A Brief, Poetic Tour of Modern Physics

27:22 minutes

Quick: What’s the difference between a quark and a gluon? Theoretical physicist Carlo Rovelli explains that and more in the most poetic book you’ll ever read about modern physics, his Seven Brief Lessons On Physics. Rather than expounding, he’s aimed for concision, in hopes of keeping the non-physics expert engaged. The result is a tour through some of the greatest ideas in physics, in just over 80 pages.

Segment Guests

Carlo Rovelli

Carlo Rovelli is the author of The Order of Time (Riverhead Books, 2018). He’s at Aix-Marseilles University in Marseilles, France.

Segment Transcript

IRA FLATOW: This is Science Friday, I’m Ira Flatow. Have you still been digesting the gravity wave discovery? You know, general relativity and quantum loop theory– have you heard of that? Well, they’re probably not topics you’d expect to find in the culture pages of the Sunday paper. But that’s what readers of an Italian financial newspaper were treated to– a series of weekend columns by the theoretical physicist Carlo Rovelli on the greatest discoveries of modern physics. And they weren’t real dense explanations for the cognoscenti– instead, they were written in hopes that anyone could grasp these foundational ideas and appreciate their beauty.

And that is the philosophy behind his new book, Seven Brief Lessons On Physics, which clocks in at just over 80 pages. You can read this in a very short ride. And rarely has physics had so eloquent an interpreter. Carl Rovelli is the author of Seven Brief Lessons On Physics and a theoretical physicist at Aix-Marseille University in France. Welcome to Science Friday.

CARLO ROVELLI: Hi, thank you very much. Thank you very much.

IRA FLATOW: You’re very welcome. Was it a pretty brave editor who agreed to publish your physics essays in the paper?

CARLO ROVELLI: He didn’t agree, he asked me. I was pretty shocked by the idea at the beginning. But it went very well. In fact, we first talked about a single article. And then it became a series of articles. And it worked very well– people liked it then, obviously. And later a book publisher asked me to turn these articles in a book.

IRA FLATOW: Right. Let me give out our number so people can phone in and ask you questions, if I might interrupt– 844-724-8255, 844-SCITALK, 844-744-8255. You can ask Carlo questions about things you’d like to know about physics. Because he only has seven brief lessons on physics in here.

And Carlo, that’s my next question. How did you narrow down everything there is about physics into just seven lessons?

CARLO ROVELLI: Well, the choice of the topics was pretty easy, because this is a book about fundamental physics, just how we think is the fundamental structure of the world. It’s not about the physics of material, or complex systems, or anything like that. And I just made a short list of what we know about the world.

There’s not much that we know about the world– we about spacetime, we know about mental particles, we know about the cosmos, you know something about heat and time. And we don’t know much about quantum gravity, so I also described what he really don’t know. And these are the main chapters of our knowledge of fundamental physics. The hard part was to squeeze each one down to a few pages.

IRA FLATOW: I’ll bet. But you did it very, very well and eloquently. In your first chapter on Einstein and general relativity, you’re write, “And it is at this point that an extraordinary idea occurred to him, a stroke of pure genius. The gravitational field is not diffused through space, the gravitational field is that space itself. This is the idea of general theory of relativity.”

This was a whoa moment for me, I got to tell you. The gravitational field is equivalent– you know, wow. Can you explain that a little bit more? It’s, let me repeat it, the gravitational field is not diffused through space, the field is that space itself. What an idea.

CARLO ROVELLI: What an idea, yeah. It’s a fantastic idea. It’s one of the greatest ideas in science.

Well, Newton had imagined that there is this space where things are moving. Now we give it for granted. But it’s really an idea by Newton. It was a great idea.

So particles move in space. Space is like a big table over which things move. What exactly is this space is not clear.

And then Einstein had to figure out how gravity works. And he thought it’s more or less like electricity. And electricity is carried by an electric field and a magnetic field. So he had to find the analogous of the electric field, magnetic field for gravity– had to find what is a gravitational field.

Except, instead of adding a field– a field is something which is diffused all over in space– Einstein had this idea that the field was already there, and it is space. The table on which we live is not a fixed thing, but is something that moves. So that’s the key idea of Einstein.

