Claudia de Rham really knows gravity, both intellectually and physically.
As a professor at Imperial College London, she studies gravity and its vexing relationship with quantum mechanics; as an adventurer, she experiences gravity by flying, diving, and otherwise viscerally feeling its power.
In her first book, The Beauty of Falling, published in April by Princeton University Press, she explores the relentless pull of gravity in her work, hobbies, and imagination. Check out de Rham’s interview with FirstPrinciples to find why she wrote the book, and the unexpected things she learned along the way.
From The Beauty of Falling:
In my life, I have experienced the joy of flying and diving and came within a hair’s breadth of making it to outer space.
But we don’t need a fancy plane, scuba gear, or space shuttle to experiment with gravity. In fact, whether we are doing something as simple as dropping a ball, swinging in a hammock, or skipping a stone, we’re all scientists conducting our own personal experiments and drawing our own conclusions about this universal yet mysterious phenomenon.
But what exactly is going on in those moments? What is gravity?
It seems like such an innocent question, yet the answer always seems to be hidden behind abstruse laws of physics. Physical phenomena are often portrayed as a set of obscure fundamental rules — Archimedes’ principle, Newton’s inverse square law, Bernoulli’s principle, and the like — that nature must unquestionably and rigidly obey.
These laws are, of course, central to our understanding of the world and the structure of our reality. They have revealed how buoyancy allows boats to float, and how the difference in pressure caused by the motion of the air beneath their wings allows birds and planes to navigate the skies. They have enabled us to send a man, and hopefully soon a woman, to the Moon.
Yet the presentation of these laws as being set in stone belies our scientific history. Far from being immutable and unchanging, our understanding and appreciation of these laws — what they mean, where they come from, and what lies behind them — is continuously unfolding before us.
Galileo Galilei, Johannes Kepler, Sir Isaac Newton, Albert Einstein, Stephen Hawking, Sir Roger Penrose, Andrea Ghez, and countless other brilliant scientists have each brought a new perspective to our understanding of gravity, but our journey is far from finished.
With their realization that gravity must be a universal force, acting on everything and accelerating everyone in the same way, regardless of their mass, Galileo, Kepler, and Newton provided the first crucial piece of the puzzle. This insight was made possible by a new perspective on what it means to be free, a perspective that discarded centuries of Aristotelian dogma and radically transformed the concept of inertia.
This new perspective was brought to light in 1632, with the publication of Italian astronomer and physicist Galileo Galilei’s Dialogue Concerning the Two Chief World Systems (Dialogo sopra i due massimi sistemi del mondo). In the dialogue, Galileo championed a new Copernican revolution, one that went beyond merely denying that the Earth occupied a special place in the solar system, by further dismissing the idea that any person or object could ever hold a privileged position with respect to the laws of nature.
To make this argument, Galileo considered the world through the eyes of a sailor confined to the main cabin below the decks of a moving ship. Unable to see the world outside, the sailor was entertained by watching the motion of “some flies and butterflies” with whom she shared the cabin.
Galileo realized that the sailor would not be able to tell whether the ship was at rest or in motion at constant speed, at least not from observing these small flying animals. Why? Because if the ship moves at constant speed, so does everything on board, including the air in which the flies and butterflies flutter about. The sailor, trapped below deck, can only observe the motion of the flying creatures relative to the inside of the ship’s cabin.
Galileo used this thought experiment, which highlighted the importance of relative motion, to explain how the Earth could rotate without us being able to feel it. Moreover, once we recognize that we cannot tell the difference between the lower deck of a ship at rest and that of one in uniform motion, we can infer that the laws of physics should be the same for any observer moving at constant velocity, no matter the speed.
It is precisely this notion of “Galilean relativity” — the realization that the laws of nature are the same regardless of who describes them — that is enshrined in Newton’s first law of motion, which holds that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.
Newton realized that being free is the privilege to carry on undisturbed, pursuing our journey at the same velocity, uniformly. Building on the work of Kepler, who developed the laws of planetary motion, this insight would later lead to Newton’s 1687 law of universal gravitation, also known as Newton’s inverse square law. According to this law, the force of gravity exerted between any two massive particles (that is, particles having mass) is a universal and instantaneous force, whose intensity decays as the square of the distance between the two particles.
Newton’s law, as many of us have been taught, describes how an object, when dropped, is inexorably attracted by the mass of the Earth.
But the universal nature of gravity extends far beyond this simple phenomenon. It applies to everything and everyone, no matter the object, no matter the separation.
In 1798, Henry Cavendish was among the first to test it formally in a laboratory, and more than three centuries after its discovery, Newton’s inverse square law has been scrutinized with impeccable precision, from distances smaller than a tenth of the width of human hair to separations that extend billions of kilometers.
In fact, Newton’s law of universal gravitation is so fundamental that it can still be used to predict how gravity has governed most of the evolution of our Universe, from the gravitational collapse of dark matter to the formation of clusters of galaxies and the creation of the solar system.
Centuries passed before observational evidence began to cast a sliver of doubt on Newton’s law of gravity. However, in retrospect, the idea that gravitational attraction between any two objects happens instantaneously should have raised a red flag.
According to Newton’s simple law, if two particles were to appear, they would be immediately attracted to one another without any delay.
No matter what your views on attraction may be, we all know that this phenomenon cannot be immediate. Even when it comes to love at first sight, you first need to “see” the other person (that is, to “communicate,” even if not verbally) for attraction to take place.
Newton himself, in a letter to Richard Bentley, expressed his discomfort with the concept of an instantaneous law:
“Tis unconceivable that inanimate brute matter should (without the mediation of something else which is not material) operate upon & affect other matter without mutual contact; as it must if gravitation in the sense of Epicurus be essential & inherent in it. And this is the reason why I desired you would not ascribe {innate} gravity to me.”
Excerpted from THE BEAUTY OF FALLING: A Life in Pursuit of Gravity © 2024 by Claudia de Rham. Reprinted by permission of Princeton University Press.
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