Do We Need a New Theory of Gravity?

Observations of the cosmos suggest that unseen sources of gravity — dark matter — tug at the stars in galaxies, while another mysterious force — dark energy — drives the universe to expand at an ever-increasing rate. The evidence for both of them, however, hinges on assumptions that gravity works the same way at all scales. What if that’s not true? In this episode, theoretical physicist Claudia de Rham explains her work on an alternative explanation called “massive gravity” to host Janna Levin.

Listen on Apple Podcasts, Spotify,  TuneIn or your favorite podcasting app, or you can stream it from Quanta.

Transcript

[Theme plays]

JANNA LEVIN: Within our cosmos, one force is inescapable: gravity. Dark matter can hide from light. Dark energy, apparently, from nearly everything.

Yet nothing can evade gravity. It’s the omnipresent puppeteer, patiently tugging on everything in the universe. Newton described gravity as a force. Einstein famously abandoned gravity as a force in favor of a curved space-time.

But what conceptually is beyond space-time? Is there a quantum gravity? Can gravity be rewritten once again,?

I’m Janna Levin, and this is “The Joy of Why,” a podcast from Quanta Magazine, where I take turns at the mic with my cohost Steve Strogatz, exploring the biggest questions in math and science today.

In this episode, we ask physicist Claudia de Rham, what might gravity be hiding at a particle level? And how might this change our fundamental understanding of space-time?

Claudia is a professor of theoretical physics and the deputy head of the physics department at Imperial College London. Her research lies at the intersection of particle physics, gravity and cosmology. For her work on massive gravity, she has received numerous honors, including the Blavatnik Award for Young Scientists, a Simon’s Foundation Award and election to the American Academy of Arts and Sciences. She’s also the author of a deeply personal new book, The Beauty of Falling, about her lifelong quest to understand gravity. Claudia, thanks so much for joining us.

CLAUDIA DE RHAM: Pleasure. Thanks.

LEVIN: I’m really looking forward to talking. I kind of want to begin, if not at the beginning, at least as far back as Newton. Newton makes this incredible leap in realizing that if gravity can make apples fall from a tree, maybe it can keep the Moon in orbit around the Earth, this vastly big leap in scales. Now, how does Einstein then make the leap away from that to something as abstract as a curved space-time?

DE RHAM: I mean, it’s incredible. You’re already absolutely right in saying from Newton to realizing that there’s so much more to gravity than just the pull from the Earth. It’s really something already very universal. And that leap forward in thinking from Newton and throughout the ages was really remarkable.

It’s something that really requires us to sit down, think about it, how something can just make an apple fall on the surface of the earth, and is not just responsible for that. It’s also responsible for the orbit of the planets on the solar system and of how the galaxies got formed, how the universe got formed. Just the understanding that gravity is something that acts on absolutely every scale and in everybody, in everyone, in some sort of universal way, that’s incredible. And going from Newton to Einstein, there’s an even deeper leap forward.

I think there’s really two leap forwards, one in the level of universality in unifying not only how gravity behaves and acts on everybody in sort of the same way, but also in unifying the aspect of space and time in the level of special relativity. You don’t think of it as being related to gravity, but it was really important.

LEVIN: Yeah, it’s a pretty big jump.

DE RHAM: It’s a huge jump.

LEVIN: It’s a huge jump. Like, why space-time? I mean, here we’re talking about forces between things that have mass. And then Einstein is like, “Ah, the mass isn’t really the most important thing.”

DE RHAM: It’s incredible.

LEVIN: It’s really any kind of energy, including massless things. It’s really about how things affect space-time, and then how space-time affects things.

DE RHAM: Yeah, the fact that gravity is universal is key because that means that it’s affecting everybody. And there’s some sort of equivalence between some motion of being accelerated, being in an accelerated frame of reference and having a feeling of gravity or having a gravitational pull.

The notion of motion is somehow ubiquitous to the notion of gravity. And therefore gravity can’t be a force by itself because you can relate to it by changing your perspective, changing your frame of reference. And so this is how you can make the leap between this gravitational pull and the gravitational attraction, acceleration and a motion in space and time, and how the structure of space-time around yourself is really what tells you how gravity affects you. That’s absolutely incredible and yet it works. It works remarkably well.

