THE fans roar into life, pumping air upwards at 260 kilometres per hour. Decked out in a baggy blue jumpsuit, red helmet and plastic goggles, steps forward into a glass chamber and… whoosh! Suddenly she is suspended in mid-air, a wide grin on her face, thrilling to the simulated experience of free fall.
I had persuaded de Rham, a theoretical physicist at Imperial College London, to come indoor skydiving with me at iFLY London. It seemed fitting, given that much of her life has been dedicated to exploring the limits and true nature of gravity – and launching ourselves out of a plane wasn’t an option, at least on this occasion.
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As she describes in her new book, , de Rham trained to be a pilot and then an astronaut, only for a medical problem to scupper her chances of the ultimate escape from gravity. But she has gone on to explore this most familiar and mysterious force in a more profound way, as a theorist, and made an impression by asking a radical question: what does gravity weigh?
By that she means the graviton, the hypothetical particle thought to carry this force. If it has mass, as de Rham suspects, that would open a new window onto gravity. Among other things, we might finally spot a “gravitational rainbow” that would betray the existence of gravitons – and with them, a long-sought quantum description of gravity.
As de Rham floats on air, she makes it look easy. She is soon ascending to the top of the chamber, some 10 metres up. “That was incredible, really fun,” she says, as she emerges, giving high fives to the instructors. “What you feel is not really gravity,” she explains. “It’s the pressure of the air. But it’s fascinating how you can play in there, by balancing the pressure of the air and gravity itself.”
After we both get our feet back on the ground, de Rham tells me that she has always been drawn to the force that keeps them there and orchestrates the motion of planets and galaxies throughout the universe. It started at the age of just 5, while living in Iquitos in the Peruvian Amazon. She remembers swinging in a hammock there and noting the feeling of weightlessness for the first time. “As I gazed up at the stars… I could almost imagine floating in outer space, out of time and conquering gravity,” she writes in her book. “This moment sparked what would become a lifelong fascination with the subject.”
As a child, de Rham moved frequently from country to country with her parents, who worked in sustainable development. Thinking about something so universal as gravity was strangely comforting amid all the upheaval, she says. “There was a sense of stability that came from being part of something much bigger than me.”
Later, while studying for a PhD in Canada, she trained as a pilot and went on to apply to be an astronaut with the European Space Agency. She made it through several rounds of screening, one of which involved a simulated rescue mission through a mock jungle. Of more than 8000 candidates, she got down to the last 42. But then a battery of medical tests revealed that she had a latent tuberculosis infection, meaning she was disqualified.
Thankfully, de Rham had also been working towards another way to conquer gravity – not by escaping its clutches in planes or rockets, but by trying to figure out how it works at the most fundamental level.
She was drawn to study gravity by its simplicity. When it comes to describing its workings at cosmological scales, she says, you have to remove all complications. “I mean, it is a little hard to believe when you first look at the equations. But for me, they are very pure, and they almost transcend any form of communication.”
General relativity
The equations to which she is referring are those of Albert Einstein’s general theory of relativity, which describes gravity as the result of mass warping space-time. At the heart of the theory is the equivalence principle, which essentially says that you can’t distinguish gravity from acceleration. More specifically, it posits that an object’s resistance to acceleration and the gravitational force it experiences are both proportional to its mass. It is an odd coincidence, but one that has always been borne out in experiments. Together with the constancy of the speed of light, it underpins our current understanding of gravity.
You can think of this equivalence as Einstein did in a famous thought experiment. Imagine being in an elevator in space, accelerating “upwards”. Inside the elevator, you would feel a force pinning you to the floor, but it would be impossible to tell if this was the normal effect of Earth’s gravity or if you were in space, but accelerating. Indeed, part of the fun of indoor skydiving was to conjure something roughly akin to this thought experiment. When I shut my eyes in the cylinder, with the air rushing past my ears, it is indistinguishable from the free fall of real skydiving.
Today, gravity, as described by general relativity, is one of the four known fundamental forces of nature. Yet it is the outlier. The other three forces are described within quantum theory, meaning that they come in discrete chunks. For the majority of physicists, gravity should fit the same mould. But we still don’t have proof of that, never mind a quantum description of gravity.
