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

by Carlo Rovelli

During my entire research life, friends and curious people have asked me to explain what was going on in quantum gravity research.

book describing the research on the quantum nature of space and time, and on loop quantum gravity in particular, did not yet exist.

This is the region at the boundary between what we have understood and what we do not yet understand, and is still far from achieving consensus.

INTRODUCTION WALKING ALONG THE SHORE

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.

emerging today from the study of the main open problem in fundamental physics: quantum gravity.

synthesizing what we have learned about the world with the two major discoveries of twentieth-century physics: gener...

It tells of the picture of the world emerging today from research in quantum gravity, taking into account the latest indications given by nature, such as the confirmation of the cosmological Standard Model obtained from the Planck satellite and the failure at CERN to observe the supersymmetric particles that many expected. And it discusses the consequences of these ideas: the granular structure of space; the disappearance of time at small scale; the physics of the Big Bang; the origin of black hole heat—up to the role of information in the foundation of physics.

are unaccustomed. But we try. This is science. Scientific thinking explores and redraws the world, gradually offering us better and better images of it, teaching us to think in ever more effective ways.

This book begins in Miletus, twenty-six centuries ago.

Figure 1.1 The journey made by Leucippus of Miletus, the founder of the atomist school (circa 450 BCE).

The greatest of these thinkers was Anaximander.

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.

is the key to the subsequent development of philosophical and scientific thinking: from this moment onward, knowledge begins to grow at a vertiginous pace, nourished by past knowledge but at the same time by the possibility of criticism, and therefore improving knowledge and understanding.

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.

Democritus, the great pupil who wrote dozens of works on every field of knowledge, was deeply venerated in antiquity, which was familiar with these works. “The most subtle of the Ancients,” Seneca called him.

The Milesians had understood that the world can be comprehended using reason. They had become convinced that the variety of natural phenomena must be attributable to something simple and had tried to understand what this something might be.

alphabet in different ways we may obtain comedies or tragedies, ridiculous stories or epic poems, so elementary atoms combine to produce the world in its endless variety. The metaphor is Democritus’s own.6

ethical ideal of Democritus is that of a serenity of mind reached through moderation and balance, by trusting in reason and not allowing oneself to be overwhelmed by passions.

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.

One possible answer is that Zeno is wrong because it is not true that by accumulating an infinite number of things, one ends up with an infinite thing. 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.

Zeno had met Leucippus and had become his teacher. Leucippus was therefore familiar with Zeno’s riddles. But he had devised a different way of resolving them. Maybe, Leucippus suggests, nothing arbitrarily small exists: there is a lower limit to divisibility.

The first of these articles contained the definitive proof that atoms exist and calculated their dimensions, solving the problem posed by Leucippus and Democritus twenty-three centuries earlier.

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,

It is as if the small particle is receiving blows randomly from each side of it. In fact, it isn’t “as if” it were being hit; it really is hit. 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 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.

It is indeed possible, with a little mathematics, to work back from the amount of movement of the granule, which can be observed, to the dimensions of the molecules. Einstein does this at the age of twenty-five. 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.

It is the splendid poem De rerum natura (The Nature of Things, or On the Nature of the Universe), by the Latin poet Lucretius.

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:

Observe what happens when sunbeams are admitted into a

light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways in the empty space within the light of the beam, as though contending in everlasting conflict, rushing into battle rank upon rank with never a moment’s pause in a rapid sequence of unions and disunions.

2 THE CLASSICS ISAAC AND THE LITTLE MOON

Ptolemy’s book is a major work of science. Rigorous, precise, and complex, it presents a mathematical system of astronomy capable of predicting the seemingly random movements of the planets in the sky with almost complete precision, given the limitations of the human eye.

Even today, with a little studying, it is possible to open Ptolemy’s book, learn its techniques and calculate, for example, the position that Mars will have in a future sky. Today: two thousand years, that is, after the book was written.

Nicolaus Copernicus. The young Copernicus studies Ptolemy’s Almagest and falls in love with it. He decides to spend his life doing astronomy, following in the footsteps of the great Ptolemy.

but thoroughly improving it—with the courage to change it in depth. Instead of describing heavenly bodies turning around Earth, Copernicus writes a sort of revised and corrected version of Ptolemy’s Almagest, in which the sun is at the center and Earth, together with the other planets, runs around it.

Kepler shows that a few new mathematical laws can describe with exactitude the movements of the planets around the sun, with a degree of accuracy even greater than any obtained in antiquity.

Exuberant, Italian, polemical, argumentative, highly cultured, exceptionally intelligent, and overflowing with inventiveness, Galileo gets sent from Holland a new invention—the telescope—and makes a gesture that changes human history. He points it toward the sky.

But Galileo’s grand idea is to make the logical deduction from the cosmic revolution brought about by Copernicus: convinced that Earth is a planet like all others, Galileo reasons that if movements in the heavens follow precise mathematical laws, and if Earth is a planet like all

others, and thus part of the heavens, then there must also exist precise mathematical laws governing the movements of objects on Earth.

Confident of the rationality of nature and of the Pythagorean-Platonic vision that nature is understandable through mathematics, Galileo decides to study how objects move on Eart...

For the first time in the history of mankind, an experiment is made.

In this phase, what is constant is not the speed of the fall but rather the acceleration—that is to say, the rate at which speed increases. And magically, this acceleration turns out to be the same for all objects.

Its value is approximately 9.8 meters per second per second, which

second that an object falls, its speed increases by 9.8 ...

With a simple proportion, we can compute the period of the orbit of the little moon. The result is one hour and a half. The little moon would complete its orbit around Earth every hour and a half.