LEVIN: You really reference this in the title of your book, but to feel gravity is really to feel like you’re falling in a free space-time. That you’re just — you take everything else out that’s interrupting your fall. Right now, in my case, it’s the chair and the floor. But if you took everything, I would experience gravity as falling freely. And of course, this is a really beautiful reference in your title.

DE RHAM: That’s right. There’s actually such a beauty in falling, and I think being here on Earth, that’s a beautiful thing in itself. But also, it prevents us from fully appreciating what free fall is.

We experience gravity as, for instance, your chair or the sole of your shoes or anything that holds you on the surface of the Earth. This is the pressure in which we feel gravity, but the reality is, gravity feels [like] nothing. It’s just freedom, in some sense.

When something falls down and crashes on the floor, if you have a broken leg or a broken vase, you may think of this being gravity, but that’s actually not gravity itself. That’s just the end. The end of the fall, the end of gravity.

The beauty of falling is actually, it’s completely free. Orbits are what it is, in some sense. You are experiencing gravity in one of its purest possible ways, with nothing stopping you from this experience of complete gravity. In itself, that is something really profound and really beautiful.

LEVIN: Yeah, the astronauts in the International Space Station are floating, not because they don’t feel gravity, right? But because they’re falling. They’re falling around the Earth.

DE RHAM: That’s right. That’s right, exactly that’s right. Because they’re feeling nothing but gravity and that’s what allows them to experience it.

LEVIN: Now, Einstein called this concept, this equivalence that you’re describing, as the happiest thought of his life. And it opened up a view of the cosmos that we really did not have before, like the Big Bang and the black holes and expanding universe. All of this comes out of this transfer from Newton to space-time.

DE RHAM: Yeah, I think it’s, as you say, it’s really incredible. Because it makes you realize that it’s not just mass attracting each other, which is a beautiful concept in itself, but it’s so much more powerful than that. It’s anything, anything at all living in your space-time, then will have an effect on gravity and therefore will have an effect on itself and on others through gravity.

And so, as you say, thanks to this structure of space-time and how gravity manifests itself, anything that we can even see with our own eyes we know they are present because they have an effect on the structure of space-time on all scales that you can imagine.

And that opens up, indeed, understanding how it’s not just about the motion of the planets or the structure of galaxies. It’s about the whole being of the universe by itself. Just the notion of universe, the notion of having a Big Bang requires some notion of time and so is ubiquitous to the notion of gravity itself. And then setting up alive that whole structure of the universe, and something that has a life of its own, in some sense.

It’s very dynamical. It’s not this static notion that something falls because you let it drop. It’s really keeping alive the whole structure of the universe in some sense.

LEVIN: So here’s this incredibly, unbelievably successful description of the entire universe. From all of a sudden, we have a scientific notion of a Big Bang, which surprised even Einstein, the universe is expanding, there’s this dark energy and dark matter, as you’ve said, which we cannot see, but we can only infer from their gravitational effects.

And yet, you say in The Beauty of Falling that general relativity is forthright about its failure. And you say that there’s a point where the theory must fail, where a brand-new layer of physics waits to be unveiled. Will general relativity really fail and reach its limit?

DE RHAM: Oh, yeah. I think I’m not the only one to think that. I think I would say the beauty of gravity as described by general relativity is that we know there will come a point where this general relativity description fails to provide an accurate description of what’s going on.

You’d think that failure should be disregarded or it’s a defect of the theory. And actually, I would think it’s something extremely profound. It’s an opportunity. It’s very perfect for a theory to come up with its own signal of when it will break down. If we had that of absolutely every model we had, we would go much further, much faster. We’d know when to stop. We’d know when we have to open eyes for something new.

But we know something is not quite right in Einstein’s theory of general relativity. When it comes to black holes, for instance. We know that if we were to trust Einstein’s theory of general relativity, as we go very, very close to the very center, the inside of the black hole, there are some observables that in principle we could measure that would seem to become as large as you want. Almost infinite.

