What is massive gravity & how is it saving life on earth? Imperial College of London physics professor explains

In her book, 'The Beauty of Falling–A Life in Pursuit of Gravity' (Princeton University Press, 2024), De Rham explains why gravity is so cool that she learnt to deep-sea dive, fly planes and almost became an astronaut to study it. In a virtual interview ahead of the Jaipur Literature Festival 2025, where De Rham will be in conversation with Mukesh Bansal on "The Incredible Gravity of Being" on February 1, De Rham spoke to Moneycontrol about gravity, Einstein's theory of special and general relativity, why blackholes are so incredibly interesting and the idea behind massive gravity. Edited excerpts:

What is the coolest thing about gravity that most people don't know?

The coolest thing about gravity is that we all connect to it. I can say these words, but probably most people won't know what I mean by that. Really, the coolest thing about gravity is that you can never be immune to it. If you take electromagnetism... you can create Faraday cages, so you are shielded from electromagnetism. For gravity, that can never happen—no matter what you are, no matter who you are, no matter whether you're big or small, very massive or not, you can be very light, you can be a blackhole. You will always interact with gravity—always.

Gravity is an equalizer. It puts everything, everyone on the same footing. It really doesn't matter what your size is, or your where you come from. Even at a fundamental level, it doesn't matter what your charge (negative, positive or neutral) is, or anything at all. Because of this equivalence in which it connects with everything and anyone, this universality, we're all equal with respect to it, so we all can use gravity to see beyond our typical senses, and to connect with the dark sectors of the universe.

On earth we have the impression that gravity affects different things in different ways, and that's because we have the friction of the air. Like when I drop a piece of paper and another object, a pen, for instance, then the piece of paper in its fall is going to be subject to the friction of the air more so than other things and that's why it's not affected in the same way. But we can do that experiment on the moon, and then you would see that if you drop a feather and a hammer on the moon, they experience gravity in exactly the same way. So that's one picture of universality where you see that gravity affects different things that have very different shapes, that have very different masses in exactly the same way.

And you can even push this a bit further and ask yourself what if the feather was even lighter? What if it didn't even have any mass? We typically think that gravity is how different masses attract each other. But you don't even need to have a mass. You can be something massless like light, and you will still have an effect on gravity. You will still, just by your presence, by your energy, by your pressure, you will have an effect on things around you through gravity, and you will be affected gravitationally by all the things.

We do see that, for instance, you can have the sun or a star, and light from behind the star gets bended from the gravitational pull of the star. So gravity is not just a connection between two different masses. It's a connection between anything in the universe; anything at all.

It's very much this notion of exploring things all around us. Of course, we can understand gravity here on earth. But what I'd like to understand more is how nature behaves at a more fundamental level. The idea of becoming an astronaut was to merge bit of this physical idea of exploration of a different environment altogether, with, of course, understanding that in this (other) environment, you take away some of the distractions you have on earth.

Yes, you have gravity on earth, and you can test it. But you have so much of it that you get distracted by all the pull from the gravitational field of the earth. You can't let yourself go; to really experience gravity fully, you should stop the friction with air, you should stop this being pushed by the ground, by the chair, by your shoes (, etc.).

To really appreciate gravity, you need to put yourself in an environment that's free from all of those distractions, and then you can really let yourself go; really let yourself fall completely, and that's when you can actually experience gravity without all of the other artifacts that come along with it.

Speaking of space, people seem to be endlessly interested in black holes. Are they really as interesting, even for physicists?

Oh, they really are precious. They really like pearls. Let me say, first of all, that black holes are not just theoretical objects. They're not just mathematical objects. They really have been observed. We have very strong evidence nowadays observationally that there are black holes in the universe. There is, for instance, a supermassive black hole at the centre of our galaxy which is called Sagittarius A*. And we understand very well how light gets deflected by that blackhole; or, for instance, when stars go around the black hole; we also understand when black holes merge together, they produce gravitational waves. So we have a deeper and deeper understanding of black hole.

