Seven Brief Lessons on Physics

by Carlo Rovelli

These simple and clear lines are the real birth certificate of quantum theory. Note the wonderful initial “It seems to me . . . ,” which recalls the “I think . . .” with which Darwin introduces in his notebooks the great idea that species evolve, or the “hesitation” spoken of by Faraday when introducing for the first time the revolutionary idea of magnetic fields. Genius hesitates.

Heisenberg imagined that electrons do not always exist. They only exist when someone or something watches them, or better, when they are interacting with something else. They materialize in a place, with a calculable probability, when colliding with something else. The “quantum leaps” from one orbit to another are the only means they have of being “real”: an electron is a set of jumps from one interaction to another. When nothing disturbs it, it is not in any precise place. It is not in a “place” at all.

It is not possible to predict where an electron will reappear but only to calculate the probability that it will pop up here or there.

Those who use the equations of the theory in the laboratory carry on regardless, but in articles and conferences that have been increasingly numerous in recent years, physicists and philosophers continue to search. What is quantum theory a century after its birth? An extraordinary dive deep into the nature of reality? A blunder that works, by chance? Part of an incomplete puzzle? Or a clue to something profound regarding the structure of the world that we have not yet properly digested?

The illustration below is not a drawing; it’s a photograph taken by the Hubble telescope in orbit, showing a deeper image of the sky than any seen previously with the most powerful of our telescopes; seen with the naked eye, it would be a minute piece of extremely black sky. Through the Hubble telescope a dusting of vastly distant dots appears. Each black dot in the image is a galaxy containing a hundred billion suns similar to ours. In the past few years it has been observed that the majority of these suns are orbited by planets. There are therefore in the universe thousands of billions of billions of billions of planets such as Earth.

Both protons and neutrons are made up of even smaller particles that the American physicist Murray Gell-Mann named “quarks,” inspired by a seemingly nonsensical word in a nonsensical phrase in James Joyce’s Finnegans Wake: “Three quarks for Muster Mark!” Everything we touch is therefore made of electrons and of these quarks.

Electrons, quarks, photons, and gluons are the components of everything that sways in the space around us. They are the “elementary particles” studied in particle physics. To these particles a few others are added, such as the neutrinos, which swarm throughout the universe but have little interaction with us, and the “Higgs bosons,” recently detected in Geneva in CERN’s Large Hadron Collider. But there are not many of these, fewer than ten types, in fact. A handful of elementary ingredients that act like bricks in a gigantic Lego set, and with which the entire material reality surrounding us is constructed.

The very way in which the equations of the Standard Model make predictions about the world is also absurdly convoluted. Used directly, these equations lead to nonsensical predictions where each calculated quantity turns out to be infinitely large. To get meaningful results, it is necessary to imagine that the parameters entering into them are themselves infinitely large, in order to counterbalance the absurd results and make them reasonable. This convoluted and baroque procedure is given the technical term “renormalization.” It works in practice but leaves a bitter taste in the mouth of anyone desiring simplicity of nature.

In addition, a striking limitation of the Standard Model has appeared in recent years. Around every galaxy, astronomers observe a large cloud of material that reveals its existence via the gravitational pull that it exerts upon stars and by the way it deflects light. But this great cloud, of which we observe the gravitational effects, cannot be seen directly and we do not know what it is made of. Numerous hypotheses have been proposed, none of which seem to work. It’s clear that there is something there, but we don’t know what. Nowadays it is called “dark matter.” Evidence indicates that it is something not described by the Standard Model; otherwise we would see it. Something other than atoms, neutrinos, or photons . . .

A group of theoretical physicists scattered across the five continents is laboriously trying to settle the issue. Their field of study is called “quantum gravity”: its objective is to find a theory, that is, a set of equations—but above all a coherent vision of the world—with which to resolve the current schizophrenia.

The central result of loop quantum gravity is indeed that space is not continuous, that it is not infinitely divisible but made up of grains, or “atoms of space.” These are extremely minute: a billion billion times smaller than the smallest atomic nuclei. The theory describes these “atoms of space” in mathematical form and provides equations that determine their evolution. They are called “loops,” or rings, because they are linked to one another, forming a network of relations that weaves the texture of space, like the rings of a finely woven, immense chain mail. Where are these quanta of space? Nowhere. They are not in space because they are themselves the space. Space is created by the linking of these individual quanta of gravity. Once again, the world seems to be less about objects than about interactive relationships.