And that is not to say that there is a physical observable which can be infinite in nature. You wouldn’t have that. But what it means is that the tools that we’re using to describe what’s going on there have failed. And that’s Einstein’s way of telling us, “OK, I’ll stop now.”

LEVIN: “You have to do something else.”

DE RHAM: Yeah, you have to take something else. It’s really beautiful. It’s really profound for a theory to predict its own failure. If we had had that from Newton’s theory of gravity, we would have been able to predict, possibly, general relativity or what would have come next well before we needed observations to tell us that they didn’t match anymore.

LEVIN: Mm hmm. So these singularities, like in the Big Bang or at the center of black holes, which you’re saying you don’t believe are physically true, you take them instead as the theory telling us that it’s failing — what is it provoking us to reach for? It’s telling us, “OK, at these singularities, I’m not doing so well.” Is it also telling us what we should reach for to try to fix the problem?

DE RHAM: I think what’s really interesting is that what doesn’t work is our description of space and time. So using some notion of space and some notion of time to describe what’s going on is probably not the right way to go. And so as you go very close to the center of the black hole, at that position, something breaks down. Even the way I’m phrasing the question requires me to have a notion of space.

When I say very early on in the history of our universe, very close to the Big Bang at that time, general relativity stopped breaking just in saying these words. It doesn’t make sense because the notion of time probably does break down. And so we’ll have to probably come up with new tools to even phrase the questions we want to ask ourselves, and then possibly we’ll be able to go ahead and make progress.

It really challenges us in the very way we’re trying to formulate the questions, let alone trying to solve them.

LEVIN: I’m going to ask before we even get into the quantum aspect to step back and think about gravity in analogy with something like electromagnetism.

So we know that in our other theories of matter, like electromagnetism, that there are waves in the electromagnetic fields, and that those waves create light. It’s one of the most familiar phenomena human beings ever come across, light. And that really is just waves in the electromagnetic fields. There is something similar for general relativity, even before we get to the quantum level.

DE RHAM: That’s right, that’s right. So for Einstein’s theory of general relativity, gravity is very different from the other fundamental forces of nature. But we also know how to describe gravity, even in the context of general relativity, in a very similar way to the other fundamental forces of nature. Like, you say, electromagnetism.

In electromagnetism, we know that there’s some propagation of electromagnetic waves, for instance, which we call light. That’s just light. And for gravity, actually, the same thing has been observed now. We have a propagation of a wave of gravity. We call them gravitational waves. We’ve almost seen them. We’ve felt them with our instruments here on Earth. And that in some sense tells you that this is where to look for the fundamental force.

For electromagnetism, the fundamental messenger at the particle level is the photon. And so the same thing happens for gravitational waves. Lots of quantum of gravitational waves, you would call the graviton, a propagation of gravitons. So there’s a direct analogy of how you think of the electromagnetic wave and of the photon to how you can think of gravity and gravitational waves and the graviton.

LEVIN: Now, it is one of the most profound dualities of modern physics that light is simultaneously a wave and a particle. Endlessly fascinating and confusing. And here you’re telling me, well, that’s the same for gravity, that there is a duality. There is a gravitational wave. A wave in the shape of space-time. And then there’s a particulate version, the graviton, a particle. Now we’ve never seen the graviton, but we have seen the waves. Is that right?

DE RHAM: And that’s exactly right. When we detected gravitation waves here on Earth, you may think of it as you having detected many, many, many gravitons, something like 10⁴⁰ gravitons.

LEVIN: That’s a big number.

DE RHAM: A big number. So for us to say we’ve detected the quantum nature of gravity per se, we should be able just to look at the effect of one, one of them, out of this huge classical wave of 10⁴⁰ gravitons.

Now that’s a little challenging because if you just want to look at the effect of one of them, you need to have something which is 10⁴⁰ times more sensitive.

But it’s not even possible because the way it would deform space and time, just one graviton through this experiment, it would be below Heisenberg’s uncertainty principle. So it’s not experimentally challenging, it’s simply theoretically impossible to just detect a graviton with the instruments that we have.