But the reason we're really so fascinated about blackhole is that—well, there are different reasons—one, which is almost more conceptual, is that it's almost a separate region as compared to us. Once you enter into a blackhole, you can never come out of it. So there's a little bit of mystery from that. There's also that space and time start becoming quite weird when you go inside a blackhole, and the notion of space and time get interchanged as you fall inside a blackhole. So it's mysterious, but that's also part of the fascination.

One of the most essential elements as a theorist for me, as a cosmologist, as to why we're so fascinated by better understanding blackholes is because at the very centre of blackholes, the curvature of spacetime becomes so important that we know that it goes beyond our reach using Albert Einstein's theory of general relativity. So we know that the theory that we have at our disposal does break down at the centre of the blackhole. And so, being able to better understand what goes beyond the notion of Einstein's theory of general relativity, and possibly it's a theory that goes beyond the notion of space and time, would allow us to uncover new layers of physics and cover new layers of nature, go beyond our current understanding.

And for me, as a cosmologist, it's very interesting, because understanding what happens at the very centre of black holes, where the (time space) curvature is extremely high, those environments are very similar to what happens at the very, very beginning of the universe, at the Big Bang, at the moment of singularity, where we also think that our current description of what happens there using Einstein's theory of general relativity, breaks down so we can no longer use it. If we want to understand our origin, the origin of the universe, even the origin of space and time altogether, we need to have a framework that allows us to go beyond Einstein's theory of general relativity, and being able to understand: What is that framework? What is this underlying layer of physics of nature?

Being able to understand what made the whole universe, what made you and me, and what possibly is the fate of the universe. So it's uncovering all of the questions of humanity. Some of the deepest questions you may ask yourself. Also, this is very universal. We may come from very different civilizations—some civilizations have died out, some new civilizations come—there's many different cultures. And in all cultures, in all civilization, we always have this fascination to try to understand and make sense as to why we are here, where we're coming from. We have this need to understand that. This is a long way to tell you that understanding nature at the very center of the universe, is the key to us understanding our origins.

Is the theory we are talking about here that the speed of light is universally the same, and nothing can go faster than light?

That is Einstein's theory of special relativity, which he wrote down in 1905. In special relativity, we don't have gravity; it's more of a notion of how different observers that move with respect to each other will observe a different flow of time, as well as a different flow of space, in such a way that the speed of light is always the same for everyone. But this is in what I'll call a flat space without gravity, without being accelerated.

In 1915, 10 years later, Einstein promoted the theory of special relativity, which was a theory without gravity, to a theory of general relativity that enables us to understand not only what happens when different people are moving with respect to each other, but also when they accelerate with respect to each other, which is a much more complicated situation. And also, when there is matter present, acceleration present because of these masses, encoding the gravitational attraction, encoding all of the forces of nature; well, encoding, at the very least, the gravitational force through the curvature of spacetime.

The theory I'm talking about, that we need to have a better understanding for at the centre of black hole, goes beyond special relativity, which tells you that the speed of light is universal. And what goes beyond special relativity is general relativity. But what we need to have is something that goes even beyond general relativity. People have different candidates for what this theory could be; in particular, string theory is one of the most fully fledged candidate theories for a theory of quantum gravity that would surpass Einstein's theory of relativity. But precisely understanding the connection between all of this is not fully there yet.

Cover of 'The Beauty of Falling' by Claudia de Rham (Princeton University Press, 2024)

If humanity could figure out a way to travel at the speed of light, would we be ageing slower? Would this be the ultimate anti-ageing formula?

It absolutely would be. The faster you go, the more relative your time becomes. To the point that if you are traveling at the speed of light like a photon, then time will have frozen altogether. That's absolutely right: If you're travelling at the speed of light, you won't even feel time lapsing. That's true, but of course you can't get there so precisely what you would feel is hard to say.

You've talked about the speed of gravity, and you've also mentioned that the speed of gravity could theoretically be more than the speed of light. What is the speed of gravity? And what is the mass of a graviton?

