Science Friday - Why physics and philosophy need each other

Episode Date: September 9, 2026

Once upon a time, much of what we call “science” was performed by people called “natural philosophers,” delving into mysteries of nature, physics, chemistry, and more. Physicist and author Carlo Rovelli argues that there’s still a great need for philosophy, and that the two fields can and should learn from each other in investigating reality.  Rovelli joins Host Ira Flatow to talk about big ideas in physics, and his new book, “On The Equality of All Things: Lessons on Physics and Philosophy.”Read an excerpt from “On The Equality of All Things.”Guest:Dr. Carlo Rovelli is an Italian theoretical physicist and the author of “On The Equality of Things.”Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Follow our show on Instagram, TikTok, Facebook, and Bluesky @scifri and sign up for our newsletters. Got a science question that’s keeping you up at night? Call us: 877-472-4374 Hosted by Simplecast, an AdsWizz company. See pcm.adswizz.com for information about our collection and use of personal data for advertising.

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 Carla with me again. Welcome back. Thank you very much. It's wonderful being here. 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.

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're 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. What do you mean by that? You say that dialogue between philosophy and science is fruitful and indeed necessary. Oh yes, it's necessary. It's easily to see this historically. All major figures in science have got a lot of inspiration from philosophy. Schrodinger was reading philosophy Heisenberg was Einstein read Kant when it was 15 actually on space and time and clearly influences science of space and time. But also vice versa. If you think of the great philosophers ever, like Kant. Kant was started off by thinking about the Newtonian revolution,

what could it mean and try 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. Well, but a lot of quantum mechanics and a lot of the new kinds of thinking involves ideas

that we lay people really can't comprehend it. I mean, are we too caught up on our limited human experience of the world to be able to grasp the way things truly are as quantum mechanics people want us to? Quantum mechanics, it's a bit complicated, but not more complicated than classical mechanics, and it's 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 talked 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 is telling us that the world is not exactly the way we thought before. And how can philosophers help us bridge that understanding? Philosophy has always been a wide exploration of ways of thinking, right? When philosophy is not just one, it's multiple, there are many philosophers, many ideas, in many centuries, in many countries, and many continents, and explores a way of thinking. And sometimes they're able to point out assumptions which are not obvious.

Think about what Einstein did. Einstein, when he understood relativity, what he did was not math. special activity, the math was already done by others. Not even 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're simultaneous or not,

it's natural for us to think that this is an absent fact, whether they happen the same time or not. And Einstein realized that that doesn't mean anything, because simultaneity depends of who is looking at it. It's relative. That's the core of thinking he understood. How did he understand that? He learned from the philosopher, the idea,

be careful about your intuition, about time. Perhaps time is not the way it looks usually to us. You see, let me put it in this way. We grow up on the earth. And so we get used to the way things are on the earth. Earth. But then we're like a little kid who grows up in his own village and thinks that the entire planet is like

its own village, right? It thinks that everybody speak that language, it's those things. Then the kid goes out and say, oh boy, there are different languages that are different ideas. It grows up. That's growing up. And so science is a little bit is humanicky growing up. Here on Earth

we're 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 his 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 the amount of time that we perceive, right? So in terms of time of light of light, Earth is so minusculus,

and because of that, we got the illusion that we're all living at the same time, because you can call Beijing and have the sense that we are together. So the reason we have 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 smallest, the smallness of the earth give us a false impression about reality. So this openness of mind sometimes is helped by philosophy.

You write about a conversation with a student. on this topic about how there is, you know, 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 such a catalyst for you? It was because, of course, I am a great fan of philosophy, but it doesn't mean that I agree with old 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 they are 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 ground under 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. This is a profound philosophical rethinking of reality. And this kind of steps, 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,

respect to me, respect to you, with respect to each perspective thinks are different. What moved to me is, it 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. Well, would you expand that to include one of the core ideas, uncertainty? You know, 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. Yes, that's right. That's right. Let me put it in 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 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 question 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, a single field? from which everything derive. Well, I think that the best answer to that is that this is a bad question. We're not interested in the ultimate explanation. There's no ultimate explanation.

I do quantum gravity, right? Is quantum gravity ultimate explanation or 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, with biological phenomenon, with biology. There are no really contradictions between these pictures, but it's not that one is a fundamental one and the other 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 is this network of reciprocal knowledge that makes reality. After the break, if there's so much uncertainty, what about a theory of everything? Stay with us. They used to talk about coming up physicists did with a theory of everything. Do you remember that those days? Yeah, yeah.

Is that a fool's errand to look for that? 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 at fundamental level. We don't have a quantum theory of gravity. It's 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 should ask ultimate questions. We should ask one question after the other. This is wisdom. You said that quantum gravity is something you study, and it seems to be one of the great unfulfilled discoveries, right? I mean, what are a couple of the big questions?

you want to be answered eventually? One of the big holes we have in the physical description of reality is exactly quantum gravity. That has been the central research of my life. This means that we know that gravitation 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 what determines the geometry of space, how big are things,

what shape they have, and of time, how clock run. So quantum aspect of gravity means quantum aspect of space and time. So understanding 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. the attempt, 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, 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 the life of the black hole. It might work, it might explain, dark matter, it might do stuff, but for the moment, there's no hard confirmation at all. Do you find this search, this uncertainty about the universe troubling or is it energizing saying, hey, you know, I love the chase? I definitely find it energizing, but even more, I find the idea that we don't need certainty.

We have open questions, very relaxing, right? I mean, look, if there was a way to know everything finally, fundamentally, the old truth, and I don't know it, that would be dramatic. 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, you know, 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's so much we don't know.

And we can know more. That's a beauty of knowledge and increasing knowledge without any need, in fact, of the anguish of knowing the final thing, get uncertainty or anything like that. Carlo, always great to have you on the program. It's a great book. Thank you very much, Erich. It was precious this talk. 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 Revelli, 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 slash equality. This episode was produced by Charles Berkwist, and if you have a head scratch and question about the universe, give us a call.

8774 SciFri. That's 877 the number four, SciFri. Thanks for listening. I'm Ira Plato.