LEVIN: I want to pause to appreciate the fact that these particles of the gravitational force, gravitons, even though they’re exchanged between massive particles and energetic particles, might not have any mass themselves.

DE RHAM: That’s correct. Yeah. Just like if you think of two electrons, for instance, and the electric — the static force, between the two of them, you can think of it as being mediated by the photon. And that mediator is completely neutral. It doesn’t carry any charge itself. If it had a charge in itself, it would sort of compromise a little bit the way the electromagnetic force or phenomenon would occur.

And so if you think of how two masses communicate their gravitational attraction to one another, that should go through gravitons to some extent, and the messenger itself carries no charge per se with respect to gravity. And you may think the analogy of that is to have no mass, so the messenger itself has no mass.

But the analogy is not perfect because we’ve seen that, for gravity, everything is charged with respect to gravity. Everything has an effect on gravity. And so gravitational waves themselves and the graviton itself will have an effect on itself. And so once you understand that, the reason as to why the graviton is actually a massless particle may be questioned.

LEVIN: It’s not so sacred.

DE RHAM: It’s not so sacred anymore from that perspective. That’s right.

LEVIN: So if I were to just look naively at general relativity, it seems to predict a massless graviton. And that is to say that both the gravitational waves and the graviton would travel at the speed of light, if that were true. And is that something we can test, whether or not these gravitational waves from colliding black holes are coming to us at the speed of light?

DE RHAM: Yeah, it’s incredible, right? What we know about the universe and about fundamental particles is so deep, it amazes me every day. We do know, we do know that we had observation of gravitational waves being emitted by two, not black holes, but two stars merging.

And because it was two stars, two neutron stars, as they were merging and emitting gravitational waves, at the same time, they also emitted light, electromagnetic waves. And so we saw it with our instruments. We saw it with some telescopes and we felt them with an instrument on Earth, the gravitational waves.

So we saw the same event through different channels of communications, one being gravitational, and the other one being electromagnetic, being light. It’s a multi-messenger event. And so those two waves, light and gravitational waves, were emitted at the same time and received here on Earth at the same time as well. So they traveled exactly at the same speed.

LEVIN: Mm hmm. And how do we know that they were emitted at the same time?

DE RHAM: Ah, yes. So there’s a little “cheep” in a little signal of when the merger happens, for instance. And then, from the gravitational point of view, we know, when the two neutron stars bubble and they become actually black holes and the bubbles emit a very specific type of pattern. So we know precisely when the merger itself happened.

And similarly for light, there’s a particular “cheep” that happens precisely at the merger itself, before the two become a black hole. But what’s interesting, actually, is that when I say they came in exactly at the same time, light came in a little later, um, 10^-15 seconds later, because light actually in that medium, it propagated — got slowed down.

So it’s light that wasn’t quite traveling at the speed of light. Gravitation waves were doing fine, but light was traveling a little bit slower.

LEVIN: Right, it’s hard to stop the gravitational waves, but you can easily keep the light trapped by scattering it around.

DE RHAM: That’s right. That’s right, exactly.

LEVIN: We’ll be right back after this message.

[Break]

LEVIN: Welcome back.

So I’m with you so far, that there’s this beautiful theory of space-time, there are waves in the shape of space-time, that when quantized, I should imagine that those waves are made up of lots and lots of these little particles, these gravitons.

Why did scientists in the 20th century begin to speculate that gravitons might actually have mass after all?

DE RHAM: So I think people came through in various ways. I think originally people were just exploring, right? This theory of Einstein’s theory of general relativity came up, and it was a theory of a massless graviton. But we know some of the force carriers do carry a mass and that’s the case for the weak force.

The W and the Z bosons are some force carriers for the weak force, and we may not be as familiar to this weak force because it is weak. And the reason it’s weak is because the particles mediating that force carry a mass, the W and the Z bosons. And nowadays we know that the Higgs mechanism actually plays a crucial role in understanding how to give a mass to those particles. So by mass, we mean an inertial mass. It means that they don’t want to be pushed along for as far a distance as normal light, for instance.

