The Gesture That Changed Human History
“Overall, discovering the world is a complicated business,” says the theoretical physicist Carlo Rovelli. “And it goes slowly. It has always gone slowly. And it’s still going slowly now, I believe.”
Rovelli’s putting it simply, but that’s all the better for the rest of us: He specializes in quantum gravity, a way of describing gravity using the principles of quantum mechanics. In his recently translated book “Reality Is Not What It Seems: The Journey to Quantum Gravity,” he calls quantum gravity “the main open problem in fundamental physics.”
What makes quantum gravity so thorny? It unites two fundamental theories describing the behavior of the universe: Quantum mechanics, and Albert Einstein’s general relativity.
“We have quantum mechanics that works extraordinarily well, and we use it for describing matter, for doing computers, for describing particle physics, the atoms, the nuclear forces and all that,” Rovelli says. “And then we have general relativity. That also works very well, and describes galaxies and stars and how things fall and the gravitational waves.”
The problem is that the theories describe different things well. In fact, “these two theories, at least at first sight, seem incompatible with one another,” he explains
“It’s like students of physics that go to listen to classes in physics university. In the morning, they get one professor that says, ‘Look, the world is so and so and so.’ And in the afternoon, they get another professor that says, ‘The world is so and so.’ … They cannot be both true. One of the two, or both, should be adapted to go with the other. So definitely we have a hole, I think we don’t understand the texture of reality.”
Uniting the two theories has preoccupied physicists for more than a century, in just one example of the slow progress of discovery, Rovelli says. So far, it’s been unsuccessful, in that no unifying theories have been proven.
“There’s a paper by Einstein in 1916 — so a century and one year ago — in which he says, ‘Look, there is a problem, and we have to figure out how gravity stays together with quantum mechanics.’ And nobody for the moment has solved that problem. Or maybe not — maybe I have solved the problem. Maybe somebody else has solved the problem. There are theories around, there are tentative theories around … but we don’t have experiments supporting them, and we don’t agree with one another.”
Rovelli says that the main problem between quantum mechanics and general relativity appears when we examine space at a very small scale. He helped found a theory of quantum gravity, called loop quantum gravity, which offers one way of approaching the puzzle. Loop quantum gravity indicates that although we’re immersed in space, space isn’t continuous, Rovelli explains. Instead, space has an atomic structure: “So there are grains of space, quanta [of] space.”
“Imagine we’re not immersed in a continuous three-dimensional thing, but we are immersed in a granular thing. So there is a minimal possible bit of space, and that’s what loop quantum gravity describes: The way these chunks of space interact with one another, their size, their shape … and how they make networks to one another. They form a kind of net, being attached to one another in these net loops. That’s where the name ‘loop quantum gravity’ comes from.”
Or to look at it another way, Einstein discovered that space is like a rubber sheet that can bend and stretch. “And if you fold quantum theory in, this rubber sheet … actually has a texture, like a T-shirt,” Rovelli says. “It’s made by little small sort of particles, except careful — these particles are not in space. They are space themselves.”
The loop quantum gravity theory hasn’t been tested yet. But if space is actually granular, it would mean “changing notions like energy and all that,” Rovelli says. It would also count space among other foundational things, like matter and light, which we now know are made up of particles.
“We thought that matter was continuous, and it took quite some time to realize that matter is not continuous,” he explains. “It’s atomic, it’s made by atoms and molecules, by particles. This was only understood about the beginning of the 20th century, not before that. It’s one of those ideas that is very old. It was suggested in ancient times. But it was realized recently.”
“So matter is discrete, and light is discrete, it’s made by photons. A wave of light is actually a cloud of photons. And it’s not surprising, after all, that space itself is sort of a cloud of little teeny small things.”
That’s not to say that Rovelli doesn’t expect space to keep surprising us. In “Reality Is Not What It Seems,” he approaches the complexity — and mystery — of quantum gravity head-on.
“The world is more complicated than our intuition,” he says. “There’s no doubt about that. Our intuition is that Earth is flat and round … that we don’t move. But actually Earth moves, and so on and so forth.”
“So science is realizing that our intuitions are often naive. Things are more complicated, they are more beautiful, and they’re very surprising when you go into them.”
Carlo Rovelli is the author of The Order of Time (Riverhead Books, 2018). He’s at Aix-Marseilles University in Marseilles, France.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. Now, I know you’ve heard of Galileo, the father of modern physics, and of course Isaac Newton and Albert Einstein. But have you ever heard of Paul Dirac? Does the name John Wheeler, maybe John Archibald Wheeler, ring any bells? If you’re a science geek, they are familiar names, both key players in the world of quantum physics.
