Claudia de Rham on The Beauty of Falling

Claudia de Rham on The Beauty of Falling

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Claudia de Rham has been playing with gravity her entire life. As a diver, experimenting with her body’s buoyancy in the Indian Ocean. As a pilot, soaring over Canadian waterfalls on dark mornings before beginning her daily scientific research. As an astronaut candidate, dreaming of the experience of flying free from Earth’s pull. And as a physicist, discovering new sides to gravity’s irresistible personality by exploring the limits of Einstein’s general theory of relativity. In The Beauty of Falling, de Rham shares captivating stories about her quest to gain intimacy with gravity, to understand both its feeling and fundamental nature. Her life’s pursuit led her from a twist of fate that snatched away her dream of becoming an astronaut to an exhilarating breakthrough at the very frontiers of gravitational physics.

What is gravity exactly and why are you so passionate about it?

CdR: You’re raising the bar quite high with this first question! We all have some more or less concrete notion of what gravity is; yet understanding precisely how to describe it at the fundamental level is one of the most challenging questions there is, one for which we may not even have yet developed the right language to even contemplate fully. So I am afraid I won’t be able to tell you what gravity is exactly, no one can. What I can do is explain why it is so challenging, and that is part of why it fascinates me.

Most of us probably associate gravity in our everyday lives with the phenomenon that makes us fall to the ground. You may even have a negative impression of it, after all gravity leads to some level of challenges, which we feel when climbing up a steep mountain. But to me there is a real beauty in falling and embracing gravity, to me it’s also a witty character. Despite, or perhaps because of, the challenge of its strain we all love to defy it. As children, gravity is the first physical phenomenon we are able to identify. Rather than being overtaken by it, we all quickly learn to play with, whether it is by relentlessly dropping and throwing toys to see them fall, or to let ourselves go on playground swings. We learn from an early age to play with and pit ourselves against gravity.

Of course, gravity is much more than just a game provided here on Earth for our own amusement. You may think of gravity as what keeps our feet firmly on the ground, but it’s also what has allowed the Earth and the Solar system to form. In fact, it’s what has allowed the whole galaxy and its billions on stars to form. It is what has allowed the whole Universe to take life and evolve in the way it has done. But that’s not even scratching the surface of how fundamental gravity is. It is what makes space and time come to life and be real actors in the unfolding drama of reality.

Deep down what fascinates me the most about gravity is how universal it is. You feel and experience gravity independently of your mass, your size, your color, your charge or any other arbitrary characteristics. You can be a feather or a hammer or a planet or a black hole, a speck of dust or a smartie, or even something as light as light itself and you will still feel the gravitational pull of the Earth. You can never shield yourself from gravity. It connects everything to everyone, everywhere, all the time, it is universal in every sense of the term.

Your book is quite personal and makes connections between how gravity works in nature and how it has affected your life and how you ended up studying it, can you tell us more about it?

CdR: We understand from Einstein’s theory of General Relativity that we experience gravity through the curvature of spacetime in which we live in. Gravity is the thread that connects different points of spacetime together.

If we compare the curvature of spacetime with that of the curvature of the surface of the Earth, we know that wherever we are, if we just look at things close enough to us, the surface of the Earth appears flat. We only start appreciating the curvature once we compare between different places on Earth. The notion of curvature manifests itself through the way different points on the surface are connected with one another. If you want to fly from one side of the planet to the other, you may end up taking what may naively appear as a funny trajectory, for instance going close to the north pole or flying over landscapes you never thought you would experience, but this will be the most direct way to connect two cities when accommodating for the curvature of the Earth.

To me, this notion of curvature and how it affects our evolution is very similar to how we create the trajectory of our own lives. Our life is not just a series of disconnected events, it is how we choose to connect between them, accommodating for the complex situations in which we find ourselves, that its meaning emerges.

From the outside your life’s path may seem quite meandering and uncertain, particularly if you have a non-standard career trajectory, or come from what can be considered as an underrepresented background. The complexity of the reality we live in is what makes a straight path look so complicated and sometimes unconventional just like the curvature of the spacetime we live in is what makes a straight path look curved from an outside perspective.

Another aspect which is quite personal to me but also intrinsically shared with gravity and with scientific research is that of failure. In research, science and perhaps in life in general, the importance of making mistakes and growing from them can never be overstated. As a researcher, we only learn by trial and errors, allowing ourselves to fall and fail and standing up again.

Einstein’s theory of General Relativity is our best description for gravity, at least so far. It has been tested with impeccable precision on the largest range of scales. Yet there is one thing we know for sure about this theory, one thing which I believe makes it a truly remarkable theory: the fact that it predicts its own failure. We don’t know precisely what comes beyond General Relativity, but we know for sure that General Relativity cannot be the full description of Nature. That is because some of its predictions are theoretically inconsistent, they would seem to suggest that measurable quantities can become infinite, something we call a ‘singularity’. By that point classical physics breaks down and we must embrace the quantum realm. What comes next is as yet unclear but what is for sure is that there is much to uncover from embracing this failure.

This is a little more than just metaphorical for me as for more than 20 years of my life I strove to achieve my number one goal of becoming an astronaut, only to see it fail for reasons that were beyond my control. Such is life. Would I still have trained for years and put myself through so much pressure if I had known from the beginning that I was fated to fail? I’d like to think so. Facing failure is an essential part of the human experience, in scientific research and in understanding gravity, facing failure is an integral part of the process and the route to progress.