So I think originally, definitely around 1939, [Markus] Fierz and [Wolfgang] Pauli started exploring what could happen with the quantum field theory level for some similar particles as the graviton, which would carry a mass.

So it was very much some investigation, because we wanted to understand better what are the fundamental blocks of nature and not necessarily to think of them as being a substitute for gravity, but just to understand, to put structure of the fundamental blocks of nature all around ourselves and better understand what could happen in principle.

But as the time went on, in observing how our universe started evolving, people realized that the expansion of the universe was actually accelerating. And so that led to some wonder, wonder at what is happening in the universe. And we can postulate, as you mentioned at the beginning, the presence of dark energy, which is a driver.

It’s a funny type of fluid that would fill the whole universe, which would have negative pressure. And it should be everywhere in the universe, in almost constant energy density. The bigger the universe is, the more it is, because the energy density is constant. If the volume of the universe increases, then the total energy stored in dark energy increases as well.

Of course, I’m making analogies here in terms of the size of the universe increasing. The universe itself is infinite in size. We think so, or if it is finite in size, it’s still very huge. But the universe could be infinite in size and still its physical size between the way it stretches, the structural space-time, can increase as the structure of the universe expands and even accelerates due to the presence of dark energy in our universe.

So these were observations. But it goes against the natural thought that we should have gravitational attraction with everything present in the universe. So if it wasn’t for dark energy, we would have thought that the expansion of the universe would have slowed down, not started accelerating. So on one hand, we would need to postulate the presence of dark energy to explain this acceleration of the universe.

But another thing that we know is that the nothingness, if you will, the energy in the vacuum, is not empty. And we know that very well from the detection of the Higgs.

LEVIN: The Higgs particle, or Higgs boson, was first discovered in 2012. This particle, and its field complement, is responsible for giving other particles in the universe their mass.

DE RHAM: There’s that funny joke that says, the discovery of the Higgs is discovering what nothingness is made out of. And it’s quite something.

It’s this presence of vacuum energy, present everywhere in space and time. And just from the — for instance, from the presence of the Higgs field, we know that we should have a contribution from the Higgs itself in the vacuum energy of the universe. Within our understanding of quantum field theory, we would expect the contribution from the Higgs field and from all of the other massive particles to play a role in the vacuum energy. And that looks exactly like dark energy. And that should lead to an accelerated expansion universe, which is way, way bigger than what we’re actually observing.

And so that led many scientists at the beginning of this century, actually, to wonder whether this observation or this realization is coming from the fact that gravity is modified at very large distances. Whether our observations of the accelerated expansion in the universe and the need of dark energy, which seems to be not completely consistent with the vacuum energy, that we expect being there, if this is a pitfall from using general relativity as a description of gravity in a regime where we never tested it before.

And if instead gravity could have a finite range — and by that what we mean is, behave on very large distance scales and time scales as a weaker force — then it could reconcile our understanding of the vacuum energy and dark energy and the acceleration of the universe.

LEVIN: So, would that mean if I give mass to the graviton, that I make it less effective on larger scales?

DE RHAM: Exactly.

LEVIN: And as a result, this conclusion by looking at the behavior of space-time — namely that it’s expanding at an accelerated rate — is no longer requiring dark energy at all?

DE RHAM: It doesn’t require dark energy at all. What it would require is simply what is actually already present in the universe, which is the vacuum energy of known particles.

LEVIN: And that would be sufficient.

DE RHAM: Exactly. So the effect of the known particles in the universe and their contribution to the vacuum, to nothingness, could lead to the accelerated expansion universe by an amount which could then no longer be in tension with our observations.

LEVIN: So beyond just — well, “let’s explore what else could be, let’s add a mass to the graviton, just see what happens,” there was also a sense in which the graviton mass hypothesis was difficult to reconcile with a cosmological framework consistently. What were the problems that arose?

DE RHAM: Yes. So one thing, and you touched upon it already in discussing, for instance, about the speed of gravitational waves. So let me just say, for instance, if the graviton had a mass, a massive graviton, then we wouldn’t expect necessarily that gravitational waves would all propagate at the speed of light.