Most people have heard of the concept of speed of light. But that is not very intuitive, because for us, light is just instantaneous. Just like gravity is just instantaneous: We have the earth, we are here, and we are attracted by the earth is just something that happens all the time. The reality is that light takes some time to propagate. When I switch on the light, it's not that instantaneously I start seeing clearly. It takes a very small amount of time, but there's a propagation of this electromagnetic wave—light is an electromagnetic wave. They are fluctuations in an electromagnetic field.

To understand this better, imagine dropping a stone on the surface of a pond. Now, there're going to be some waves travelling on the surface of the pond, and they'll be travelling at a given speed and in all the directions. So, waves travel at a given speed, quite slowly on the surface of the pond. For the electromagnetic waves, they are fluctuations on the structure of the electromagnetic field, and they travel at a given speed.

This is also related: if I have an electron here, and, all of a sudden, I put another electron there, it's not that instantaneously they're going to feel an electric force between the two of them. It takes some small amount of time which is related to the speed of light and the distance between those two electrons, for them to know about the existence of each other, and to start repulsing each other from the electric force. So there's a speed at which the electric force is being carried out. It's not an instantaneous phenomenon in principle.

For gravity, it should be the same thing. It's not an instantaneous phenomenon.

Let's say I am in a science-fiction movie. I have a planet here and, all of a sudden, another planet appears here. It is not that automatically, instantaneously, the two planets start attracting each other gravitationally. There's a small amount of time that elapses which is related to the distance between the two planets and the speed of gravity, which in Einstein's theory of relativity is expected to be the same as the speed of light in a vacuum. So there's a speed at which the gravitational information gets carried out.

Now, the same analogy with light being fluctuations in the electromagnetic field—that they are ripples and they propagate at a given speed—happens for gravity as well. We know, for instance, that when two blackholes are about to merge around each other, or two stars are about to merge around each other, they create fluctuations in the gravitational field. Those fluctuations are gravitational waves, and those gravitational waves travel through space at a given speed which is the speed of gravity. In Einstein's theory of general relativity, the speed of gravity is the same speed as the speed of light.

We have very strong evidence that within some given error (margin), the speed of propagation of gravitational waves, or the speed of gravity, is the same as the speed of light. (We've been able to observe this in) two neutron stars—neutron stars are special kind of stars—(that) have been merging with each other. During the merger, they emitted not only gravitational waves but also light, and we received both the light and the gravitational waves here on Earth at almost the same time. The stars were very, very far away, and they (light and gravitational waves) both propagated at almost exactly the same speed, and so we have a very tight constraint on the speed of gravity being the same as the speed of light, within some very, very small error bars.

Gravity is not instantaneous. Gravitational waves take a very small amount of time to propagate. (Image credit: Lilartsy/Pexels)

What is a gravitational rainbow?

When you go to the law of frequencies, if you can think of an analogy with light, light has different frequencies, you can see light of different colours. It's the same thing for gravitational waves. There would be different colours for gravitational waves if we could see them with our eyes. But we can't. We can observe gravitational waves of a particular frequency, a particular colour. But if we were able to observe gravitational waves at a much lower frequency... they could be a bit slower or a bit faster, depending on the exact theory of gravity, and then depending also on the environment in which they travel...  they would have different colours.

What we do know is that as light and gravitational waves travel through the universe, light gets much more easily slowed down as compared to gravitational waves. (This is) because light interacts more strongly—not with everything, but with some things light interacts more strongly—whereas gravity interacts with everything uniformly but very weakly.

So as gravitational waves and light travel through the universe, it can be that light gets slowed down by whatever (environment or thing) they're going through, whereas the gravitational waves interact very weakly and so they just go through. So what we would expect typically is, in fact, that the gravitational waves would travel a little bit faster than light, if anything, but depending on the frequency.

You were invited to speak on the 10th anniversary of Christopher Nolan's 'Interstellar' releasing in theatres, right? The film is re-releasing in India on February 7. How do you rate it in terms of the science it depicts?