The beauty is that out of this simple idea all sorts of incredible predictions came out, like the gravitational waves that we have discovered a couple weeks ago, or the black holes, or the expansion of the universe, a lot of marvels that nobody believed at the beginning and they all turned out to be true. That’s the marvel of physics.

IRA FLATOW: And one of the great ideas of physics now is trying to unite Einstein’s, I guess, geometrical view of gravity– it’s geometry here– with the quantum mechanics particle view of gravity. Is it possible? Should we be able to detect these graviton particles that are predicted to be carrying the force of gravity?

CARLO ROVELLI: For the moment, we are not capable of doing so. And not only that, but for the moment we don’t even have a consensus of a good idea of a good theory for that. That’s a big problem with quantum gravity– it’s one of the chapters of the little booklet.

And it’s not something we have understood. It’s something that we know we have to understand. And so there are different attempts– I work on one of these, loop quantum gravity, which you mentioned at the beginning.

But this is a theoretical idea– it’s not something we know, it’s nothing we think is going to be true. The idea is that this space, which is moving, which is also a field, is itself made by grains. So there are grains of space, [? quantum ?] space.

I think the magic of science, even more than what we have understood, is that we’re always on the boundary between what we know and what we don’t know. So we are always like explorer trying to look into the dark and try to see a little bit ahead. And we’re not sure.

IRA FLATOW: Tell me a little bit more about your idea of the loop.

CARLO ROVELLI: Oh, the loops is because. The effort as a scientist– I’m a scientist before being a writer, that’s what I really am– the effort to understand quantum gravity is to understand this granular structural space that’s very, very, very small scale, much, much smaller than the elementary particles. And in this theory we have, the structure is like if it was made by little loops or by little sort of first three dimensional chainmail, little loops attached to one another, which together weave space like a t-shirt is weaved by threads.

A t-shirt seems nothing continuous, but if you look carefully it has a structure. And space itself, we believe, has a structure– this is a quantum structure of space. So it’s not really particles, it’s not really grains of space, because these grains are attached to one another, weaved one with the other in the form of little loops. That’s the idea of loop, to bring together general relativity and quantum mechanics.

IRA FLATOW: So this is not string theory. String theory is a loop theory. It’s a bit different.

CARLO ROVELLI: Exactly. We are in a domain where there are ideas around, there is a lot of work, there is people all over the world in the university that study, but there’s no consensus yet. So there’s more than one idea. And probably strings and loops are the most studied idea at that point. So you find people who think that one is right and the other is wrong and vice a versa. So there’s a big debate. And that’s science at its best, when there are different ideas. And so far big questions scientists have ended up converging and getting to a consensus. I hope is going to happen soon.

IRA FLATOW: You know, you talk to scientists– I talk to a lot of them– and they would rather not know the answer. I mean the chase, the hunt is more fun than actually finding out the answer.

CARLO ROVELLI: You’re stretching it, maybe. I would love to know the answer. But I think it’s right, it’s true. Like people who go hunt– you don’t really care about having hunted something. It’s a search which is fascinating.

And that’s what I try to do in the book– not just say, look, we know this, we know this, we know that, but tell the story of the passion of trying to see what we don’t yet know. Make a list of what we know, but then the bigger, larger list of things we do not know.

IRA FLATOW: Let me go through some of the questions that were raised while we were reading the book. And that is why does time pass faster in the mountains way above sea level than at sea level?

CARLO ROVELLI: Oh, that’s a great question. It’s a great question because it’s one of those typical questions whose answer is very simple, it’s why not.


IRA FLATOW: Next question.

CARLO ROVELLI: The real story that we have the habit, we sort of believe that time passes the same for everybody. But why do we believe so? Well, because we have seen this all our life. But we have seen this all through our life because we don’t have very precise sense of time. It’s like why the earth is round– well, why should it be flat?

IRA FLATOW: You know, it doesn’t satisfy. I know that may be an answer. Why not– I could say that to anything, you know?