And we can add another name to that list– theoretical physicist Carlo Rovelli. Well, he’s not quite Einstein or Dirac just yet, but Rovelli specializes in quantum gravity, a way to describe gravity using the principles of quantum mechanics. No one, not even Einstein, has been able to unite the quantum and gravity world views.
And in his aptly named new book Reality is Not What it Seems, he explores and explains how the universe is really a strange place. Carlo, welcome back to Science Friday.
CARLO ROVELLI: Thank you very much, Ira. Thank you for the wonderful introduction. I deserved it.
IRA FLATOW: We had you on for your last book, Seven Brief Lessons, where you explained physics through discrete scientific discoveries. But in this book, you took a tour, an historic tour. Is that the best way to understand modern physics, by looking backwards into history?
CARLO ROVELLI: I do believe so. By following the path [? of ?] discovery, we do get a grasp and we do get an understand of modern physics much better, in my opinion.
IRA FLATOW: And you describe how there are these large gaps of time, sometimes hundreds of years, between discoveries. Looking back, was that unusual for those periods?
CARLO ROVELLI: There have been periods in which there was a faster pace of discovery in history. There was certainly one at the beginning of the 20th century with Einstein and Schrodinger and Dirac and those people. There was one of them Newton and Galileo, and that was one of the ancient times, with Greek time.
But overall, discovery of the world is a complicated business, I believe, and it goes slowly. It has always gone slowly, and it’s still going slow now, I believe.
IRA FLATOW: Why do you say that? Why do you say it’s going slowly now? We keep hearing of all kinds of new discoveries.
CARLO ROVELLI: Well, there is obviously a defective perspective, right? One has always the impression that a lot of things are happening right now because the thing of the past are more diluted. We only see the important things.
I certainly think they’re happening. Very interesting things are happening. I believe quantum gravity is among this. But if you look at the major discoveries– for instance, from now, as you go back, you have to go back to basically to the beginning of the 20th century. It’s a century for the big, big steps.
So big steps require time. And new ideas are proposed, they have to be digested, they have to be checked. There’s a long discussion, different schools debating. All this takes time. It’s beautiful, but takes time.
I think the sense of everything is happening fast now, it’s a little bit misdirected, I would say.
IRA FLATOW: You mentioned quantum gravity. Let’s slowly tiptoe way into this. And I want to get to, by asking first, the second half of your book is about quantum physics and questions we don’t have answers to.
So what is wrong with what we currently know about how the universe is put together? We have a theory, do we not? What is wrong? What doesn’t fit?
CARLO ROVELLI: We have two basic fundamental theories, and that’s a problem. We have quantum mechanics that works extraordinarily well. And we use it for describing matter, for doing computers, for describing particle physics– the atoms, the nuclear forces, and all that. It works fantastically well.
And then we have general relativity that also works very well and describes galaxies and stars that how things fall. And the gravitational waves that were a big excitement last year. So all this is great, except that these two theories, at least at first sight, seem incompatible with one another.
So clearly, we have a problem. It’s like students of physics that go to listen to classes in physics at the university. In the morning, they get to one professor that says, look, the world is so and so and so. In the afternoon, they get another professor that says the world is so and so. It’s a completely different story.
They cannot be both true. One of the two or both should be adapted to go with the other. So definitely we have a hole, something we don’t understand in the texture of reality.
IRA FLATOW: But isn’t this a question that, as you say, for the last 100 years has occupied physicists, to try to unite these two ideas? Unsuccessfully so far.
CARLO ROVELLI: Exactly. That’s why things are slowly. That sets an example of how the progress is slow.
There’s a paper by Einstein in 1916, so essentially one year ago, in which he says, look, there is a problem. We have to figure out how gravity stays together with quantum mechanics. And nobody, for the moment has solved, that problem.
Or maybe not. Maybe I have solved the problem. Maybe somebody else has solved the problem.
There are theories around. There are tentative theories around. I describe one of these theories in my book, loop quantum gravity. But we don’t have experiments supporting them, and we don’t agree with one another– “we” I mean the physicists– on which one is the right one.
IRA FLATOW: Now that you brought it up, Carlo, let’s get into loop quantum gravity a little bit, as much as we can. Let’s first talk about things that have been talked about in recent past. String theory, trying to explain everything. Is string theory no longer working for you?