'Interstellar' is probably one of my favourites (sci-fi films dealing with gravity), because in a lot of aspects, it is actually very accurate. Then there's a little bit of a fiction that comes in when it comes to coming out of the blackhole and propagating information in the past, and everything that they have to do to make it interesting. But overall, there's a lot of very, very accurate elements in 'Interstellar', which is very close to my heart.

Also, this idea that you want to go inside a blackhole to explore what happens at the singularity of the blackhole - of course, we're not going to do it the way they show in the movie, but it's still very true that there's some knowledge that is accessible somewhere in the universe, and we don't have access to it right now. But if we were able to, who knows what we would be able to get out of that?

But in some sense, it's not about the application. It's not so much about the survival of the human race. Maybe it is in the long, long term, but it's mainly about having access to that knowledge, being able to uncover some new layers of science, of physics which is present throughout the movie. It also shows how important it is to keep exploring. It can't just be that we have a society where we're just focusing on one particular area, because this is what we need to do for pragmatic reasons, for survival reasons. We'll be doomed as a society if we do that; it's really important to keep exploring all sorts of different directions, because who knows what may come out of that.

There's also an idea in sci-fi that perhaps you could weaponize gravity. Is that even theoretically possible?

Do I say that? Who says that?

This is people talking about sci-fi ideas on social media platforms; since we were talking about sci-fi taking off from what is really happening in physics a little earlier.

There're different things you can do in principle, theoretically think of doing (it), in practice, I don't think (it could work).

Let me give you two gravity or gravity-related things you can think of trying to use in futuristic science-fiction movies.

One is, because gravity is all related to the curvature of spacetime, you can imagine engineering your own curvature of spacetime by adding some particular matter with some particular properties and pressure and energy so that you (re)engineer the shape of the spacetime in which we live. And even more futuristically, you can imagine creating a shortcut in how you want to connect different points in the universe. That's something which is sometimes used in science-fiction movies - in principle, theoretically, you can imagine doing it. But the reality is that to start engineering our spacetime so that it has a particular shape to our advantage, we will need to put blackholes there. Or even more than blackholes, some kind of masses or forms of matter that have either negative energies or energy properties that could then become quite unstable, so you wouldn't be able to keep them - they'll just blow in your face.

The other thing has to do with some of the aspects we discussed at the very beginning, related to the dark sectors of the universe. If we look at the night sky, we see is stars and galaxies and clusters of galaxies. But this is just ordinary matter. We know that most of the matter present in the universe is in the shape of dark matter. So it's a separate sector as compared to us, with which, at the moment, we're only interacting gravitationally; we have no direct interactions with dark matter.

And then an even bigger proportion of the energy budget in the universe is under the shape of dark energy. So it's some energy present here, there, everywhere. It's present in very, very small quantity, but absolutely everywhere. And so you can start thinking about whether you can start harvesting that kind of energy which is everywhere - it's here on earth, it's in space, in the whole solar system, in the whole galaxy, in all of the cosmic voids. Could you use this energy to your advantage? We're going through energy crisis. And there's all of this other energy which dominates the energy budget of the universe. Why are we not using it? But we're not going to be doing that anytime soon, because we don't interact with it. It's not like we can just grab it. We have no instrument to actually grab it, and it's so diluted that it will be really hard to do anything with it.

The first idea you explained of using the curvature of timespace, and introducing elements to it, is that also, for example, the idea of time travel, or travelling to different dimensions, or the idea of using portkey for instant transportation in Harry Potter - is that something that would be possible sometime in the future?

The portkey, they would be using that for the extra dimensions. You can imagine even that our universe is embedded on a surface in an extra dimension, but this surface can bend, and through the extra dimension you can go through different points in the universe. Or you can possibly travel through the extra dimension, possibly the way we travel through the extra dimension is not with us physically, but gravity itself can travel through the extra dimension, so you would have to encode all the information about your existence and what you're thinking about, and everything, all the cells in your body, how they interact, through gravity. You encode all of this information in gravity. You let gravity go through the extra dimension, maybe to another parallel universe. And then in that parallel universe, you decode the information into who you are, and then you try to export yourself there.