What were the differences between working with or even fighting gravity as a pilot or astronaut candidate, and then trying to investigate it as a physicist—was there any benefit to trying to get to grips with the true nature of gravity having spent a number of years working with it?

CdR: What I do as a theoretical physicist working on gravity and other fundamental aspects of the laws of nature is naturally quite abstract. You rely a lot on a mathematical formalism to model the world around you and make predictions. What I found fascinating about flying for instance is to see some of these abstract concepts just come to life. You know for instance that to compensate the torque from the propeller of the airplane, the propeller needs to be off-centered by just a tiny bit. Being able to predict it, apply it, and see it working in practise in a way that you can feel makes it very physical and concrete, it is just beautiful. There are so many subtleties and constant complications in what I work with that being able to make a simple prediction in a real system and see it work as it should and feel it with your own senses is really refreshing and allows you to connect back to reality.

There’s also real value in learning to be thrown out of your comfort zone, learning to rely on your own inner logic in situations where your instincts fail. Imagine for example attempting to recover from spins or other emergency procedures while learning to fly. You’re thrown completely off-guard. In these situations, it is hard to even tell which way is up and down, the only thing you know for sure is that following your basic instincts or succumbing to your fears would almost inevitably lead you to make disastrous manoeuvres. So instead, you must learn to take all your feelings out of the equation, to rely simply on logic, and to have confidence in what you have learnt; the confidence that deep down you know you have what it takes to make it work.

Research relies on a lot of creativity and innovation. When investigating fundamental concepts like the nature of gravity, there will always come a point where things can get quite uncomfortable, where you will be thrown out of your comfort zone. That is an inevitable part of exploring the unknown. Knowing that you can deal with challenging situations where your life literally depends on it makes it much more concrete to know that you will be able to work methodologically through very abstract and counterintuitive concepts in any other challenging and uncomfortable situation.

Another personal analogy you draw in the book is related to the role of women in physics.

CdR: I think there’s definitively a value in recognizing that different elements in the Universe work in different ways. It is not because some are not as ordinary or as familiar, or as loud or visible as to what we are used to, that they may not actually end up having a very significant if not essential effect on the Universe.

Dark energy is an invisible substance that fills the Universe. In our everyday life, it is utterly imperceptible. It is the most misunderstood and unnoticeable element in the Universe and yet it is the one that currently drives its evolution. There are many cases in my career and in that of many other people I have witnessed, where their contributions may seem unnoticeable because their characteristics are sometimes different to what we are typically looking for. Yet these contributions are sometimes the most fundamental and ultimately influential.

The book concludes with a final journey towards proving the quantum nature of gravity, what does it mean and what would this require?

CdR: Today, there is little doubt among the scientific community that gravity should be regarded fundamentally as a quantum phenomenon, just like the electron, light and all the other fundamental forces of nature. It may be useful if we start by drawing a parallel between the force of gravity and other fundamental forces of nature like the electric and magnetic forces, or what we call electromagnetism. We know that if we take two electrons or two electric charges that act on each other through the electromagnetic force, and shake them around (accelerate them), they radiate, i.e. they produce what we call electromagnetic waves, but that’s just another word for light. The light from the Sun is a wave in the electromagnetic field, a little bit like ocean waves although they don’t need a medium like water to propagate.

In a similar way, if we take two masses and shake them around (say two stars or two black holes about to merge), they will also radiate. This time they will produce what we call gravitational waves which were first detected here on Earth in 2015, providing one of the most remarkable confirmations of the theory of General Relativity.

For light or electromagnetism, it has been known for a century that if you have light of a particular color (or frequency), you can only ever get finite quantized amounts of energy out of it, no matter the intensity. This quantum of light is called a photon. If you shine light onto an atom, it can absorb energy, but it can only do so in discrete (quantized) amounts, by absorbing one photon, or two photons or any discrete number of photons but never half a photon for instance. We say that the energy in the electromagnetic field is quantized.

There is a direct analogy between electromagnetism and gravity, and this quantized structure is also valid for gravity. It has to be the case, otherwise by interacting with gravity light would be able to evade its quantum nature. To actually confirm experimentally that gravity is quantized, we would need to show that we can only ever absorb a finite quantized amount of energy when swallowing a gravitational wave of a given frequency. We call that finite quantum the graviton.

The issue is that it’s actually quite challenging to produce gravitational waves with sufficiently large amplitude to detect them. For that we need to rely on astronomical cataclysmic events, like the merger of two black holes. And if we want to prove that the gravitational waves emitted in these events are quantized you need to sit next to them with our little atom until it fancies absorbing a graviton of just the right frequency (or color). The tricky part is that to make sure our dear little atom has really absorbed a graviton and not anything else, for instance a cosmic ray, or a photon, we need to shield our whole experiment. This means that all the while we’re gently standing next to our astronomical cataclysm, rather than running for our lives we should instead shield the system of merging Black Holes and our atom within a cavity surrounded by multi-kilometre wide walls. As a theorist, this is not yet where I would necessarily give up. However, with current known forms of matter, the mass of these shielding walls would be so large that they would collapse into a black hole themselves, destroying our whole experiment along the way. This certainly doesn’t prove that the quantum nature of gravity cannot be directly confirmed experimentally but it illustrates why this is not likely to be an experimental project we can yet offer to our first year undergraduate physics students at the moment. Who knows, maybe in a few millennia?

Claudia de Rham is professor of theoretical physics at Imperial College London and a member of the American Academy of Arts and Sciences. The recipient of numerous prizes and awards, she is ranked among the most influential researchers in fundamental physics of the past decade.