Seven Brief Lessons on Physics

by Carlo Rovelli

As if by magic: as if a friend were whispering into my ear an extraordinary hidden truth, suddenly raising the veil of reality to disclose a simpler, deeper order. Ever since we discovered that Earth is round and turns like a mad spinning-top, we have understood that reality is not as it appears to us: every time we glimpse a new aspect of it, it is a deeply emotional experience.

once you understand how it works, the theory has a breathtaking simplicity.

But a few years before the birth of Einstein two great British physicists, Michael Faraday and James Maxwell, had added a key ingredient to Newton’s cold world: the electromagnetic field. This field is a real entity that, diffused everywhere, carries radio waves, fills space, can vibrate and oscillate like the surface of a lake, and “transports” the electrical force.

For me, the electromagnetic/thermodynamic interaction of particles at the subatomic level is where our souls reside

he soon came to understand that gravity, like electricity, must be conveyed by a field as well: a “gravitational field” analogous to the “electrical field” must exist. He aimed at understanding how this “gravitational field” worked and how it could be described with equations.

at this point that an extraordinary idea occurred to him, a stroke of pure genius: 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.

A momentous simplification of the world: space is no longer something distinct from matter—it is one of the “material” components of the world. An entity that undulates, flexes, curves, twists. We are not contained within an invisible, rigid infrastructure: we are immersed in a gigantic, flexible snail shell. 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...

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. That is it. The equation fits into half a line, and there is nothing more. A vision—that space curves—became an equation.

All of this is the result of an elementary intuition: that space and gravitational field are the same thing.

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.

It was Bohr who understood that the energy of electrons in atoms can take on only certain values, like the energy of light, and crucially that electrons can only “jump” between one atomic orbit and another with determined energies, emitting or absorbing a photon when they jump. These are the famous “quantum leaps.”

In 1925 the equations of the theory finally appeared, replacing the entire mechanics of Newton.

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.

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.

“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’s as if God had not designed reality with a line that was heavily scored but just dotted it with a faint outline.

In quantum mechanics no object has a definite position, except when colliding head...

In order to describe it in mid-flight, between one interaction and another, we use an abstract mathematical formula that has no existence in real sp...

The question of probability goes to the heart of physics, where everything had seemed to be regulated by firm laws that were universal and irrevocable.

Patiently, Bohr explained the new ideas to Einstein. Einstein objected.

In the end Bohr always managed to find an answer with which to rebut these objections. For years, their dialogue continued by way of lectures, letters, articles . . . During the course of the exchange both great men needed to backtrack, to change their thinking.

Ultimately, Einstein conceded that the theory was a giant step forward in our understanding of the world, but he remained convinced that things could not be as strange as it proposed—that “behind” it there must be a further, more reasonable explanation. A century later we are at the same point.

For they do not describe what happens to a physical system but only how a physical system affects another physical system.

What does this mean?

Or does it mean, as it seems to me, that we must accept the idea that reality is only interaction?

When Einstein died, his greatest rival, Bohr, found for him words of moving admiration. When a few years later Bohr in turn died, someone took a photograph of the blackboard in his study. There’s a drawing on it. A drawing of the “light-filled box” in Einstein’s thought experiment. To the very last, the desire to challenge oneself and understand more. And to the very last: doubt.

before experiments, measurements, mathematics, and rigorous deductions, science is above all about visions. Science begins with a vision. Scientific thought is fed by the capacity to “see” things differently than they have previously been seen.

Copernicus understood and showed that our Earth is not at the center of the dance of the planets but that the sun is there instead. Our planet becomes one among the others, turning at high speed upon its axis and around the sun.

The things we see are made of atoms. Every atom consists of a nucleus surrounded by electrons. Every nucleus consists of tightly packed protons and neutrons. 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.”

Electrons, quarks, photons, and gluons are the components of everything that sway...

They are the “elementary particles” studied in p...

The nature of these particles, and the way they move, is described by quantum mechanics. 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. Even if we observe a small, empty region of space in which there are no atoms, we still detect a minute swarming of these particles. There is no such thing as a real void, one that is completely empty.

so the fields that form the world are subject to minute fluctuations,

and it is possible to imagine its basic particles having brief and ephemeral existences, continually created a...

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.

This is what its all about... This is where the "sausage" gets made.

led to an intricate theory, based on quantum mechanics and bearing the not very romantic title of “the Standard Model of elementary particles.” The Standard Model was finalized in the 1970s, after a long series of experiments that confirmed all predictions. Its final confirmation occurred in 2013 with the discovery of the Higgs boson.

For now, this is what we know of matter:

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.

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.

General relativity has taught us that space is not an inert box but rather something dynamic: a kind of immense, mobile snail shell in which we are contained—one that can be compressed and twisted. Quantum mechanics, on the other hand, has taught us that every field of this kind is “made of quanta” and has a fine, granular structure. It immediately follows that physical space is also “made of quanta.”

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.”

This is utterly fascinating

These are extremely minute: a billion billion times smaller than the smallest atomic nuclei.

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.

This fact - not objects independent of one another; but in dynamic engagement like the metaphor of the Yin/Yang Symbol

The equations describing grains of space and matter no longer contain the variable “time.”

but elementary processes cannot be ordered in a common succession of “instants.” At the minute scale of the grains of space, the dance of nature does not take place to the rhythm of the baton of a single orchestral conductor, at a single tempo: every process dances independently with its neighbors, to its own rhythm.

The passage of time is internal to the world, is born in the world itself in the relationship between quantum events that comprise the world and are themselves the source of time.

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.”

Another of the consequences of the theory, and one of the most spectacular, concerns the origins of the universe.

Well, the equations of loop theory allow us to go even further back in the reconstruction of that history.

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. And the final image of the third lesson is transformed