Carlo Rovelli on Philosophy and Physics
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This is Philosophy Bites with me, Nigel Warburton. And me, David Edmonds. Philosophy Bites is unfunded. Please help us keep it going by subscribing or donating at www.philosophybites.com or you can become a patron at Patreon. Not so long ago, the renowned astrophysicist Stephen Hawking declared that philosophy is dead. Philosophy, he claimed, had nothing to teach physics.

But another distinguished scientist, the Italian theoretical physicist Carlo Rivelli, is eager to come to philosophy's defence. Carlo Rivelli, welcome to Philosophy Bites. Thank you very much. The topic we're going to focus on today is philosophy and physics. What do you think the relationship between philosophy and physics is and should be?

I think the relationship exists and should be much stronger than what it is today. It was stronger in the past. Heisenberg, Einstein, Newton read philosophy with enormous interest and with enormous relevance to their work.

It has weakened after the war in the second half of the 20th century and I think this has not done good to physics and perhaps neither to philosophy. If we go back to the classical period, it's impossible sometimes to distinguish between natural scientists and philosophers. They were the same people speculating about the nature of the cosmos.

Yes, Newton was considering himself a philosopher and a scientist. And the kind of questions he would address are questions which are between philosophy and physics, like what is space, what is time, Kant discussed the same questions, Einstein discussed the same questions, being aware that, of course, there is a physical scientific side to this question and there is a philosophical side to this question. I don't want to say that there is no difference. There is an obvious difference in methods, in questions.

But there is a conversation and there is an enormous learning from one another. I don't think Kant would have thought for a moment that he wasn't supposed to know in depth the physics of his time. I don't think Newton or Einstein would have thought for a moment that it wasn't good to know in depth what the philosopher was saying about these problems. So there is a back and forth between two manners of trying to get to a better knowledge that worked very well.

So in a simple view of what scientists do, they make empirical observations of the world. But it strikes me that that's not what theoretical physicists do.

The simple view of what scientists do is the following. They have their own knowledge, culture, conceptual structure whatsoever, language. And using this, they do experiments, they do observations, they build up theories, and then with the theories they interpret observations, they change the theory, they have more observations. There's this back and forth between theory and observation, which has been studied a lot.

I think this is wrong, because what happened historically in theoretical physics in particular, perhaps in all sciences, but especially in theoretical physics, is that during this process, the conceptual structure, the language, the basic grammar we use to think about the world evolves as well. The idea of what space is, of matter is, of what causality is, of what we mean by understanding the world, changes.

The big steps in science, and sometimes also the small steps in science, are not: "Well-posed problem within a certain framework, solve it." It's a problem within a certain framework and then you have to change the framework.

And in doing that, very often, philosophy helps. That sounds very close to Thomas Kuhn's notion of scientific revolution, his famous book, The Structure of Scientific Revolutions. In that, he argued that there's a period of normal science where, within a framework, people make observations and understand the little experiments they need to run to clarify points about their bigger theory. And then there come these revolutionary moments where,

where the theory itself is up for grabs. These moments aren't actually completely justified by empirical observation. Sometimes the empirical observation, as with the Copernican revolution, doesn't actually warrant the change of world picture.

I think that we have learned enormously from Kuhn. Kuhn has clarified this historical aspect in the acquiring of knowledge, has clarified how much there is a change of perspective. But I also think that in the world of science there is an over-reading of Kuhn, especially the idea that this means that there is no cumulative growth of knowledge. I think it's badly wrong.

The idea of incommensurability, which is at the core of Kuhn, namely that when you go through one of these steps, one of these revolutions, the previous theory is not really related to the next theory. I think this is really wrong. Classical mechanics is perfectly recognized within quantum mechanics or within relativistic physics.

Aristotelian physics is still recognizable in Copernican Newtonian physics. There is an undeniable cumulative aspect in science. Theories talk to one another very well. Scientists should know in which sense generativity is related to Newton theory. So I don't think that we learn by throwing away completely a theory. I think we learn step by step.

by changing bits of our previous conceptual structure, but keeping most of it. But nevertheless, we do change bits of conceptual structure, crucial ones, and usually ones that we were not aware of, like simultaneity. When Einstein modifies the notion of simultaneity, nobody expected that that was one of the things up to change. Einstein was reading...