They would propagate at a speed very close to light at a very high frequency, and at very, very low frequencies, we would expect gravitational waves to slow down considerably

And so that understanding by itself tells you that if gravitational waves in principle don’t need to propagate at the speed of light anymore, we can have some sort of longitudinal polarization of gravitational waves.

And we are familiar with that with, for instance, sound waves. Sound waves are pressure waves in the air, and they are longitudinal waves along the line of propagation, of the sound wave. It gets closer together and then further apart. These are longitudinal waves.

For Einstein, in the gravitational waves, you wouldn’t have a longitudinal wave coming in because all the waves have to follow a very strict regimen in terms of the speed they have to follow. So you have no freedom in having this sort of longitudinal mode, or waves. But as soon as you give that up, it doesn’t matter what the actual number is.

It opens you a possibility for new phenomena to occur. And in particular, for this longitudinal mode to arise. And that in itself, you may think, well, that’s just potentially a signature. This is something to look for. But this is where the beauty again of space-time comes in.

These are not simple sound waves that you can listen for. These are messing up the way space and time, particularly time, flows. And if you make time flow and you speed it up and you slow it down, you actually mess up energy itself, and it leads to actually a wave that can have negative energy because you mess up with the flow of time along this propagation.

So you end up with a phenomenon which opens up a possibility for energy to decrease. You send those longitudinal waves and that decreases energy. And so nothing would prevent you from sending an infinite number of those longitudinal waves and compensate them with the normal gravitational waves at no energy cost at all.

And so in some sense, you can have a huge explosion of gravitational waves in your face at no energy cost at all. And that in itself is something which is meaningless. You know, this is not occurring.

LEVIN: That’s so unstable that we would see that happening in the universe.

DE RHAM: Exactly, exactly. So one of the key things was understanding if you have — if you’re thinking of a theory where the graviton could potentially acquire mass, you need to make sure to stabilize it in a very precise way, so as not to allow for this possibility. Not to allow for this new channel to come in and destroy the very notion of energy. And therefore the very notion of space-time along its way.

LEVIN: Now, you proposed, with collaborators, a new theory of “massive gravity.” Is that what you were working towards? A way to make it stable against basically destroying the universe [laughing] as we know it?

DE RHAM: [laughing] That’s right. It’s called being a ghostbuster.

LEVIN: Yeah.

DE RHAM: That’s a scientific term, actually. I’m not, I’m not making it up.

LEVIN: These were called “ghosts” because they were negative energy?

DE RHAM: Yeah, think of it as a particle with negative energy. Really, it tells you it can’t exist. If you have a theory where in principle it could be there, then you know that theory is simply not valid. It doesn’t make sense. So what we had to do was to think of a way to come up with a model of massive gravity where naturally this mode would not propagate.

We sort of attach the theory itself in such a way that it’s stable in its own right without wanting to propagate this mode in itself.

LEVIN: And in this model, where you free it, as you say… This really is a technical term of these “ghosts,” these negative energy modes that are really unstable. How does that reshape our understanding of space-time?

DE RHAM: That to me is the key. You can ask yourself a question. What would happen if the graviton has a mass? And you can make prediction against observations. You can think of having the vacuum energy as leading the accelerated expansion of the universe.

So the idea there is really to try to reconcile two of the modern pillars of physics today, which is quantum field theory on one side — and it’s been tested to incredible accuracy. And then gravity on the other side, which also has been tested to an incredible accuracy. But put together, they don’t seem to match up, not only at very small scales but also on large cosmological scales.

And so in this theory where the graviton would have a mass, first of all, you think of gravity much more at a quantum field theory level, like the other fundamental forces of nature. And then you are able to reconcile potentially the vacuum energy from particle physics with the effects on gravity.

There are other things as well that you can try to learn in a theory of massive gravity, because you’re thinking of gravity very much at the quantum field theory level. You can start looking at some of the effect of what quantum gravity means for some of the excitations of gravitational waves in a regime which would happen well before what you would expect it to happen for general relativity. So in some cases, it is a toy model for looking for a theory of quantum gravity.

