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

by Carlo Rovelli

If we try to put together what we have learned about the physical world in the course of the twentieth century, the clues point toward something profoundly different from what we were taught at school.

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. This strange new world is slowly emergi...

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

Science is a continual exploration of ways of thinking.

This adventure rests upon the entirety of past knowledge, but at its heart is change.

I’ve written with a particular reader in mind: someone who knows little or nothing about today’s physics but is curious to find out what we know, as well as what we don’t yet understand, about the elementary weave of the world—and where we are searching.

I’ve sought to outline the general landscape of the structure of the physical world, as seen by the double lights of relativity and of quantum physics, and to show how they can be combined.

This is not only a book of divulgation; it’s also one that articulates a point of view, in a field of research where the abstraction of technical language may sometimes obscure the wide-angle vision.

This book gives an account of the current state of the search for our new image of the world, as I understand it today. It is the reply I would give to a colleague and friend asking me, “So, what do you think is the true nature of things?”

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.

According to tradition, in the year 450 BCE, a man embarked upon a ship traveling from Miletus to Abdera (figure 1.1). It was to be a crucial journey for the history of knowledge.

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.

Perhaps the decisive discovery is that of a different style of thinking, where the disciple is no longer obliged to respect and share the ideas of the master but is free to build on those ideas without being afraid to discard or criticize the part that can be improved.

It 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.

This new approach to knowledge works quickly and impressively.

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. Thus gradually founding the basis of a grammar for understanding the world, which is substantially still our own today.

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.

They had conceived of a kind of elementary substance of which ...

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. “Sweetness is opinion, bitterness is opinion; heat, cold and color are opinion: in reality only atoms, and vacuum.”

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

random and accidental—of

of this movement, and this combin...

When atoms aggregate, the only thing that matters, the only thing that exists at the elementary level, is their shape, their arrangement, and the order in which they combine. Just as by combining letters of the 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.

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.

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.

The 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.

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 string cannot be cut as many times as we want; matter is not continuous, it is made of individual “atoms” of a finite size.

Perhaps if all the works of Democritus had survived, and nothing of Aristotle’s, the intellectual history of our civilization would have been better.

But centuries dominated by monotheism have not permitted the survival of Democritus’s naturalism.

You must understand that they all derive this restlessness from the atoms. It originates with the atoms, which move of themselves. Then those small compound bodies that are least removed from the impetus of the atoms are set in motion by the impact of their invisible blows and in turn cannon against slightly larger bodies. So the movement mounts up from the atoms and gradually emerges to the level of our senses, so that those bodies are in motion that we see in sunbeams, moved by blows that remain invisible.23 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.

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.

There is none of this in the world of Democritus as sung by Lucretius. There is no fear of the gods; no ends or purposes in the world; no cosmic hierarchy; no distinction between Earth and heavens. There is a deep love of nature, a serene immersion within it; a recognition that we are profoundly part of it; that men, women, animals, plants, and clouds are organic threads of a marvelous whole, without hierarchies.

There is a feeling of deep universalism, in the wake of the splendid words of Democritus: “To a wise man, the whole earth is open, because the true country of a virtuous soul is the entire universe.”25

the idea of free and rectilinear motion in space; the idea of elementary bodies and their interactions, out of which the world is constructed; the idea of space as a container of the world.

And there is the simple idea of the finite divisibility of things.

The granular quality of the world. The idea that stops the infinite between our fingers. 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 i...

Plato divested Pythagorism of its cumbersome and useless mystical baggage. He absorbed and distilled its useful message: mathematics is the language best adapted to understand and describe the world.

The reach of this insight is immense; it is one of the reasons for the success of Western science. According to tradition, Plato carved on the door of his school the phrase: “Let no one enter here who is ignorant of geometry.”

Mathematics allows the world to be described and the future to be predicted: the apparently wandering and disorderly movements of the planets can be precisely predicted by using mathematical formulas that Ptolemy, summarizing the results of centuries of work by Greek astronomers, presents in a systematic and masterly way.

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

Experimental science begins with Galileo. His experiment is simple: he lets objects fall; that is, he lets them follow what for Aristotle was their natural movement, and seeks to measure precisely their falling speed.

The result is momentous: objects do not fall always at a constant speed, as everybody thought. Their speed, instead, gradually increases during the early part of the fall. 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 incr...

Its value is approximately 9.8 meters per second per second, which is to say, that for every second that an object falls, its speed increases by 9.8 me...

But the greatest result is still to come, and it will be Isaac Newton who will attain it. Newton studies in depth the results of Galileo and Kepler and, combining them, finds the hidden diamond.

We can follow his reasoning in terms of the “little moon,” as he himself tells it in The Mathematical Principles of Natural Philosophy, or Principia, the book that founded modern science.