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

by Carlo Rovelli

I began to design the entire book in my mind while driving, and got increasingly excited, until I heard a police car’s siren telling me to pull over: I was driving far above the speed limit. 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.

the more we discover, the more we understand that what we don’t yet know is greater than what we know.

Today we see almost to the Big Bang, the great explosion from which, fourteen billion years ago, all the galaxies were born—but we have already begun to glimpse something beyond the Big Bang. We have learned that space is curved but already foresee that this same space is woven from vibrating quantum grains.

An elementary structure of the world is emerging, generated by a swarm of quantum events, where time and space do not exist. Quantum fields draw together space, time, matter, and light, exchanging information between one event and another.

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.

The problem of synthesizing what we have learned about the world with the two major discoveries of twentieth-century physics: general relativity and quantum theory.

gradually offering us better and better images of it, teaching us to think in ever more effective ways. Science is a continual exploration of ways of thinking.

Science is made up of experiments, hypotheses, equations, calculations, and long discussions; but these are only tools, like the instruments of musicians. In the end, what matters in music is the music itself, and what matters in science is the understanding of the world that science provides.

The connection between problems posed by the scientists of antiquity and solutions found by Einstein and quantum gravity is, as we shall see, surprisingly close.

The man was probably fleeing political turmoil in Miletus, where the aristocracy was violently seizing back power. Miletus had been a prosperous and flourishing Greek city, perhaps the principal city of the Greek world before the golden age of Athens and Sparta.

During the preceding century, a revolution in thinking that would prove fundamental to humanity had taken place in Miletus. A group of thinkers had reformulated the way questions were asked about the world—and the way answers were sought.

Then, at Miletus, at the beginning of the sixth century before our era, Thales, his pupil Anaximander, Hecataeus, and their school find a different way of looking for answers. This immense revolution in thought inaugurates a new mode of knowledge and understanding and signals the first dawn of scientific thought.

The Milesians understand that by shrewdly using observation and reason, rather than searching for answers in fantasy, ancient myths, or religion—and above all by using critical thought in a discriminating way—it is possible to repeatedly correct our worldview and to discover new aspects of reality that are hidden to the common view.

The dazzling incipit of Hecataeus’s book of history goes to the heart of this critical thinking, including as it does the awareness of our own fallibility: “I wrote things which seem true to me, because the accounts of the Greeks seem to be full of contradictory and ridiculous things.”

Within a matter of a few years, Anaximander understands that Earth floats in the sky and the sky continues beneath Earth; that rainwater comes from the evaporation of water on Earth; that the variety of substances in the world must be susceptible to being understood in terms of a single, unitary, and simple constituent, which he calls apeiron, the indistinct; that animals and plants evolve and adapt to changes in the environment, and that man must have evolved from other animals.

Situated at a point of conjunction between the emergent Greek civilization and the ancient empires of Mesopotamia and Egypt, nourished by their knowledge but immersed in the liberty and the political fluidity that is typically Greek; in a social space without imperial palaces, or powerful priestly castes, where individual citizens discuss their destinies in open agoras, Miletus is the place where for the first time men decide collectively their own laws; where the first parliament in the history of the world gathers—the Panionium, meeting place of the delegates of the Ionian League—and where for the first time men doubt that only the gods are capable of accounting for the mysteries of the world.

THIS LUMINOUS MILETUS shortly afterward came to a calamitous end. The arrival of the Persian Empire, and a failed anti-imperial revolt, lead to a ferocious destruction of the city in 494 BCE and to the enslavement of a large number of its inhabitants.

The idea of Democritus’s system is extremely simple: the entire universe is made up of a boundless space in which innumerable atoms run. Space is without limits; has neither an above nor a below; is without a center or a boundary.

Atoms have no qualities at all, apart from their shape. They have no weight, no color, no taste.

On this foundation Democritus wrote dozens of books articulating a vast system, dealing with questions of physics, philosophy, ethics, politics, and cosmology. He writes on the nature of language, on religion, on the origins of human societies, and on much else besides. (The opening of his Little Cosmology is impressive: “In this work I treat of all things.”)

We know of his thought only through the quotations and references made by other ancient authors, and by their summaries of his ideas.7 The thought that thus emerges is a kind of intense humanism, rationalist and materialist.8 Democritus combines a keen attention to nature, illuminated by a naturalistic clarity in which every residual system of mythic ideas is cleared away, with a great attention to humanity and a deep ethical concern for life—anticipating by some two thousand years the best aspects of the eighteenth-century Enlightenment.

Plato and Aristotle were familiar with Democritus’s ideas and fought against them. They did so on behalf of other ideas, some of which were later, for centuries, to create obstacles to the growth of knowledge.

Aristotle speaks extensively about the ideas of Democritus, with respect. Plato never cites Democritus, but scholars suspect today that this was out of deliberate choice,

Plato was an ass. Brilliant but an ass.

Criticism of Democritus’s ideas is implicit in several of Plato’s texts, as in his critique of “physicists,” for example. In a passage in his Phaedo, Plato has Socrates articulate a reproach to all “physicists,” which will have a lasting resonance. He complains that when “physicists” had explained that Earth was round, he rebelled because he wanted to know what “good” it was for Earth to be round; how its roundness would benefit it.

Without needing anything from modern physics, Democritus had already arrived at the idea that everything is made up of indivisible particles.

Democritus observed that matter could not be a continuous whole, because there is something contradictory in the proposition that it should be so.