CARLO ROVELLI: Well, we are in the realm of what is not clear yet, so there are different opinions around. And that’s good. That’s how science works, with different bands fighting one another. And from the discussion, clarity emerges, has always emerged in the past. And we are in one of these periods.
So there certainly are people around who work on string theory and still hope that string theory could provide some good answers. But there are people around who work in different direction, and work in loop quantum gravity, and are far more hopeful in different directions. String theory raised a lot of hopes in the ’80s. I think it’s a little bit more humble now.
Some of the things that string theory people expected did not happen, did not work out. One example is the discovery of supersymmetry. String people were expecting the discovery of supersymmetry. Didn’t arrive.
Other things that people hoped to compute with string theory turned out to be impossible. So we don’t know for sure, but more and more people, I would say, are looking in other directions.
IRA FLATOW: And so you’re looking toward loop quantum gravity. Can you describe why you think that works? What is quantum gravity?
From reading your book, it seems like you’re trying to get to the tiny world that is down there and find the gravitational or the actual elements in smaller and smaller tiny places?
CARLO ROVELLI: Exactly. So the problem between quantum on the one hand and general relativity on the other hand appears when we look at small things. And more precisely, when we look at space at the very, very small scale.
What the theory says, what the theory indicates is the case, is that we are immersed in space, but space is not continuous like we study at school. It’s not well described by standard geometry. It actually has a structure, has an atomic structure. So there are grains of space– quanta space.
Imagine we’re not immersed in a continuous three-dimensional things, but we are immersed in a granular thing. So there is a minimal possible bit of space. And that’s what loop quantum gravity describes.
The way this chunk of space interact with one another, the size, the shape, so to say, and how they make networks to one another. They form a kind of net being attached to one another. In this net are loops. That’s where the name “loop quantum gravity” come from. And that’s a minute, minute structure of space or, more precisely, of space time.
And the use of that is that if we have this theory, we should be able to understand what happens in places of the universe like the center of black holes or the Big Bang, which are places where we don’t yet understand what’s going on because we need a quantum theory of gravity for that.
IRA FLATOW: So you’re saying that in order to unite quantum mechanics and gravity, gravity– the Einsteinian way of looking at it is geographical, so to speak. It’s bending of–
CARLO ROVELLI: It’s exactly geographical.
IRA FLATOW: If you get small enough, if you go down tiny enough, you’re saying we can actually see that the gravity is not this continuous bending that Einstein has, but it’s like tiny, tiny, weeny little particles.
CARLO ROVELLI: Exactly.
IRA FLATOW: It’s granular. And that way, it’s a quanta. Like everything else that’s tiny, it’s really united that way. It’s not as smooth as you think. It’s really quantized. Would that be a way of summarizing it?
CARLO ROVELLI: It’s a very good way of summarizing it. Einstein discovered that space is like a rubber sheet that can bend and stretch. And if you fold quantum theory in this rubber sheet, it actually has a texture, like a T-shirt. It’s made by his little, small sort of particles, except– careful– these particles are not in space. They are space themselves. It’s their collection that make up space.
Like a T-shirt, a thread of a T-shirt is not in the T-shirt. If you take away all the threads, there’s no T-shirt anymore. So here, if you take away all this grain of space, there’s no space anymore. There’s nothing.
So these grain of space are the constituents of space, so to say.
IRA FLATOW: Wow. In your book, there are a lot of really interesting and provocative things looking at quantum mechanics and looking at universe. I mean, your book is titled Reality is Not What it Seems, and it quite really took me by surprise about some of the things you said, and some of the things we take for granted in high school that we learn. And one of those is the notion of what an electron is.
I mean, from our high school class, we think it’s this object. And you say an electron is nowhere when it is not interacting. Things only exist by jumping from one interaction to another. And you also say, “Does it all seem a little absurd?”
So this electron is not what we were all taught about it is in school, going around a little nucleus and doing that stuff?
CARLO ROVELLI: Well, if you think about what you look at school, it’s a little bit contradictory, right? On the one hand, the electron is a little stone. On the other hand, that there are these orbitals, these little clouds. The electron opens up in cloud.
This is quantum theory. Quantum theory is a complicated theory that works very well. It’s 100 years old, but it’s a theory that when you look at it carefully, it’s extraordinarily surprising. And I try my book not to hide its mystery, its complication, and what it says about the world.
The world is more complicated than our intuition. There’s no doubt about that. Our intuition that that Earth is flat and is round. Our intuition is that we don’t move, that actually the Earth moves, and so on and so forth.
