Reality Is Not What It Seems: The Journey to Quantum Gravity

by Carlo Rovelli

The Italian policeman asked me politely if I was crazy to drive at that speed. I explained that I had just found the idea I’d been seeking for so long; the policeman let me go without a ticket, and wished me good luck with the book. This is that book.

This is not a book about certainties; it is a book about the adventure of moving toward the unknown.

Reality is a network of granular events; the dynamic that connects them is probabilistic; between one event and another, space, time, matter, and energy melt into a cloud of probability.

Science is a continual exploration of ways of thinking. Its strength is its visionary capacity to demolish preconceived ideas, to reveal new regions of reality, and to construct new and more effective images of the world.

The incompleteness and the uncertainty of our knowledge, our precariousness, suspended over the abyss of the immensity of what we don’t know, does not render life meaningless: it makes it interesting and precious.

Science is about reading the world from a gradually widening point of view.

Atoms are indivisible; they are the elementary grains of reality, which cannot be further subdivided, and everything is made of them. They move freely in space, colliding with one another; they hook on to and push and pull one another. Similar atoms attract one another and join. This is the weave of the world. This is reality. Everything else is nothing but a by-product—random and accidental—of this movement, and this combining of atoms. The infinite variety of the substances of which the world is made derives solely from this combining of atoms.

There is no finality, no purpose, in this endless dance of atoms. We, just like the rest of the natural world, are one of the many products of this infinite dance—the product, that is, of an accidental combination.

Our life is a combination of atoms, our thoughts are made up of thin atoms, our dreams are the products of atoms; our hopes and our emotions are written in a language formed by combinations of atoms; the light that we see is composed of atoms, which bring us images. The seas are made of atoms, as are our cities, and the stars.

If, in some cataclysm, all scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis, or the atomic fact, or whatever you wish to call it, that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.

Could small particles of extended dimension remain? No, because if this were the case, the piece of matter would not yet be broken up to infinity. Therefore, only points without extension would remain. But now let us try to put together the piece of matter starting from these points: by putting together two points without extension, you cannot obtain a thing with extension, nor can you with three, or even with four. No matter how many you put together, in fact, you never have extension, because points have no extension. Therefore we cannot think that matter is made of points without extension, because no matter how many of these we manage to put together, we never obtain something with an extended dimension.

The subtle point is the following: there are an enormous number of molecules of air. On average, as many hit the granule from the left as hit it from the right. If the air’s molecules were infinitely small and infinitely numerous, the effect of the collisions from right and from left would balance, and thus cancel out at each instant, and the granule would not move. But the finite size of the molecules, the fact that these are present in finite rather than infinite number, causes there to be fluctuations (this is the key word): that is to say, the collisions never balance out exactly; they only balance out on average.

Democritus’s insight: matter is granular.

The beauty of the poem lies in the sense of wonder that pervades the vast atomistic vision. The sense of the profound unity of things, derived from the knowledge that we are all made of the same substance as are the stars, and the sea:

Do you not see that nature is clamoring for two things only, a body free from pain, a mind released from worry and fear for the enjoyment of pleasurable sensations?17

religion is ignorance: reason is the torch that enlightens.

The very idea of Einstein’s, that the existence of atoms is revealed by the Brownian motion of minute particles immersed in a fluid,

It was not merely a luminous and serene meditation on the beauty of the world. It was much more: it was an articulate and complex structure of thinking about reality, a new mode of thinking, radically different from what had been for centuries the mind-set of the Middle Ages.

To comprehend how light may be simultaneously an electromagnetic wave and a swarm of photons will require the entire construction of quantum mechanics. But the first building block of this theory has been established: there exists a fundamental granularity in all things, including light.

Bohr makes the hypothesis that electrons can exist only at certain “special” distances from the nucleus, that is, only on certain particular orbits, the scale of which is determined by Planck’s constant h. And that electrons can “leap” between one orbit with the permitted energy to another. These are the famous “quantum leaps.” The frequency at which the electron moves on these orbits determines the frequency of the emitted light, and since only certain orbits are allowed, it follows that only certain frequencies are emitted.

This is a second cornerstone of quantum mechanics, its hardest key: the relational aspect of things. Electrons don’t always exist. They exist when they interact. They materialize in a place when they collide with something else. The “quantum leaps” from one orbit to another constitute their way of being real: an electron is a combination of leaps from one interaction to another. When nothing disturbs it, an electron does not exist in any place.

Area is not continuous; it is granular. There is no such thing as an arbitrarily small area. Space appears continuous to us only because we cannot perceive the extremely small scale of these individual quanta of space, just as when we look closely at the cloth of a T-shirt, we see that it is woven from small threads.

the theory of loop quantum gravity, or “loop theory,” combines general relativity with quantum mechanics in a rather conservative way, because it does not employ any other hypotheses apart from those of the two theories themselves, suitably rewritten to render them compatible. But the consequences are radical.