Schopenhauer and Kant and of course Leibniz. And I think that a lot of this freedom to him to do that, Mach, came from reading this philosopher. Another striking example for me is Heisenberg. Heisenberg is perhaps the real creator of quantum mechanics. He made an unbelievable jump by... Everybody was trying to understand atoms by changing the dynamical equations.

And he said, well, let's keep the dynamical equation they are and change the kinematic, change what we mean by an electron.

change the notion of observable, of entity. The entity is something else before and after Heisenberg. And he could do that because he was impregnated by the philosophy of his time. It doesn't matter if it's good or bad philosophy, but the freedom that the philosophy, the strong empirical, even anti-realist, in the case of Heisenberg, atmosphere, in which he was breathing philosophy, allowed him to do that. There's one sentence that

Physicists don't do anything unless philosophers first allow them to do that. I think it's important. I would even get to the point of saying that there's a certain sterility in modern physics which comes from the fact that there's not enough philosophy in it.

That's really intriguing. You're giving a kind of psychological explanation of the value of philosophy in that it liberates people to think in new ways. That's a bit like those architects that are influenced by, say, deconstructionism and then produce buildings which are supposedly inspired by Derrida. That doesn't really treat it as philosophy but more as a catalyst for physics.

Yes, part is that, but this maybe will be underplayed in philosophy. Philosophy is openness of mind. There is a second aspect, if I may, which is related to that, which is methodological awareness. The physicists that say, I don't have a philosophy, of course have a philosophy, and they work in an interpretation of what they themselves do or are supposed to do, which completely determine what problem they address and how they address their problems.

Kuhn is a great example because physicists learned a lot of Kuhn, but in my opinion, they were misled by Kuhn. Today, there's a lot of theoretical physics which is, let's imagine that the world is like that. Why? Because it's a great revolution and Kuhn told us that we jump into something very different.

And then there are articles and articles and articles in Physical Review D, which in my opinion are a waste of intelligence, time and energy, because that's not the way science works. Science works building on what we have, and of course changing in what we have, but you get to new knowledge by well digesting the science we have.

You're completely right by saying that the great steps ahead in science are not necessarily motivated by new empirical data. Copernicus is an example, no empirical data whatsoever. General relativity is an example, basically no empirical data. Of course, a huge amount of science is based on new empirical data, bringing new information, so we get new theories.

But there's also this constructing without empirical data. But how is this possible? Well, it is possible because the empirical data are the previous successful theories, of course. So generativity builds on taking seriously special relativity and Newton theory.

Copernicus builds on Ptolemy. So by building on previous theories, one gets to new ideas. Not out of the blue, like Kuhn seems to tell us. Out of the blue, never good science came in. So that's a good example, in my opinion, in which

A good dialogue about methodology, what's the best way of getting new knowledge, in the way he's addressed by a philosopher and in the way he's practiced by a physicist, is always good. And lack of methodological awareness, again, it brings to sterility in theoretical physics. Now, physicists talk about

about space and time, but when they start talking, they seem to be talking about something quite different from my subjective experience of time passing, or the kind of things that philosophers talk about in relation to time. And I wonder if there's a terminological thing here, that they're doing different sorts of things. When a philosopher talks about time, they're not necessarily talking about the same thing as a physicist. I think you're right. I think there is a mistake in communication sometimes.

When a physicist talks about time, he talks about the best notion of time that can be used to make sense of the world, not just everyday world, but also atomic world, cosmic world, quantum gravity world, and so on. So you need a different notion of time. And the mistake in communication is that very often the physicists, I'm the first one, express this by making a strong point about how reality is different from what we experience.

And this creates a misunderstanding, which is like physics denies the reality of our experience, for instance, of time. In Newton's theory, there is no up and down. Newton's equations are perfectly symmetrical about directions. There's no preferred direction in time. So according to Newton and all the theories that come after that, there's no up and down. In the universe, in fact, there's no up and down. Astronauts don't have an up and down.