So science is realizing that our intuitions are often naive, things are more complicated and more beautiful, and they’re very when you go into them. It doesn’t mean they are absurd or incomprehensible. It means that you take time to digest, and to accept the complexity, and to discover the beauty of reality beyond its appearances.
IRA FLATOW: I’m Ira Flatow. This is Science Friday from PRI, Public Radio International. Talking with Carlo Rovelli, author of Reality is Not What it Seems. It really isn’t when you read the interesting things in this book.
For example, we mentioned before, taking this idea further about tiny little bits of space, that you quantize the space itself. And you say that there is a limit to how small you can make those little pieces of space. Why is there a limit? Why can’t I just cut it in half again, you know?
CARLO ROVELLI: Why shouldn’t there be a limit? Do you really believe that there is an infinite, infinite tower of smallness going into the small? That’s very hard to believe.
IRA FLATOW: That’s why you get paid to think of that and not me, because I– you know. But you have an interesting idea about what happens when you get too small, what happens to the energy in the space. And something happens there, right?
CARLO ROVELLI: Yes. Well, let me say one thing. It already happened with matter, right? We thought that matter was continuous, and it took quite some time to realize that matter is not continuous. It’s atomic. It’s made by atoms and molecules, by particles.
This was only understood about at the beginning of the 20th century, not before that. Again, it’s one of those ideas that is very old and was suggested in ancient times, but it was realized recently.
So matter is discrete, and the light is discrete. It’s made by photons. Wave of light is actually a cloud of photons. And it’s not surprising, after all, that space itself is sort of a cloud of little teeny, small things.
And yes, this requires changing notions like energy and all that. In a sense, physics as a whole has to be rewritten on the basis of these discreteness of space.
IRA FLATOW: In the modern theory, not only are we talking about space but time, right? Time is in the equations of Einstein. Is time divisible, then, into tiny little quanta? Little pieces?
CARLO ROVELLI: Yes, I think so. And time is absolutely magic and mysterious things, because time is a mystery. We don’t really understand time in the sense that the more we study it, the more we discover that it works differently than our intuition, right?
For instance, clocks go faster in the mountains than near the ocean, and clocks go slower if you move them fast, and so on. There’s a long story about time that tell us that time is a extremely tricky concepts, and I think we still don’t have total clarity about time. It’s one of these beautiful open questions in science on which so many people thinking, and it seems to be connected with open problems like what is consciousness, what is life, what is inside a black, hole how the universe began.
All this has to do with understanding what time really is. And I think we don’t know yet.
IRA FLATOW: Well, you have a lot of time to take those up in your next books. Stay with us, Carlo. We’re going to take a short break and come back and talk lots more with Carlo Rovelli.
If you want a really interesting book about physics, time, all kinds of stuff in there, Reality is Not What it Seems: The Journey to Quantum Gravity. It’s a great book to read. As I write on my Facebook page, I keep reading more and more quantum mechanics books, so maybe someday I’ll understand it. This one will actually make you think about some interesting stuff.
Stay with us. We’ll be right back. More with Carlo Rovelli after this break.
This is Science Friday. I’m Ira Flatow. We’re talking with Carlo Rovelli, author of Reality is Not What it Seems. And he is sort of shattering our concepts of space and time and all kinds of interesting things, and trying to make all those quantized.
What about, Carlo, let’s move on to something that didn’t exist up until a few years ago, or the knowledge of it– dark energy, dark matter. How do those fit into a larger shape of the mathematics and the workings of the universe?
CARLO ROVELLI: I think that dark matter is a good proof that there’s still a lot we don’t know. There’s still a lot we don’t understand, and we’re very far from the end of physics. The universe is still full of mysteries.
We have understood a lot. It’s unbelievable how much we have understood about the universe. We have the basic equations that describe so much and what happens. But these basic equations are clearly not complete results.
And dark matter has come as a surprise. Nobody ordered it. It’s just astronomers looking at the sky with their telescopes and figuring out that there is stuff there which is not stones, it’s not stars, it’s not powder, it’s not light. It’s something else. And none of the theories we have seem capable of explaining what dark matter is.
Well, great. Fantastic.
IRA FLATOW: You like that?
CARLO ROVELLI: It is something else to discover. There are some of my colleagues who says, well, we write down the final theory of everything. Now, come on. We are far away from the final theory of everything.