LEVIN: Mm hmm. And would you say it’s the beginnings of probing the quantum aspect but not yet at the highest energies where the full quantum behavior would be revealed?

DE RHAM: That’s absolutely right. With that theory, we’re very far away from understanding how the standard gravitational waves would be real quantum gravitons in the full quantum regime.

LEVIN: Now, you’ve mentioned dark energy, which was, as we discussed in the opening of this show, able to hide from light but not from gravity. And also the other example, dark matter, which is able to hide from light, but not from gravity. Could these massive gravitons themselves be an example of dark matter?

DE RHAM: Yeah, some people have explored that possibility as well. And then one possibility in that case is not necessarily that gravity itself is massive, but in addition to gravity, you could have other particles that look very much like the graviton and themselves have a mass. People have been exploring models of multi-gravity. In some sense, you have multiple copies of gravity and the other copies have themselves a mass and those different copies would act as dark matter.

LEVIN: So, if everything hangs together, you’ve got a pretty good approach to some of the big quandaries in modern cosmology, at least about the dark sector, the hidden sector. What was the scientific community’s response to theories of massive gravity?

[Both laugh]

DE RHAM: That’s a good question.

LEVIN: We’re chuckling to ourselves.

DE RHAM: You’re in the field as well. So I think I’m sure you can imagine.

So, I mean, in all honesty, there was very mixed feelings. The thing is this idea, the idea of massive gravity per se, or the idea that graviton could potentially have a mass, was not new. It has started being explored by Fierz and Pauli to some level, not quite at the level of gravity, but at the particle level, already in 1939.

And then throughout the past centuries, people have been exploring it over and over again. People thought that there was no way to avoid this ghost, to avoid this longitudinal mode from ever being excited. And so throughout the ages, people were just convinced that it wasn’t a possibility.

And I have to admit that myself, when I started working on it as well, I didn’t think it was a possibility. It’s quite easy to convince yourself that there’s some proofs out there that prove [to] you that it doesn’t work.

And it’s beautiful to think that general relativity is unique. And if you grow up with this feeling that general relativity is unique, and if someone comes and tells you that, “Actually, I’ve come up with something else,” I wouldn’t believe them. I wouldn’t believe them. You really need to go through the details of what made general relativity so unique to really understand and what can happen next.

So there was a part of the community that simply did not want to believe it because it was preconceived that general relativity was unique. There was no other possibility. And they knew there were theorems out there, that proved so without necessarily having gone through all the details of those proofs themselves.

But then there was another part of the community that was a little bit more open to understanding if that were true, what would have been going on wrong with the other proofs that were out there?

And that required a lot of work to go through the nitty gritty of everything. It was hundreds and hundreds and hundreds of pages of calculations. We can talk about it. We can talk about the concepts, but at the end of the day, you need to go down to the mathematical proofs, and you need to understand really the details of them, and then lift yourself back up again and try to explain them in more intuitive ways.

LEVIN: So these pages of calculations convinced you that it was possible. What would convince you that it’s actually true?

DE RHAM: Oh, you would need to see observations that would be consistent with massive gravity and would no longer be consistent with general relativity.

LEVIN: Like very long wavelength gravitational waves traveling slower than the speed of light? Would that be something?

DE RHAM: Yeah, for instance, you could do that. Yeah, exactly. So very long wavelength gravitational waves is definitely one of the ways to go because they explore what happens at very low energy scales, much closer to the scale of the graviton. And so for instance, seeing if there’s a structure in the power spectrum of primordial gravitational waves would be one definite way to go forward.

LEVIN: So gravitational waves left over from the Big Bang itself.

DE RHAM: Exactly. Exactly.

LEVIN: Now, I want to ask you a little bit, because you wrote so personally in your book, about your kind of beginning in this field, and in particular, you describe a series of ups and downs — the response to the massive gravity idea as perhaps being one example. But you also wanted to become an astronaut. Can you tell me a little bit about the trials and tribulations of that campaign?

DE RHAM: Yeah, I think that’s a good example of perseverance — a little bit like dark energy, where it has to be persevering for billions of years. It wasn’t quite that long periods, but it was, it was a long period of perseverance where I had just one goal in my life before massive gravity. I had just one goal, which was to become an astronaut.