General relativity taught us that space is something dynamic, like the electromagnetic field: an immense, mobile mollusk in which we are immersed, which stretches and bends. Quantum mechanics teaches us that every field of this sort is “made of quanta,” has a fine, granular structure. It follows that physical space, being a field, is “made of quanta” as well. The same granular structure characterizing the other quantum fields also characterizes the quantum gravitational field, and therefore space.

Space as an amorphous container of things disappears from physics with quantum gravity. Things (the quanta) do not inhabit space; they dwell one over the other, and space is the fabric of their neighboring relations. As we abandon the idea of space as an inert container, similarly we must abandon the idea of time as an inert flow, along which reality unfurls. Just as the idea of the space continuum containing things disappears, so too does the idea of a flowing continuum “time,” during the course of which phenomena happen.

The closer you get to Earth, where gravity is more intense, the slower time passes.

every object in the universe has its own time, running at a pace determined by the local gravitational field.

we, in reality, never measure time itself; we always measure the physical variables A, B, C . . . (oscillations, beats, and many other things) and compare one variable with another, that is to say, we measure the functions A(B), B(C), C(A), and so on.

The quanta of gravity are the way in which space and time interact.

All of this takes us well over the edge of what we know, to the vantage point from which we look upon what we definitely don’t know: the immense mystery that surrounds us.

Belgian priest suggests that the universe was originally extremely small and compressed, and started its expansion in a gigantic explosion. He calls this initial state the “primordial atom.” Today it is known as the “Big Bang.”

His name was Georges Lemaître. In French, this name sounds like “le maître” meaning “the master,” and few names are more appropriate for the man who first understood the existence of the Big Bang. But in spite of this name, Lemaître’s character was reserved; he avoided polemics and never even claimed priority for the discovery of the expansion of the universe, which ended up being attributed to Hubble.

the Bible knows nothing about physics, and physics knows nothing about God.

fourteen billion years ago the universe was a compressed ball of fire. But what happened before this initial hot and compressed state?

Quantum mechanics prevents a real electron from falling into a nucleus. A quantum repulsion pushes away the electron when it gets too close to the center. Thus, thanks to quantum mechanics, matter is stable. Without it, electrons would fall into nuclei, there would be no atoms, and we would not exist.

the word “universe” has assumed another meaning in cosmology: it refers to the spacetime continuum that we see directly around us, filled with galaxies the geometry and history of which we observe. There is no reason to be certain that, in this sense, this universe is the only one in existence.

Science works because, after hypotheses and reasoning, after intuitions and visions, after equations and calculations, we can check whether we have done well or not: the theory gives predictions about things we have not yet observed, and we can check whether these are correct, or not. This is the power of science, that which grounds its reliability and allows us to trust it with confidence: we can check whether a theory is right or wrong. This is what distinguishes science from other kinds of thinking, where deciding who is right and who is wrong is usually a much thornier question, sometimes even devoid of meaning.

Science is the best strategy if we value reliability.

The black hole at the center of our own galaxy is currently being studied in detail. It has a mass a million times greater than our sun. Every so often, a star gets too close to this monster, is disintegrated by the gravitational distortion, and is swallowed by the cyclopean black hole,

On the horizon itself, time stops: if we get extremely close to it and then move away after a few of our minutes, a million years might have elapsed in the rest of the universe.

General relativity predicts that everything is squashed at the center into an infinitely small point of infinite density. But this is, once again, if we ignore quantum theory.

In the end, basically, this is what a black hole is: a shortcut to the distant future.

Quantum gravity is the discovery that no infinitely small point exists. There is a lower limit to the divisibility of space. The universe cannot be smaller than the Planck scale, because nothing exists that is smaller than the Planck scale.

what we call “infinite” often is nothing more than something that we have not yet counted, or understood. I think this is true in general. “Infinite,” ultimately, is the name that we give to what we do not yet know. Nature appears to be telling us that there is nothing truly infinite.

in order to understand reality, we have to keep in mind that reality is this network of relations, of reciprocal information, that weaves the world. We slice up the reality surrounding us into “objects.” But reality is not made up of discrete objects. It is a variable flux.

We are not atoms: we are orders in which atoms are arranged, capable of mirroring other atoms and mirroring ourselves.

This acute awareness of our ignorance is the heart of scientific thinking. It is thanks to this awareness of the limits of our knowledge that we have learned so much.

Science is born from this act of humility: not trusting blindly in our past knowledge and our intuition. Not believing in what everyone says. Not having absolute faith in the accumulated knowledge of our fathers and grandfathers. We learn nothing if we think that we already know the essentials, if we assume that they were written in a book or known by the elders of the tribe.

A scientist is someone who lives immersed in the awareness of our deep ignorance, in direct contact with our own innumerable limits, with the limits of our understanding.

Science is not reliable because it provides certainty. It is reliable because it provides us with the best answers we have at present. Science is the most we know so far about the problems confronting us. It is precisely its openness, its constant putting of current knowledge in question, that guarantees that the answers it offers are the best so far available: if you find better answers, these new answers become science.