We are discovering step by step, solving problem one by one. And as has always happened in history for centuries, and it’s still happening now, every time we seem to have put everything in order, there’s something new that appears. There’s something that we didn’t think about or we hadn’t seen before. And that dark matter is that.
It’s actually quite fantastic. We see these galaxies in the sky. And there’s good evidence that around each galaxy, there’s a sort of huge cloud. And we see the effect of this huge cloud, but we don’t see the cloud itself. So what is it? We don’t know.
There are explanations, but there are maybe 10 different explanations, which means that we don’t know which one is the right explanation.
IRA FLATOW: We have the supercolliders, we have CERN, we have things like that. Are we stuck without a newer tool, another toy for physicists?
CARLO ROVELLI: No, we’re not stuck at all. New data are coming in from astrophysics, from telescope. The new instruments that are coming online rapidly.
So we are not in a static situation. We are in an evolving situation. For instance, now the astronomers the radio astronomers are putting all the big radio telescopes online together to make something which is like having a single telescope as large as the Earth. And with that, they’re going to see with incredible precision, detail, very far away.
And with that, they’re going to look plug the black hole in the center of the galaxy. And in a few years, we should be able to see it, to see the actual black hole, to see the black spot in the sky, which is a black hole. Which is something that seemed impossible a few years ago.
So science is not stuck. It’s going ahead. But nevertheless, the more it goes ahead, the more it finds new things which are unclear and to be understood, like dark matter.
IRA FLATOW: The mathematics works for some of this stuff– certainly quantum mechanics. It’s been right. But how do we know that we’re not fooling ourselves? As Einstein once said, “As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality.”
You come up with these theories, and you have great thoughts and theories. How do you know it actually is describing the reality of what we know? Given that we don’t know what dark energy, dark matter, is, how do we know that it’s real, the stuff that we’re describing?
CARLO ROVELLI: It’s a very good question. It’s a very deep question. I think the answer is that we are not looking for certainty. We are looking for good theories that work well, right?
If you have a map of your country where you want to go, you don’t want to be sure that the map is absolutely perfect, absolute detailed. No, you just want it to work. And if it doesn’t work, you look for a better map. And that’s what we’re doing.
Our theory, it’s a good description of what happens that allows us to make predictions, so we check the theory. Why do we think that Newton’s theory is good? Well, because our car works, our buildings stay up, our airplane do fly, and all this is computed with Newton’s equations, and so on.
How do we know when Maxwell equation work? Because we have computers and telecommunication. All this is done with Maxwell equation. So these equations work very, very, very well.
Are they perfect? No, because we have found instances where they don’t work. In the very small, with very high energy, very high speed.
So when we find that they don’t work, well, we correct them. And we never think we’re at the end of the road, but we think we do have a better and better picture as we go ahead.
IRA FLATOW: Speaking of going ahead and getting a better picture, let’s talk about your theory, loop gravity, loop theory, predictions. How do you test whether your loops are there or not?
CARLO ROVELLI: That’s exactly what we want to do today. We have been working on the loop theory for decades now. The theory’s in pretty good shape, theoretically. The question is now how do we test it.
And there are a number of ideas, none realized yet. One can use it for describing the universe and try to find sequences of the theory in the cosmological observations. What we’re doing is studying black holes. And we’re very excited because we have realized that black hole may explode.
Black holes are not forever. It’s not like pink diamonds that are forever. Because of quantum mechanics, a black hole has a finite lifetime. And in fact, it may even explode suddenly– we believe so. We’re not sure, but we believe so.
And we are trying to figure out how it explodes, and if we can see in the sky exploding black holes. If we could see, that this might confirm the theory. And we even have a little dream that some of the signals already observed by radio astronomers called fast radio burst might perhaps be these exploding black holes.
So if we can confirm that with more observation and more calculation, that would be a way of checking the predictions of the theory.
IRA FLATOW: Well, I look forward to you getting those observations, and checking out of theory, and coming back with a new book that describes finding that. Just give us a phone call if and when that happens soon.
CARLO ROVELLI: All right, I will certainly do.
IRA FLATOW: No, I’m very serious. Carlo, it’s always a pleasure talking to you. And it’s a great book. You’ve outdone yourself. I thought the seven questions were good. This is even better.
CARLO ROVELLI: Thank you very much. Wonderful talking to you. Good luck, and if I may, good luck with your new president.
IRA FLATOW: Thank you. Carlo Rovelli, author of the new book Reality is Not What it Seems and his past book Seven Brief Lessons on Physics. Both great books, both great subjects to talk about over the dinner table.