The world described by the theory is thus further distanced from the one with which we are familiar. There is no longer space that “contains” the world, and there is no longer time “in which” events occur. There are only elementary processes wherein quanta of space and matter continually interact with one another. The illusion of space and time that continues around us is a blurred vision of this swarming of elementary processes, just as a calm, clear Alpine lake consists in reality of a rapid dance of myriads of minuscule water molecules.

This hypothetical final stage in the life of a star, where the quantum fluctuations of space-time balance the weight of matter, is what is known as a “Planck star.” If the sun were to stop burning and to form a black hole, it would measure about one and a half kilometers in diameter. Inside this black hole the sun’s matter would continue to collapse, eventually becoming such a Planck star. Its dimensions would then be similar to those of an atom. The entire matter of the sun condensed into the space of an atom: a Planck star should be constituted by this extreme state of matter.

Another of the consequences of the theory, and one of the most spectacular, concerns the origins of the universe. We know how to reconstruct the history of our world back to an initial period when it was tiny in size. But what about before that? Well, the equations of loop theory allow us to go even further back in the reconstruction of that history. What we find is that when the universe is extremely compressed, quantum theory generates a repulsive force, with the result that the great explosion, or “big bang,” may have actually been a “big bounce.” Our world may have actually been born from a preceding universe that contracted under its own weight until it was squeezed into a tiny space before “bouncing” out and beginning to re-expand, thus becoming the expanding universe that we observe around us. The moment of this bounce, when the universe was contracted into a nutshell, is the true realm of quantum gravity: time and space have disappeared altogether, and the world has dissolved into a swarming cloud of probability that the equations can, however, still describe.

Heat, as we know, always moves from hot things to cold. A cold teaspoon placed in a cup of hot tea also becomes hot. If we don’t dress accordingly on a freezing cold day, we quickly lose body heat and become cold. Why does heat go from hot things to cold things and not vice versa?

For example, for the motion of the planets of the solar system heat is almost irrelevant, and in fact this same motion could equally take place in reverse without any law of physics being infringed. As soon as there is heat, however, the future is different from the past. While there is no friction, for instance, a pendulum can swing forever. If we filmed it and ran the film in reverse, we would see movement that is completely possible. But if there is friction, then the pendulum heats its supports slightly, loses energy, and slows down. Friction produces heat. And immediately we are able to distinguish the future (toward which the pendulum slows) from the past. We have never seen a pendulum start swinging from a stationary position, with its movement initiated by the energy obtained by absorbing heat from its supports. The difference between past and future exists only when there is heat. The fundamental phenomenon that distinguishes the future from the past is the fact that heat passes from things that are hotter to things that are colder. So, again, why, as time goes by, does heat pass from hot things to cold and not the other way around? The reason was discovered by Boltzmann and is surprisingly simple: it is sheer chance.

In this way the temperature of objects in contact with each other tends to equalize. It is not impossible for a hot body to become hotter through contact with a colder one: it is just extremely improbable.

This may seem like an abstruse mental problem. But modern physics has made it into a burning issue, since special relativity has shown that the notion of the “present” is also subjective. Physicists and philosophers have come to the conclusion that the idea of a present that is common to the whole universe is an illusion and that the universal “flow” of time is a generalization that doesn’t work. When his great Italian friend Michele Besso died, Einstein wrote a moving letter to Michele’s sister: “Michele has left this strange world a little before me. This means nothing. People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.”

The flow of time emerges thus from physics, but not in the context of an exact description of things as they are. It emerges, rather, in the context of statistics and of thermodynamics. This may hold the key to the enigma of time. The “present” does not exist in an objective sense any more than “here” exists objectively, but the microscopic interactions within the world prompt the emergence of temporal phenomena within a system (for instance, ourselves) that interacts only through the medium of a myriad of variables.

The heat of black holes is a quantum effect upon an object, the black hole, which is gravitational in nature. It is the individual quanta of space, the elementary grains of space, the vibrating “molecules,” that heat the surface of black holes and generate black hole heat. This phenomenon involves all three sides of the problem: quantum mechanics, general relativity, and thermal science. The heat of black holes is like the Rosetta stone of physics, written in a combination of three languages—quantum, gravitational, and thermodynamic—still awaiting decipherment in order to reveal the true nature of time.