We know that since the Big Bang theory, the universe has just been expanding, and expanding quite fast. Why is it not contracting back into itself despite there being gravity all around?

One can, in principle, just say that there's some unknown dark energy out there that leads to an accelerated expansion of the universe. But that in itself is a little bit unsatisfying, because it's just giving a name to something that we don't know. We don't know what that would be, and it has properties which are unlike the classical kind of energy and matter that we know about.

However, what we do know is that the quantum world is much more interesting. And we do know from particle physics is that it's not just that the particle is here or there; there's constantly some bubbling in the vacuum, in nothingness, as some bubbling of quantum fluctuations and some probabilities of a particle to pop up here, pop up there, disappear, and come back.

Now, for this quantum world to work out, it means that everywhere you take empty space, you take a box and you remove anything in it and you completely shield it from everything, and yet you have probabilities in that empty box to create particles out of nothing. So it means that in nothingness, there's still some energy - we call this the quantum energy of the vacuum, because it's in a vacuum.

Even in nothingness, there's still some energy there, and it's coming from a quantum effect. And so we have strong evidence that vacuum emptiness is filled with some quantum vacuum energy.

And the key thing is this: quantum vacuum energy is not just where we are. It's not just here on the planet or in the solar system, or in the galaxy, or in the clusters of galaxies. It's actually everywhere in the universe. It's completely uniform and democratic everywhere in the universe. And so because it's not only distributed like masses here and there, the way they react gravitationally, it would be to attract each other. This quantum vacuum energy is present everywhere in the universe, and what it does, rather than making everything attract each other, is actually pushes everything in the universe. And so this push from the quantum vacuum energy would be a very natural explanation for the accelerated expansion of the universe.

Why do you call the counteracting force massive gravity?

Okay, so I haven't gone to that yet. So this is already (explained) in normal gravity, in Einstein's theory of general relativity.

Now, if I am to explain a little bit more: Let me start with quantum vacuum energy. I know that it will lead to an accelerated expansion of the universe. But does it really lead to the right level of acceleration of the universe? Or is it too fast or too slow?

The real puzzle is that when we account for the quantum vacuum energy being naturally present in the universe, we would expect the universe to have been accelerating far too fast; way, way, way too fast. So fast, that nothing could have ever got formed in the universe. Really, extremely fast. But this is not what we're observing.

This is where massive gravity comes in. It tries to reconcile the existence of quantum vacuum energy in the universe with our expectation from gravity.

So in Einstein theory of general relativity, you take this quantum vacuum energy, and you ask yourself the question: how much acceleration of the universe does it lead to? And the answer is, way too fast. Whereas if you take the theory of massive gravity, and I'll explain in a second why we call it massive gravity, but in that theory, if you take the quantum vacuum energy, and you ask yourself, how fast does it lead to a rate of acceleration of the universe? The answer is: at the beginning of the universe, it leads to a very fast rate of acceleration, but that relaxes over time, and as you wait a long enough period of time, then it leads to a smaller and smaller acceleration rate of the universe. And so that may explain why we observe that smaller rate of acceleration of the universe today, even though it's been driven by a very large quantum vacuum energy.

Well, now, we call this slightly changed theory of gravity massive gravity, because to make it work, the fundamental carrier of gravity should be massive. In Einstein's theory of general relativity, gravitational waves themselves are massless waves, whereas in massive gravity, the gravitational waves would be behave as if they were massive waves, so their dynamics would be slightly different.

So, if massive gravity weren't protecting the universe, it would all be dispelled too far away for sustaining life? Would we have life in the universe?

Oh, yeah. If you take the normal quantum vacuum energy at face value, the effect it should have on the universe according to Einstein's theory of general relativity, then we shouldn't be there.

The book cover is lovely as is the title. Could you tell us a bit about the title, why is it called 'The Beauty of Falling'? Of course, early on in the book you explain how free falling is the only time that you can experience lack of gravity on earth.