Now, if I say up and down are an illusion, then a philosopher comes to me and says, well, we're talking about different words because I'm talking about the real world where there is an up and down.

Why this is a misunderstanding? Because, of course, up and down are real. It's just their parochial reality. I cannot walk on the ceiling, I have to walk on the floor. So up and down are real and true, and here, and Newtonian theory is perfectly compatible with the existence of an up and down here. In fact, it explains why there is an up and down here. So if I want a theory of the world, like the one I'm writing in quantum gravity, without time, it's my job as a physicist

to explain how you can think about the world without time, but also to match this with the existence of time here, the time we have for our interview, I mean it's just limited,

and see how this notion of time emerges or whatever, or appears, or locally appears in the global larger scheme of things where time is not defined, up and down, not defined. So our experience of the world, which is our common sense experience, is not contradicted by getting a larger picture. In fact, it's supported, and I believe there is no contradiction whatsoever

between me living in time, I'm obviously an entity living in time, my brain lives in time, I think in terms of past, future, and a world where perhaps time exists, but only exists locally and not as a universal feature from the largest cosmological scale to the smallest quantum gravity scales. In recent years, there's been a lot of criticism of philosophy that's come from very eminent physicists, something of Stephen Hawking particularly, but there are others too. Why do you think that is?

Can I be blunt? Yes, physicists said things like that. Steven Hogan said that. Steven Weinberg, Nobel Prize, one of the architects of the Standard Model, said exactly things like that. I think this is just stupid things to say. There has been a separation between disciplines. As I said, in the second half of the 20th century,

The great physicists of the first half of the century, Dirac, Heisenberg, Einstein, would have never talked against philosophy. They loved philosophy, they talked philosophy all the time. Then came this fracture between physics and philosophy. To some extent, theoretical physics itself went from a mood of "let's find the fundamental equation of the universe, the fundamental concept of the universe" - this is what quantum mechanics and general relativity are all about -

Two, let's use these fantastic things that Einstein and Heisenberg have found out in Dirac to understand the helium atom or the standard model. And you don't need philosophy for that. Einstein and Heisenberg have already done the job for you of finding the right concept of structure. You just have to use it. Great physics was done, of course, but not the same kind of physics.

Now, in a sense, we're back to the foundation problems because generativity and quantum mechanics don't talk to one another. So we have to rethink what is space, what is time, what is matter. So I think the mood is shifting. But many physicists were educated in this just compute and, you know, the basic equation, use them. And plus there's philosophy and philosophy.

Philosophy has been attacking physics a lot for a long time. There's been this war developing. It's particularly strong in the United States. There is a part of philosophy today which nicely takes us granted that the best description we can give about the world is the one that science gives.

But there's a big part of philosophy which, in different degrees from Heidegger on, has blamed science for being a wrong knowledge, with the idea that there is a natural world described by physics which is limited, but then there's a world of our experience, of course there's all the phenomenological tradition, and plus there's all the world of sociology, and...

which have tried to portray scientific description as a very partial description of the world. So the clash is on both sides, I think. But surely it's true that a scientific description, while it can account at a certain level for everything, doesn't really tell me very much about, for instance, the nature of my own consciousness.

The scientific picture of the world is not a picture of the world that solves everything. There are an immense number of things we just don't know. The more a scientist is a good scientist, the more is as clear the amount of things we don't understand well. Consciousness is a typical thing.

We have learned enormously about consciousness from brain science, from cognitive science, from neuroscience and so on and so forth. So we know much more than we knew just 10 or 20 years ago. But I don't think that we can say, all right, we do understand what is this physical phenomenon, which is consciousness. We certainly don't. Like we don't understand what happened before the Big Bang, what is inside a black hole, or how exactly life started on Earth, or what sort of life is in the galaxy or whatever.

So the number of unsolved questions is immense. The amount of fractures, we know this, but we don't know how to connect to that, are clear. So if the blame is science doesn't explain everything, yes, of course it doesn't explain everything. It's completely obvious to say so, and it's completely true to say so. If the point is that, ah, therefore we have better knowledge of getting to knowledge and to understanding these things than the scientific one...