CARLO ROVELLI: No, no, no. The point is that what Einstein really understood is not that space is curved– the precise story is that spacetime is curved. So what one has to think is that when you go high time is sort of stretched, like if you take a piece of rubber and the higher part of the rubber you stretch it. As that’s similar for spacetime. So the higher part is stretching [INAUDIBLE] lower part. And the reason is because the presence of the Earth, which is a big mass, curves spacetime, so it slows down clocks near itself, so to say.

IRA FLATOW: That I get. It’s a greater curvature at the top there where you’re stretching spacetime. That’s a good answer. A couple of more questions. Let’s go to Jeff in Denton, Maryland. Hi, Jeff.

JEFF: Hi, Ira, thanks for having me. I had a quick question. For the recent discovery of the gravitational waves with the black holes emerging, my question is how did they qualify that. As in, was it a direct measurement, or was it more so like a proxy of the actual phenomenon itself. Thank you.

IRA FLATOW: You’re welcome. Let me quantify– is there a unit that we call a gravitational wave? Does it have a name, a unit, a dimension?

CARLO ROVELLI: Yes, yes. It’s a precise quantity which is measured, which is not hard to get. The antenna is something long, four kilometers. And when a gravitational wave goes through, there is a change in this length of four kilometer. So out of four kilometer there’s a change of a teeny, teeny fraction of a centimeter. And that’s what is measured. That measurement is 10 to the minus 14 centimeters. So the strength of the wave is 10 to the minus 14 centimeters over four kilometers. It’s very small– it’s smaller than an atom.

IRA FLATOW: Now they played a little audio clip when they made the discovery.

CARLO ROVELLI: Yeah, they did, yeah.

IRA FLATOW: Could you hear it? Is it in the audio range, or did they have to fool around with that?

CARLO ROVELLI: They have to fool around a little bit with that. It’s not too far from the audio. The one that they played on the radio, it’s quite emotional to hear it. It’s a little bit stretched– the real one is a little bit shorter, but not much. So on the other hand, it’s very low volume, so they to amplify it.

IRA FLATOW: Right. Talking with the Carlo Rovelli, author of the great little book Seven Brief Lessons On Physics on Science Friday from PRI, Public Radio International.

You write so poetically about the stuff, it really makes it easy to read. You have a passage at the end of your particles chapter that I especially like. I wondered if you could read it for us.

CARLO ROVELLI: Sure, it would be a pleasure. All right, a couple of paragraphs?


CARLO ROVELLI: So for the moment we have to stay with the standard model. It may not be very elegant, but it works remarkably well at describing the world around us. And who knows– perhaps on closer inspection, it is not the model that lacks elegance. Perhaps it is we who have not yet learned to look at it from the right point of view, one which would reveal its hidden simplicity.

For now, this is what we know of matter. A handful of types of elementary particles, which vibrate and flock to it constantly between existence and non-existence, and swarm in space, even when it seems there’s nothing there, combine together to an infinity like the letters of a cosmic alphabet to tell the immense history of galaxies, of the innumerable stars, of sunlight, of mountains, woods, and fields of grain, of the smiling faces of the young at parties, and of the night sky studded with stars.

IRA FLATOW: That’s beautiful, beautiful.


IRA FLATOW: How complete is the standard model? It’s not perfect yet, is it?

CARLO ROVELLI: No, it’s not perfect. It’s not perfect, because it looks patchy. There’s different pieces attached together. There’s a lot of constant numbers inside of it, not very clear where they come from. It has the look of something not nice and clean.

But on the other hand, it’s right, it’s right, it’s right. It’s all my life I’ve witnessed this– people say, oh, we’re going to discover it’s wrong. And it keeps being right. It has happened even recently– people expect the supersymmetric particles outside the standard model, and they were not found. So the standard model seems to be correct.

IRA FLATOW: What’s it going to take to get your loop chainmail as an accepted part?

CARLO ROVELLI: That’s a big question with a capital B for me. We are working on an attempt of a measurement of an effect which is predicted by the theory which is about exploding black holes. So we expect that black holes might explode because of quantum gravity. We’re trying to figure out if they explode fast enough so we could measure it. And I have a little hope before dying to see the measurement done. And that would be great happiness.

