Seven Brief Lessons on Physics

by Carlo Rovelli

You don’t get anywhere by not “wasting” time—something, unfortunately, that the parents of teenagers tend frequently to forget.

theory of relativity (known today as “special relativity”), the theory that elucidates how time does not pass identically for everyone:

the gravitational field is not diffused through space; the gravitational field is that space itself. This is the idea of the general theory of relativity. Newton’s “space,” through which things move, and the “gravitational field” are one and the same thing.

The sun bends space around itself, and Earth does not turn around it because of a mysterious force but because it is racing directly in a space that inclines, like a marble that rolls in a funnel. There are no mysterious forces generated at the center of the funnel; it is the curved nature of the walls that causes the marble to roll. Planets circle around the sun, and things fall, because space curves.

properties of a curved space are captured by a particular mathematical object, which we know today as Riemann’s curvature and indicate with the letter R. Einstein wrote an equation that says that R is equivalent to the energy of matter. That is to say: space curves where there is matter.

the energy of a light ray spreading out from a point source is not continuously distributed over an increasing space but consists of a finite number of “energy quanta” which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.

why does the periodic table have this particular structure, with these periods, and with the elements having these specific properties? The answer is that each element corresponds to one solution of the main equation of quantum mechanics. The whole of chemistry emerges from a single equation.

Science begins with a vision. Scientific thought is fed by the capacity to “see” things differently than they have previously been seen.

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. The force that “glues” quarks inside protons and neutrons is generated by particles that physicists, with little sense of the ridiculous, call “gluons.”

These particles do not have a pebble-like reality but are rather the “quanta” of corresponding fields, just as photons are the “quanta” of the electromagnetic field. They are elementary excitations of a moving substratum similar to the field of Faraday and Maxwell. Minuscule moving wavelets. They disappear and reappear according to the strange laws of quantum mechanics, where everything that exists is never stable and is nothing but a jump from one interaction to another.

Quantum mechanics and experiments with particles have taught us that the world is a continuous, restless swarming of things, a continuous coming to light and disappearance of ephemeral entities. A set of vibrations, as in the switched-on hippie world of the 1960s. A world of happenings, not of things.

A handful of types of elementary particles, which vibrate and fluctuate constantly between existence and nonexistence and swarm in space, even when it seems that there is nothing there, combine together to infinity like the letters of a cosmic alphabet to tell the immense history of galaxies; of the innumerable stars; of sunlight; of mountains, woods, and fields of grain; of the smiling faces of the young at parties; and of the night sky studded with stars.

The twentieth century gave us the two gems of which I have spoken: general relativity and quantum mechanics. From the first cosmology developed, as well as astrophysics, the study of gravitational waves, of black holes, and much else besides. The second provided the foundation for atomic physics, nuclear physics, the physics of elementary particles, the physics of condensed matter, and much, much more. Two theories, profligate in their gifts, which are fundamental to today’s technology and have transformed the way we live. And yet the two theories cannot both be right, at least in their current forms, because they contradict each other.

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.

Loop quantum gravity is an endeavor to combine general relativity and quantum mechanics.

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.

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.

A Planck star is not stable: once compressed to the maximum, it rebounds and begins to expand again. This leads to an explosion of the black hole. This process, as seen by a hypothetical observer sitting in the black hole on the Planck star, would be a rebound occurring at great speed. But time does not pass at the same speed for him as for those outside the black hole, for the same reason that in the mountains time passes faster than at sea level. Except that for him, because of the extreme conditions, the difference in the passage of time is enormous, and what for the observer on the star would seem an extremely rapid bounce would appear, seen from outside it, to take place over a very long time. This is why we observe black holes remaining the same for long periods of time: a black hole is a rebounding star seen in extreme slow motion.

Our universe may have been born from a bounce in a prior phase, passing through an intermediate phase in which there was neither space nor time.

The gravitational field, as we saw in the first lesson, is space itself, in effect space-time. Therefore, when heat is diffused to the gravitational field, time and space themselves must vibrate . . . But we still don’t know how to describe this well. We don’t have the equations to describe the thermal vibrations of a hot space-time. What is a vibrating time? Such issues lead us to the heart of the problem of time: what exactly is the flow of time?

For a hypothetically supersensible being, there would be no “flowing” of time: the universe would be a single block of past, present, and future. But due to the limitations of our consciousness we perceive only a blurred vision of the world and live in time.

Using quantum mechanics, Hawking successfully demonstrated that black holes are always “hot.” They emit heat like a stove. It’s the first concrete indication on the nature of “hot space.” No one has ever observed this heat because it is faint in the actual black holes that have been observed so far—but Hawking’s calculation is convincing, it has been repeated in different ways, and the reality of the heat of black holes is generally accepted. 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.

We are like an only child who in growing up realizes that the world does not revolve only around himself, as he thought when little. He must learn to be one among others. Mirrored by others, and by other things, we learn who we are.

I believe that our species will not last long. It does not seem to be made of the stuff that has allowed the turtle, for example, to continue to exist more or less unchanged for hundreds of millions of years, for hundreds of times longer, that is, than we have even been in existence. We belong to a short-lived genus of species. All of our cousins are already extinct. What’s more, we do damage. The brutal climate and environmental changes that we have triggered are unlikely to spare us. For Earth they may turn out to be a small irrelevant blip, but I do not think that we will outlast them unscathed—especially since public and political opinion prefers to ignore the dangers that we are running, hiding our heads in the sand. We are perhaps the only species on Earth to be conscious of the inevitability of our individual mortality.

Here, on the edge of what we know, in contact with the ocean of the unknown, shines the mystery and the beauty of the world. And it’s breathtaking.