I don't know. I don't think we really understand much more about our brain via what, meditation? Yeah, we get meditation is good for something else. But for understanding what happened in my brain when I perceive reality, I think the best is to say, I don't know, rather than to make up other stories. A little bit like thunderstorms. We don't understand them. So we have two options. Either we make a story about Zeus,

Jupiter being nasty and angry, or we say, well, let's study them better, and maybe then at some point we discover, oh boy, there is some electromagnetic phenomenon there. So what I'm saying is that the idea that there is something outside nature, and we have better access to knowledge, to deep knowledge, than just studying nature, I myself don't understand it. So we've been talking about the relationship between philosophy and physics.

Is there an example of, in your own experience, of how philosophy has influenced you in your physics? Yes. I'm doing quantum gravity, right? Quantum gravity is a theory without time.

fundamental, that's what I believe. So I was strongly trying to understand how to describe the world without time, and I think it's through the long conversation with philosophers that I realized that I should, as I said before, not just make sense of how we can think of the world without time, but also make sense how in this world we can get back to our experience of time and understand what is time in a fundamentally timeless world.

In Newtonian theory, where there is no up and down, how do we recover down there a world where I cannot walk on the ceiling? Well, we know how to do it. We are on the surface of a planet, there is attraction, so we are attracted down. And what we mean by down is toward the center of the planet. What we mean by up, it's the other direction, and that's why I cannot walk on the ceiling. So the story makes sense completely.

I realized that I have the complete story about time if I want to build a quantum theory of gravity where, as I believe, there is no space and no time at the fundamental level. Just to get clear, you're saying that from the point of view of the universe, the cosmos, there is no time. That it's a, as it were, a human creation. It's about our perspective on these things.

Is that right? Yeah, it's about our perspective on these things. I think so. It's one of the things on which quantum gravity, one of the big themes on which quantum gravity is now struggling. So I cannot say that I have total clarity about that. If I had, I would give you a clear story, which I clearly cannot.

But I think that's the direction where we are understanding. Of course, no time doesn't mean that the universe is frozen and suddenly we realize that and we are blocked in the position we are. It means that the best way to make sense of a universe is not to assume that there is a flowing time measured by a t variable and everything dances with respect to this flowing time. That's worked here. It works pretty well for here.

but it doesn't work at the Planck scale, it doesn't work at large, it doesn't work at the cosmos. The reason, intuitively, I believe, is that what really happens is that things change with respect to one another. They don't change with respect to time. In a sense, Newton had clarity of that. It's Newton who introduced the idea that time is an entity by itself, that time passes even if nothing happens. Before Newton,

if we read Aristotle or even Descartes, in a sense, if nothing happens, there's no time. Time is just the counting of the happening of things. So happening of things comes before an independent notion of time. And I think quantum gravity is taking us back to Aristotle in this sense. See, the way you're speaking, it sounds like a philosopher talking about time. It doesn't sound like there could be observations which are going to corroborate your hypothesis in any way.

Oh, there definitely should be. I mean, otherwise, I'm a physicist, so otherwise what I say is meaningless.

I'm working right now with some colleagues on a hypothetical model in which black holes collapse and then explode. And the time inside the black hole is immensely different than the time outside the black hole. So a star collapses, and if you're sitting on the star, you bounce out in a millisecond. If you look at this from the outside, 10 billion years have passed them.

So to be able to talk about that means to be able to think time very differently

But to be able to talk about that sensibly for a physicist means to end the story by saying, and therefore you astronomers are going to see the signal from an exploding black hole, check and find out if it is there. So I'm doing physics. I'm not doing philosophy. I'm trying to listen to philosophers. I'm trying to get ideas from philosophers, get criticism from philosophers. But at the end of the day, I write equations, I make predictions, and I hope they are tested and they come out right.

And if they do come out right, should the philosophers listen to you? That's what I would like to tell philosophers. If you find a discrepancy between what physics is telling you about the world at large and your intuition, it's your intuition is wrong. Carlo Rovelli, thank you very much. Thank you, thank you very much. For more Philosophy Bites, go to www.philosophybites.com. You can also find details there of Philosophy Bites books and how to support us.