IRA FLATOW: Well, sort of what Einstein was doing through the last 30 years of his life. I’m sure you’re a lot younger.

CARLO ROVELLI: Thank you for that comparison.

IRA FLATOW: Yeah. Well, we wish you great luck. We’re going to talk more about it– we have to take a break.

And this is a great little read. Let me just repeat the book– it’s Seven Brief Lessons On Physics with Carlo Rovelli. It is not hard to read– is the kind of book you can sit down and read a chapter every now and then. Or you could read the whole thing on a long train ride or an evening in bed. Carlo, will you stay with us? We’re going to take a little break and come back and talk lots more about physics.

And talk about a question we asked our listeners– 1,500 respondents– we asked our listeners on Twitter which of these physics principles they’d most be comfortable explaining– general relativity, quantum mechanics, or elementary particles. We had 1,500 respondents, and we’ll tell you what they said after this break.

This is Science Friday, I’m Ira Flatow. We’re talking with theoretical physicist Carlo Rovelli, author of the new book Seven Brief Lessons On Physics, a great little read. And you know, Carlo, these gravity waves have really ignited interest in people. I think two major science events recently– and that is going to Pluto and looking at all the photos from Pluto and the gravity waves, these two. And a lot of listeners are asking really interesting question. Let me go to Tyler in Wenatchee, Washington. Hi, Tyler, welcome to Science Friday.


IRA FLATOW: Go ahead.

TYLER: So I know we can change the electromagnetic spectrum by inputting energy, like in radio and stuff. But could we manipulate the gravitational spectrum at all with like energy or something like that?

IRA FLATOW: Great question. Carlo, can we transmit gravity waves?

CARLO ROVELLI: Yeah, it’s a great question. The answer is in principle yes, in practice no. So if we had an enormous amount of power– but enormous means much, much, much bigger what we imagine of having– yes, we could just have a gravitational radio, even. But in practice, gravitational interaction is so small that even if we move very, very fast the mass, the amount of gravity wave produced is too small that we cannot detect them, unfortunately.

IRA FLATOW: Do we know why gravity is such a weak force– in other words, it needs such a big body to see it?

CARLO ROVELLI: That’s one of the mysteries. The other forces of nature, the other fundamental forces– electromagnetism and the nuclear forces– in comparison are much, much stronger by many order of magnitudes. And one of the things that nobody understands why is why there is this huge discrepancy in strength of the forces.

IRA FLATOW: Yeah, my kitchen magnet is stronger than gravity, right?


CARLO ROVELLI: Well, put it this way– gravity pulls you down, but it takes the entire Earth to put you down, while your kitchen magnet is attracted by the refrigerator and takes just a little piece of iron to do the thing.

IRA FLATOW: Yeah, it’s so weak, the gravity. Lets go to the phones again to Samir in San Antonio. Hi, Samir.

SAMIR: Hi, how is it going?

IRA FLATOW: Hi, there, go ahead.

SAMIR: I have a question. There was a paper written in 2014 out of North Carolina. And I read it on physics.org. And it was talking about basically saying that black holes don’t exist. It said that once the star collapses it sheds so much mass that it can’t form the black hole, hence no singularity and no event horizon. And it blew my mind when I read that. So I was just wondering what his thoughts were on that paper, if he was familiar– I believe it was out of Chapel Hill, North Carolina about in 2014.

IRA FLATOW: All right. Let me get– Carlo, what do you say to that?

CARLO ROVELLI: There’s a little bit of ambiguity in this. Sometimes people say that black holes don’t form. If this means really nothing like that forms, I don’t think this is an idea that gets a lot of support in the scientific community.

On the other hand, people might mean something less strong. I do think that black holes are not exactly what people think they are– and Hawking thinks the same. Because according to a textbook, black hole is forever, while there are good reasons to believe that black hole is not forever. So in a technical sense, that’s not a black hole. But it looks like a black hole, it’s weighed like a black hole, it behaves like a black hole. It’s just its future is different. So then is a matter of words whether you call it black hole or not.

IRA FLATOW: Let’s go to another call from Tim in Washington, DC. Hi, Tim.

