so small that light can go from Beijing to New York in less than amount of time that we perceive. So in terms of time of light, of light, the Earth is so minuscule. And because of that, we got the illusion that we’re all living in the same time, because you can call Beijing and have the sense that we are together.

So the reason we have difficulty of understanding Einstein relativity is because we do not have the proper experience of the slowness of light, because we live too close to one another to perceive the time of light, of light. So the smallness of the Earth gives us a false impression about reality. So this openness of mind sometimes is helped by philosophy.

IRA FLATOW: You write about a conversation with a student on this topic about how you have to think about the relativity of where you are and the relativity of objects to themselves that led to one of the reasons you wrote this book. Tell me about that. And why was it such a catalyst for you?

CARLO ROVELLI: [LAUGHS] It was because, of course, I am a great fan of philosophy, but that doesn’t mean that I agree with all philosophers. So this was a situation which I found myself in a very interesting disagreement with a brilliant young philosopher who ultimately did not believe in relativity– in Einstein relativity, not even Galilean relativity– because he believed that motion is absolute. The velocity is absolute. He couldn’t digest the idea. I mean, he couldn’t understand the idea that motion is relative, and velocity is relative.

And the main discovery of 20th century physics is precisely that most things are relational, a relative-like velocity. When we say, this object is not moving, what we mean is not moving with respect to something else. If you’re on an airplane, you say it’s not moving, you mean you’re not moving around in the airplane. But of course, you’re moving, because the airplane is moving. And if you’re on Earth, you say, this is not moving. My tree is not moving because it’s grounded on the Earth. But the Earth is moving. So the tree is moving with the Earth.

So what is something which is really, really, really not moving? Well, it doesn’t mean anything, because to be moving means to change position with respect to something else. And I think this relationality is what quantum mechanics has discovered to be universal in reality. So reality is much more a network of relations than things with properties. So this is a profound philosophical rethinking of reality.

And this step is what we need to understand, to make sense of quantum mechanics, also, to make sense of relativity. Reality is always seen from a perspective and is what it looks like with respect to a perspective, with respect to me, with respect to you. With respect to each perspective, things are different. What moves to me doesn’t move to you if you’re moving with respect to me. And that’s universal. So this relationalism, I think, is the core of the message that modern physics is giving us.

IRA FLATOW: Well, would you expand that to include one of the core ideas, uncertainty? Science used to be talked about– we know this stuff to be sure. And then along comes quantum mechanics and says, well, it’s probabilistic.

CARLO ROVELLI: Yes, that’s right. That’s right. Let me put it this way. Question number one, what do we know with certainty? And I think that a good scientist– in fact, a good human being– should answer that the answer what we know with certainty is nothing at all. We know things with a very good credibility, things we are very convinced about. But with certainty, there is nothing that we know.

And science is not about certainty. It’s about finding good reason for believing in this or into that, and being always open to change your mind. So certainty is a bad ideal, a bad mythology. And related to that, another central question of my book– I think these are the two central questions of my book, certainty. The second one is, what is the foundation of everything? What is the basis of everything? What is a single substance, a single thing, the single field from which everything derives? Well, I think that the best answer to that is that this is a bad question.

IRA FLATOW: [LAUGHS]

CARLO ROVELLI: Right. We’re not interested in the ultimate explanation. There’s no ultimate explanation. I do quantum gravity. Is quantum gravity ultimate explanation of something? No, not at all. Is physics an ultimate explanation? No. Of course, everything is physical, but we do understand things in its proper terms. We understand chemical phenomena with chemistry, biological phenomena with biology. There are no really contradictions between these pictures.

But it’s not that one is a fundamental one and the other a derived one. They’re all fundamental. They’re all fundamental from a different perspective. Reality is much more complicated than a single foundation. Reality, it’s, I think, a rich interwoven fabric of knowledge we have about it and knowledge we have about one another. And it’s this network of reciprocal knowledge that makes reality.

IRA FLATOW: After the break, if there’s so much uncertainty, what about a theory of everything? Stay with us.

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IRA FLATOW: They used to talk about, coming-up physicists did, with a theory of everything. Do you remember that, those days?

CARLO ROVELLI: Yeah, yeah.

IRA FLATOW: Is that a fool’s errand to look for that?

CARLO ROVELLI: I think it is. Yeah, I think it’s a bad objective to look for a theory of everything, especially today. There’s so much we don’t know. We don’t know what is dark matter. We’re confused about so many things from the fundamental level. We don’t have a quantum theory of gravity.

The hope of finding a theory of everything was triggered in the ’80s– so long ago, half a century ago– by the existence of string theory. That seemed to be a very, very general theory, but it hasn’t worked. It hasn’t delivered what it gave. It found experimental difficulties. So I think we shouldn’t ask ultimate questions. We should ask one question after the other. This is wisdom.

IRA FLATOW: You said that quantum gravity is something you study. And it seems to be one of the great unfulfilled discoveries. I mean, what are a couple of the big questions you want to be answered eventually?

CARLO ROVELLI: One of the big holes we have in the physical description of reality is exactly quantum gravity. And that has been the central research of my life. This means that we know that gravitational field, gravity has quantum aspect, but we are not sure about how to describe them. And the reason this is super interesting is because what Einstein discovered is that gravity is what determines the geometry of space– how big are things, what shape they have– and of time– how clock ran.

So quantum aspects of gravity means quantum aspects of space and time. So [INAUDIBLE] quantum gravity means rethinking space and time. You see philosophy coming again. And where are we? We are not in a very bad situation, but not in a great situation either, in the sense that there are theories.

There are attempts. And I’ve been working all my life in one of these especially, which is called loop quantum gravity. There’s a tentative theory of quantum space-time, of quantum gravity. And we can try to apply the theory to black holes to see what happened inside the black holes, to see what happened at the Big Bang or at the end of life of the black hole. It might work. It might explain dark matter. It might do stuff. But for the moment, there is no hard confirmation at all.

IRA FLATOW: Do you find this search, this uncertainty about the universe troubling? Or is it energizing? Saying, hey, I love the chase.

CARLO ROVELLI: I definitely find it energizing. But even more, I find the idea that we don’t need certainty– we have open questions– very relaxing. I mean, look, if there was a way to know everything, finally, fundamentally, the whole truth– and I don’t know it– that would be traumatic.

But I don’t think that’s the case. We are limited being. We are just a piece of nature, little creatures, little critters that walk around this planet. And we know a little bit. And it’s wonderful to know more. And it’s wonderful to know that there is so much we don’t know, and we can know more. That’s the beauty of knowledge and increasing knowledge without any need, in fact, of the anguish of knowing the final thing, getting certainty, or anything like that.

IRA FLATOW: Carlo, always great to have you on the program. It’s a great book.

CARLO ROVELLI: Thank you very much, Ira. It was a pleasure, this talk.

IRA FLATOW: You’re welcome. If you want to know a lot more of what Carlo is talking about, you can pick up a copy, On the Equality of Things. Carlo Rovelli, Italian theoretical physicist and author, On the Equality of Things, a small book with lots of stuff in it.

And there’s an excerpt from the book you can read for certain. Speaking of uncertainty, we’ve got a certain spot for you at sciencefriday.com/equality. This episode was produced by Charles Bergquist. And if you have a head-scratching question about the universe, give us a call, 8774-SCIFRI. That’s 877, the number 4, SCIFRI. Thanks for listening. I’m Ira Flatow.

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