Imagine, says Democritus, that matter is infinitely divisible, that is to say, that it may be broken down an infinite number of times. Imagine then that you break up a piece of matter ad infinitum. What would be left? 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.

The only possibility, Democritus concludes, is that any piece of matter is made up of a finite number of discrete pieces that are indivisible, each one having finite size: the atoms.

Parmenides had explored an avenue to truth via pure reason alone, a path that led him to declare that all appearances are illusory, thus opening the path that would progressively move toward metaphysics and distance itself from what would come to be known as “natural science.” His pupil Zeno, also from Elea, had brought subtle arguments to bear in support of this fundamentalist rationalism, which refutes the credibility of appearances radically.

Think of taking a piece of string, cutting it in half, and then again in half, and so on ad infinitum. At the end you will obtain an infinite number of small pieces of string; the sum of these, however, will be finite, because they can only add up to the length of the original piece of string. Hence, an infinite number of strings can make a finite string; an infinite number of increasingly short times may make a finite time, and the hero, even if he will have to cover an infinite number of distances, ever smaller, will take a finite time to do so, and will end up catching the tortoise.

But is this really the correct solution in the real world? Do arbitrarily short strings really exist? Can we really cut a piece of string an arbitrary number of times? Do infinitely small amounts of time exist? This is precisely the problem that quantum gravity will have to face.

The universe is granular, not continuous. With infinitely small points, it would be impossible to ever construct extension—as in Democritus’s argument reported by Aristotle and mentioned previously. Therefore, the extension of the string must be formed by a finite number of finite objects with finite size. The string cannot be cut as many times as we want; matter is not continuous, it is made of individual “atoms” of a finite size.

Still at the beginning of the last century numerous scientists and philosophers did not consider the atomic hypothesis to be credible. Among them was the renowned physicist and philosopher Ernst Mach, whose ideas on space would come to have great importance for Albert Einstein. At the end of a lecture by Ludwig Boltzmann at the Imperial Academy of Sciences in Vienna, Mach publicly declared: “I do not believe that atoms exist!”

The definitive proof of the “atomic hypothesis” had to wait until 1905. It was found by a rebellious twenty-five-year-old who had studied physics but had not been able to find employment as a scientist and was making ends meet by working as an employee in the patent office in Berne. I will speak a lot about this young man in the rest of this book, and about the three articles he sent to the most prestigious physics journal of the time, the Annalen der Physik.

Anyone could have arrived at it, from the time of Democritus onward, if he had had Einstein’s acumen, and a sufficient mastery of mathematics to make what was not an easy calculation. The idea goes like this: if we observe attentively very small particles, such as a speck of dust or a grain of pollen, suspended in still air or in a liquid, we see them tremble and dance. Pushed by this trembling, they move, randomly zigzagging, and so they drift slowly, gradually moving away from their starting point. This motion of particles in a fluid is called Brownian motion, after Robert Brown, a biologist who described it in detail in the nineteenth century.

It trembles because it is hit by the individual molecules of air, which collide with the particle at times from the right and at times from the left.

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.

the collisions never balance out exactly; they only balance out on average.

The smaller the molecules, however, the shorter the interval between collisions and the better would hits from different directions balance and cancel one another out. And the less the granule would move.

From observations of granules drifting in fluids, from the measurement of how much these “drift”—that is, move away from a position—he calculates the dimensions of Democritus’s atoms, the elementary grains of which matter is made. He provides, after twenty-three hundred years, the proof of the accuracy of Democritus’s insight: matter is granular.

The closure of the ancient schools such as those of Athens and Alexandria, and the destruction of all the texts not in accordance with Christian ideas was vast and systematic, at the time of the brutal antipagan repression following from the edicts of Emperor Theodosius, which in 390–391 declared that Christianity was to be the only and obligatory religion of the empire.

Epicurus is interested more in ethical than scientific questions and does not have Democritus’s depth. He sometimes translates Democritean atomism a little superficially. But his vision of the natural world is substantially that of the great philosopher of Abdera.

There is a sense of luminous calm and serenity about the poem, which comes from understanding that there are no capricious gods demanding of us difficult things, and punishing us.

For Lucretius, religion is ignorance: reason is the torch that enlightens.

Lucretius’s text, forgotten for centuries, was rediscovered in January 1417 by the humanist Poggio Bracciolini, in the library of a German monastery.

Humanist and book thief.

the rediscovery of De rerum natura had a profound effect upon the Italian and European Renaissance,19 and its echo resounds, directly or indirectly, in the pages of authors ranging from Galileo20 to Kepler,21 and from Bacon to Machiavelli.

The very idea of Einstein’s, that the existence of atoms is revealed by the Brownian motion of minute particles immersed in a fluid, may be traced back to Lucretius.

Einstein resuscitated the “living proof” presented by Lucretius, and probably first conceived of by Democritus, and made it solid by translating it into mathematical terms, thus managing to calculate the size of the atoms.

The medieval cosmos so marvelously sung by Dante was interpreted on the basis of a hierarchical organization of the universe that reflected the hierarchical organization of European society: a spherical cosmic structure with Earth at its center; the irreducible separation between Earth and heavens; finalistic and metaphorical explanations of natural phenomena; fear of God, fear of death; little attention to nature; the idea that forms preceding things determine the structure of the world; the idea that the source of knowledge could only be the past, in revelation and tradition. .

This idea is at the root of the atomic hypothesis, but it will also return with augmented force with quantum mechanics, and today is revealing itself to be powerful again—as the keystone of quantum gravity.