TIM: Hi, hi. Dr. Rovelli, I feel like I’m talking to Lionel Messi, this is fantastic.



TIM: My question is have we collected enough data about that gravitational wave that LIGO detected that it’s possible to calculate harmonics from that? Thank you very much.


CARLO ROVELLI: Oh, yes, that’s a very good question. Yes, we don’t have just a single wave, we have a complicated shape of the wave. So we have a full spectrum of harmonics. We don’t have just a single signal. We have a rich signal.

That’s why we, or perhaps I should say them, have been able to understand that it comes from two black holes, with that particular mass, with that particular kind of motion. And they even figured out the distance. So the signal received, it’s a reach signal with a lot of information in it.

IRA FLATOW: Talking about black holes, the way you write about time as a factor of heat dissipating is really a beautiful, clear way of putting it. Let me read that. “The heat of black holes is like the Rosetta Stone of physics, written in a combination of three languages– quantum, gravitational, and thermodynamics– still awaiting decipherment in order to reveal the true nature of time.” It’s beautifully written. I mean I think of all the concepts that we have, the thing we get asked most about is time– how can you say that physicists say there is no such thing as time?

CARLO ROVELLI: There is a sense in which there is no such thing as time. Of course it has to be interpreted correctly. There is time for us, right?


CARLO ROVELLI: We have a certain amount of minutes to talk and no more, unfortunately, and so on. But this does not imply that time is a fundamental concept for describing nature. Think about high and low. We know that there’s up and down– up is up there, down is down there. But in the universe there’s no up and down. So up and down are not elementary, not fundamental notions, notions that makes sense only here around the planet when there’s a specific direction and so on.

So similarly, that is not something on which there is total consensus, but many physicists– me included– think that if you want to describe the world at the fundamental level time is not good notion. You should not describe how things change in time. Just describe the relation between different things.

It’s not complicated. It’s like instead of saying at 8 o’clock I wake up, at 9 o’clock I go to school, etc. I say when the sun is there, I wake up, when this happens these other things happen. So I can talk about the world in terms of duration between events without ever mentioning the word time.

IRA FLATOW: Yeah. If you didn’t have a clock, you wouldn’t know that there is time.

CARLO ROVELLI: That’s right.

IRA FLATOW: You would just say the sun rises, the sun sets, some stuff happens. But if you don’t measure it, it’s not time.

CARLO ROVELLI: Exactly, exactly. And Newton told us that a good way to describe the world is to think that there is a unique time that passes. But maybe that’s not a good way in general. It’s a good way only if our scales are not down in this smallest of things or in the largest of the universe.

IRA FLATOW: What excites you most about the universe?

CARLO ROVELLI: That it’s so different from our common sense. That time passes faster in the mountain than lower. That at fundamental scale there is no time. That space is different than we thought, matter is different than we thought, heat is different than we thought.

I feel we are like children who have an idea about the world and then go away from their little town and discover the rest of the country, the rest of the planet, and see more, and more, and more about nature. And every time, it’s wow, look, it’s not what we thought, it’s something else. It’s this continuous rearranging our view of the world which fascinates me.

IRA FLATOW: You know, people say, what good is studying, knowing more about time? It has an intrinsic beauty, does it not, understanding how the universe works? If you found a new Picasso painting, no one would ask you why is that beautiful?


We’ve now have found a beautiful way of looking at gravity. Why does that have to be explained?

CARLO ROVELLI: I completely agree. And I think that beauty is something which is there, we find in science, we find in physics. And beauty sometimes is not easy– to appreciate music or to appreciate a Picasso, a great novel, it takes an effort. One has to learn, it’s not immediate. But when you learn it, when you get it, you say, why? And you have new eyes for looking at the world and a new model of the world.

IRA FLATOW: Well there is great beauty in your new book, Carlo. I want to thank you for taking time to speak, to be with us today. Seven Brief Lessons On Physics. You really will enjoy this book. Carlo Rovelli, thank you very much for taking the time to be with us today.

CARLO ROVELLI: Thank you very much, thank you very much.

IRA FLATOW: Dr. Rovelli is of theoretical physicist at the Aix-Marseille University in France.

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