For her part, de Rham has sought to make progress by thinking deeply about gravitons, the hypothetical carrier of the force of gravity. Each of the fundamental forces is carried by an equivalent “boson” particle – some have zero mass, others have a very small mass. De Rham wanted to know: what is the graviton’s mass?
She wasn’t the first to ask that question. In previous explorations of the idea, any attempt to give gravitons mass meant that they would come in a variety of forms, one of which would have negative energy. Since that seemed physically impossible, the idea of gravitons with mass, known as massive gravity, fell by the wayside. But de Rham felt there was more to it. Working with her husband Andrew Tolley, also a physicist at Imperial College, and Gregory Gabadadze at New York University, she was able to work out a consistent new framework for massive gravity that doesn’t spit out negative energy particles. This was the first time anyone had produced a workable framework in which gravitons could have mass. In 2020, she won a for “developing an innovative mathematical framework that yields a rigorous and viable theory of massive gravity” and “profoundly impacting our understanding of many fundamental problems in cosmology and particle physics”.
Gravitational waves
One of the biggest problems remaining is whether gravity really does come in the form of gravitons. Even if these particles exist and have mass, it would be vanishingly small, which makes snaring them hugely challenging. Still, one way to find out involves gravitational waves, the “ripples in space-time” first observed using the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. This detection wasn’t proof of gravitons in itself; a gravitational wave is many, many times more energetic than a graviton. But the waves could yet reveal the presence of these hypothetical particles, says de Rham.
We often think about the speed of light, but we don’t think much about the speed of gravity – in other words, the speed at which gravitational waves travel. If gravitons are massless, then gravitational waves travel at light speed and nothing interesting happens. If, however, they have mass, then de Rham says we would expect lower frequencies of gravitational wave to move more slowly. At very low frequencies, that would create a kind of “gravitational rainbow” – so called because it would be similar to what happens when light is refracted by raindrops, albeit minus the colours.
We don’t yet have sufficiently sensitive technology to detect the very low frequencies where these rainbows would show up. But if and when we do, we could look for them and, if we find them, that would be evidence not only that gravitons exist, but also that they have mass. “In my mind, there is no doubt that there should be a graviton,” says de Rham. “But still, actually discovering it would be a big deal. It’s definitely Nobel prize territory.”
Today, de Rham is busy exploring whether her ideas about massive gravity could help to explain other mysteries of the cosmos too, such as dark energy – the mysterious force behind the accelerating expansion of the universe – and maybe even lead us to a deeper theory of gravity. We know that general relativity breaks down at very high energies, which suggests that there must be a better, more complete way to understand gravity.
“We know something has to come next,” says de Rham. “We know that we don’t even have the tools and the language to describe it, to understand how to ask ourselves the right questions. That may seem very daunting, but at the same time it is fascinating, because it tells us so much more is out there to be discovered.”
On this topic, de Rham says that fundamental physics is currently undergoing a shift. Over the past decade or so, the field has been looking for physics beyond what we already know by testing ideas about specific new particles. But with all these efforts coming back empty-handed, we need ways to broaden the search, she says. “We are being pushed to chart out the parameter space that it is interesting and useful to look at.”
Her approach is to look at the characteristics that any standard unified theory, one that unites gravity with the other forces, would need to fit with the known laws of nature, and then to work back to see what consequences these would have that we could measure. To get your head around one aspect of this work, it helps to think of how photons of light can have two polarisations – either right or left handed – depending on how the light oscillates through space. If the graviton is massless, we would expect the same thing for gravitational waves – just two polarisations. If not, then there could be additional polarisations and de Rham has worked out that carefully measuring the properties of these extra polarisations could test certain kinds of more complete theories of physics, including string theory.
As we peel off our jumpsuits, some new punters arrive to train for an upcoming skydive. They are breathtakingly good, using small hand movements to somersault and spin their bodies through the air in synchrony. They will soon experience the force of gravity at its most visceral, throwing themselves out of a plane and hurtling towards Earth. “What connects it all,” says de Rham, referring to her adventures in gravity, both physical and intellectual, “is really exploration.”
Joshua Howgego is deputy head of features at Â鶹´«Ă˝
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