ILLUSTRATION BY PHIL HACKETT
Carlo Rovelli is an Italian theoretical physicist who is currently working with the Spanish National Research Council in Madrid. He is the author of Seven Brief Lessons on Physics (Allen Lane)
I am about to walk you through what we know of the physical universe. All of which, at a basic level, can be defined by three sets of equations (with apologies to my fellow physicists for the brevity).
The ingredients that make up these equations are “fields”. A physical “field” is a funny sort of entity, spread all over space, and was first conceived of one and a half centuries ago by a lower-class Londoner lacking formal education, Michael Faraday. The fields in the basic equations I am talking about are even funnier than the magnetic field actually imagined by Faraday, because they then manifest themselves in small discrete chunks, like grains, or particles, or “quanta”. These are the characters of the equations.
The first set of equations, pretentiously named “general relativity”, regards the so-called gravitational field. This field dictates how fast a clock ticks, how far from each other the two ends of a rigid ruler keep themselves and how things fall down. Its configurations also get names such as “black hole” or “gravitational wave”; its large-scale relaxing is called “the expansion of the universe”.
The second set of equations, unpretentiously called “the standard model of particle physics”, regards a rather patchwork group of fields whose quanta we call “electron”, “quark”, “neutrino”, “photon” and similar. These happen to be the ingredients of everything around us we call material, as well as light.
The third set of equations is the most general: it governs how each of those fields — and everything else built out of those fields — affects anything else. These equations are called “quantum theory”; they are used daily by engineers, physicists, cosmologists and those designing our modern technology — so we know how to use them. But scientists and philosophers still discuss what is the best way of making sense of them. In my opinion, quantum theory says we should think of objects as defined by how they affect something else — not as they are, by themselves, in isolation.
Aside from scholastic debates on what the hell quantum mechanics says about the world, the above set of equations, rather astonishingly, seems to underpin absolutely anything we can observe in reality. We have no hint of a single phenomenon that clearly contradicts them. (The “dark matter” observed by astronomers is sometimes suspected of escaping them, but even this is not clear: it could be formed by tiny black holes or their products.)
The immense power of these three basic sets of equations, written about a century ago, is a triumph of modern physics. All the Nobel prizes in fundamental physics of the past decades have been given to confirmations of these simple equations.
However, a few considerations temper this sense of triumph. First, while the equations of quantum theory fit the standard model like a glove, we are still working on using them with general relativity. This has been the task of my own scientific life. Since no full consensus has been reached, all sorts of crazy speculations are being explored. I am sure you have heard about them: extra dimensions, supersymmetry, strings and the like. Before getting excited, please wait. This uncertainty means we do not really know, for instance, what happened at the Big Bang, or what goes on deep inside a black hole.
Second, you might feel reassured by the second and most important consideration: fundamental physics is overrated. Knowing these basic equations does not mean “understanding the world” or how to deal with it. It is like knowing the basic rules of chess: this alone is obviously not enough to call yourself a competent chess player. In fact it goes beyond that: although there is no way to be competent in chess without knowing the basic rules of how pieces move, we can be very competent about many aspects of the world while happily ignoring fundamental physics. This is what biology, chemistry, sociology, common sense, literature and art are about. And vice versa, we can know the basic equations of physics and still be incapable of fixing a broken bicycle. Fundamental physics is great but the richness of the world stems from its complexity, not its elementary ingredients.