Yeah, so the title is a little bit of a pun on different aspects. There's different ways in which we should embrace the beauty of falling. First of all, it's about exploring the beauty of gravity, because it's a really beautiful phenomenon, and that's what I want to share, and this passion for gravity, because it's a universal phenomenon which I think we should all love. But we all get a little bit scared by gravity, because it's what makes us fall down. And we associate falling down with not the fall itself, but actually hitting the ground. And it's typically when you think of falling, you think of failing, and it's a negative connotation. But actually if you imagine the fall itself, it's a very blissful, very beautiful experience. And so you should embrace this beauty of falling.

There is a beauty in falling, because in exploring what gravity is, in the research environment, there's a lot of failing. It is important to keep exploring. The ideas that you have will fail. They will fall, and there's something beautiful in that as well. It's part of the scientific exploration. And I think this is something that maybe it was better known in the past, but nowadays we have very much the impression that everything we do should be successful, should have a finality. But, in fact, it's not like that. We should embrace the beauty in experiencing something which may fail, which will fall down, and yet there is a virtue in going through that process anyways. Research does fail sometimes and it's very difficult, but there is a beauty in it as well. The beauty in all of this, trying to becoming an astronaut and failing into it, in realizing that it's actually through those failures that you improve in life, and it's an experience in itself that you should enjoy.

But really the real beauty of falling is in that of gravity itself. We know that Einstein's theory of general relativity itself does fall down, so gravity fails at some point. And again, there is a beauty in that which we should embrace. It's not an embarrassment, it's not something we should shy away and say, 'Well, we haven't quite finished, and we'll be there.' I think it is worth sharing the fact that there are things we don't know, and it's in exploring those things that we don't know that we'll make progress. So it's in trying to share how we do the research and how we're trying to explore the unknown for gravity.

Your paper on massive gravity came out in 2011. Is that right?

Correct, yes.

In the book, you also mention that when you first shared this idea of massive, it was just a theory and that you had hoped there would be experiments to prove or disprove it. What has happened in this respect till 2025?

I think the best observation is the direct detection of gravitational waves, because they really allow us to understand the dynamics of gravitational waves, the speed of gravitational waves depending on their frequency, and from the very first observation of gravitational waves in 2015 - so the paper was announced in 2016, but the discovery was made in 2015, very soon after the 1st thing that they were able to do. I didn't do it because I didn't have access to the data, but the collaboration was able to do, is put a constraint from the evolution of the gravitational waves, they put a constraint on the mass of the graviton. So I think the fact that we are having exploration and observations on gravity which are better and better, that's really going to push the constraints on the mass of the graviton.

So the current constraint from gravitational waves observations, they put - just for to give you an order of magnitude - they give you a constraint on the graviton mass to be of the order of 10 to the minus nowadays 22 electrovolt. But really to explain our cosmological observations for things to be consistent, we should have the Graviton mass to be 10 to the minus 32 electrovolts. So we still have quite a large margin in there. So the current constraints are not ruling out completely massive gravity. They're just allowing us to understand how, with future experiments, particularly with space-based interferometers, we may be able to observe gravitational waves at lower and lower frequencies, and put better and better constraints on the Graviton Mass, for instance.

One last question: Sunita Williams' return to earth has been postponed to March 2025 at the earliest. What do you think is are the main things they need to watch out for in zero gravity?

Oh, I'm not a specialist, but I would imagine that the psychological effects are probably quite strong. From what I understand, the loss in muscles is probably one of the biggest aspects. Of course, you can regrow that. So that's okay. You probably need to watch out for radiation as well. But I worry about the psychological implications as well. It must be slightly stressful, although, of course, they're trained for that. But still.

Professor of theoretical physics at Imperial College London, director of the Abdus Salam Centre for Theoretical Physics, and member of the American Academy of Arts and Sciences, Claudia De Rham will be in conversation with entrepreneur Mukesh Bansal at the 2025 Jaipur Literature Festival at 4 pm on February 1.