And I guess I’m not the only one. If you go anywhere, many people want to become an astronaut, but it really became sort of my way of trying to make sense or even take charge, or trying [to] set myself a goal, which I would work towards in almost — in a way to give me an illusion that I could do something concrete in that direction.

So for 20 years, everything I would do is really to try to become an astronaut. And I can’t quite change who I am. But you can try your best. There are activities that you can try to do. There are ways to make you feel as much as possible as if you were an astronaut in sense of free falling, like scuba diving. It’s not real free fall, but it’s almost as close as you can have it on earth.

I learned to fly as well. So I became a pilot when I was in Canada. And each one of these experiences, even though it was a little bit to do them because I wanted to become an astronaut, they became passion of their own and having a life of their own as well. And I guess it was all part of the same idea of exploration and challenging gravity at some playful level.

LEVIN: Mm hmm. And how did that interest turn to theoretical physics and cosmology?

DE RHAM: So I think I like this notion of universality of gravity. I think for me, this is very profound in how I can connect everything. It is the most fundamental thing you can think of. Everything is very complicated. So if you take everything, you try to remove all sorts of complication. The only thing you have left is space-time. The only thing you have left is gravity. So in some sense, I’m studying gravity because it’s possibly the simplest thing you can ever study.

LEVIN: Yeah, it’s hard to convince people of that, but yeah, there’s this really surprising simplicity to cosmology, the universe on the largest scale. It just yields to these simplest, most beautiful concepts.

A question we like to ask here at “The Joy of Why” is what in your research, although I hear it in your descriptions already, brings you joy?

DE RHAM: I think every little ups and downs brings you joy. There’s not a dull day, definitely not. I think sometimes I wake up in the morning and it’s the little things, it’s the little “ahh, I think I know how to do that.” And it’s not something I’m necessarily going to be worth sharing with anyone else.

It is just small discoveries for yourself and how it’s almost tricking nature. It’s almost tricking math to make them work in a particular way. And you realize that nature manifests itself in a way you never had anticipated before. Those little tricks, they’re so beautiful. Sometimes they lead to very important discoveries, like Einstein did. But for, for most of us in our everyday life, that’s not what happens. And yet, there’s a little bit of a playful way to work with theoretical physics. It doesn’t seem like that. If you look at my blackboard, most people will feel daunted by it, but it’s actually sort of a little witty game that you have with nature and with the way it manifests itself.

LEVIN: Hmm. Math is your friend when it comes to these concepts. It helps. It guides you. We call it “following the chalk,” right?

DE RHAM: Oh yes!

LEVIN: You follow the math, as opposed to trying to force it to follow you.

We’ve been speaking with Claudia de Rham about massive gravity and her new book, The Beauty of Falling. Claudia, it’s been such a pleasure speaking with you on “The Joy of Why.” Thanks for joining us.

DE RHAM: Thank you very much, Janna. A great pleasure. Thank you.

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LEVIN: Thanks for listening. If you’re enjoying “The Joy of Why” and you’re not already subscribed, hit the subscribe or follow button where you’re listening. You can also leave a review for the show — it helps people find this podcast.

“The Joy of Why” is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests or other editorial decisions in this podcast or in Quanta Magazine.

“The Joy of Why” is produced by PRX Productions. The production team is Caitlin Faulds, Livia Brock, Genevieve Sponsler and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Morgan Church and Edwin Ochoa provided additional assistance.

From Quanta Magazine, John Rennie and Thomas Lin provided editorial guidance, with support from Matt Carlstrom, Samuel Velasco, Arleen Santana and Meghan Willcoxon. Samir Patel is Quanta’s editor in chief.

Our theme music is from APM Music. Julian Lin came up with the podcast name. The episode art is by Peter Greenwood and our logo is by Jaki King and Kristina Armitage. Special thanks to the Columbia Journalism School and Bert Odom-Reed at the Cornell Broadcast Studios.

I’m your host, Janna Levin. If you have any questions or comments for us, please email us at [email protected]. Thanks for listening.

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