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

by Carlo Rovelli

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 space, time, matter and light, exchanging information between one event and another. Reality is a network of granular events; the dynamic which connects them is probabilistic; between one event and another, space, time, matter and energy melt in a cloud of probability.

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.

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.

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

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 were vast and systematic, at the time of the brutal anti-pagan repression following from the edicts of Emperor Theodosius, which, in 390–1 declared that Christianity was to be the only and obligatory religion of the empire. Plato and Aristotle, pagans who believed in the immortality of the soul or in the existence of a Prime Mover, could be tolerated by a triumphant Christianity. Not Democritus.

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 Catholic Church attempted to stop Lucretius: in the Florentine Synod of December 1516 it prohibited the reading of Lucretius in schools. In 1551, the Council of Trent banned his work. But it was too late. An entire vision of the world which had been swept away by medieval Christian fundamentalism was re-emerging in a Europe which had reopened its eyes. It was not just the rationalism, atheism and materialism of Lucretius that were being proposed in Europe. 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.

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 marvellous 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

Pythagoras was born on Samos, a small island not far from Miletus. His first biographers, Iamblichus and Porphyry, report how young Pythagoras was a disciple of elderly Anaximander. Everything originates in Miletus.

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.

Venus, Mars and Jupiter can be easily observed in the night sky. They seem to move a little at random, back and forth among the other stars. Is it possible to find a mathematics which is able to describe and predict their movements?

Ptolemy was an astronomer who lived in Alexandria in the first century of our era, under the Roman Empire, when science was already in decline and about to disappear altogether, overwhelmed by the collapse of the Hellenistic world and suffocated by the Christianization of the empire.

More or less at the same time as Poggio Bracciolini discovered the manuscript of Lucretius, the heady atmosphere of Italian humanism and the enthusiasm for ancient texts also intoxicated a young Pole who had come to study in Italy, first at Bologna, then at Padua. He signed himself in the Latin manner: 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.

if movements in the heavens follow precise mathematical laws, and if the 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.

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.

This is the first mathematical law discovered for earthly bodies: the law of falling bodies.fn4

The world of Newton is the world of Democritus, rendered mathematical.

The entire technology of the nineteenth century and of our own modern world rests largely upon Newton’s formulae.

Three centuries have passed, but it’s still thanks to theories based upon Newton’s equations that today we build bridges, trains and skyscrapers, engines and hydraulic systems; that we know how to fly planes, make weather forecasts, predict the existence of a planet before seeing it and send spaceships to Mars

The second and biggest surprise, crucial to the story I’m telling, is that understanding this force requires an important modification to the world of Newton: the modification out of which modern physics was born, and the most important notion to keep in focus, to understand the rest of this book – the notion of field.

His intuition is this: we must not think of forces acting directly between distant objects, as Newton presumed. We must instead think that there exists an entity diffused throughout space, which is modified by electric and magnetic bodies and which, in turn, acts upon (pushes and pulls) the bodies. This entity, whose existence Faraday intuits, is today called the field.

The contemporary world, based on communications, emerges from the intuitions of a poor London bookbinder – a skilful explorer of ideas with a vivid imagination – who saw some lines in his mind’s eye; and the work of a good mathematician who translated this vision into equations, understanding that in the blink of an eye the waves of these lines can carry news from one side of the planet to the other.

The deepening of our understanding of the world is based on two theories: general relativity and quantum mechanics. Both demand a daring re-evaluation of our conventional ideas about the world: space and time in relativity; matter and energy in quantum theory.

In Newton’s physics, velocity can only be velocity of something with respect to something else.

Einstein has claimed that he was not put on the right track by any experiments but only by reflecting on the apparent contradiction between Maxwell’s equations and Newton’s mechanics.

He asked himself whether there was a way of rendering Newton’s and Galileo’s core discoveries and Maxwell’s theory consistent.

This means we can say that, on Mars, there are events that in this precise moment have already happened, events that are yet to happen, but also a quarter of an hour during which things occur that are neither in our past nor in our future.

Since antiquity, the idea of empty space, halfway between a thing and a non-thing, had troubled thinkers.

Thus, Einstein addresses not one but two problems. First, how can we describe the gravitational field? Second, what is Newton’s space?

What if Newton’s space was nothing more than the gravitational field? This extremely simple, beautiful, brilliant idea is the theory of general relativity.

Space is no longer different from matter. It is one of the ‘material’ components of the world, akin to the electromagnetic field. It is a real entity which undulates, fluctuates, bends and contorts.

Without the notion of fields introduced by Faraday, without the spectacular power of mathematics, without the geometry of Gauss and Riemann, this ‘certain physics’ would have remained incomprehensible. Empowered by new conceptual tools and by mathematics, Einstein writes the equations which describe Democritus’s void and finds for its ‘certain physics’ a colourful and amazing world where universes explode, space collapses into bottomless holes, time slows down in the vicinity of a planet, and the boundless expanses of interstellar space ripple and sway like the surface of the sea …

And all this is the result only of an elementary intuition – that spacetime and the gravitational field are one and the same thing

Einstein had a unique capacity to imagine how the world might be constructed, to ‘see’ it in his mind. The equations, for him, came afterwards; they were the language with which to make concrete his visions of reality. For Einstein, the theory of general relativity is not a collection of equations: it is a mental image of the world arduously translated into equations.

But Einstein finds a third way: the universe can be finite and at the same time have no boundary.

A three-dimensional space of this kind, finite but without boundary, is called a 3-sphere.

Everything attracts, therefore the only way for a finite universe not to collapse on itself is for it to be expanding: just as the only way to prevent a football from falling to the ground is to kick it upwards. It either goes up, or falls down – it can’t stay still, suspended in the air.

the strange physics of this theory, relates how the theory came into being and the three aspects of reality it has unveiled: granularity, indeterminism and relationality.

For Max Planck, taking energy in finite-size packets was only a strange trick which happened to work for the calculation – that is, to reproduce laboratory measurements – but for utterly unclear reasons.

At first, Einstein’s idea that light could be made up of photons is regarded by his colleagues as no more than youthful waywardness. Everyone commends him for his theory of relativity, but everybody judges the notion of photons to be outlandish.

What if these were the mysterious quantum leaps which appeared to underlie the structure of the atomic spectra? What if, between one interaction with something, and another with something else, the electron could literally be nowhere.

What if the electron could be something that manifests itself only when it interacts, when it collides with something else; and that between one interaction and another it had no precise position?

a fundamental description of the movement of particles, in which they are described not by their position at every moment but only by their position at particular instants: the instants in which they interact with something else.

When nothing disturbs it, an electron does not exist in any place.

the world is not made of things, it’s constituted of an abstract mathematical structure which shows us how things appear and how they behave when manifesting themselves. It’s a magical encounter between logic and intuition.

every object is defined by an abstract spacefn20 and has no property in itself, apart from those that are unchanging, such as mass. Its position and velocity, its angular momentum and its electrical potential, and so on, acquire reality only when it collides – ‘interacts’– with another object.

The apparent determinism of the macroscopic world is due only to the fact that the microscopic randomness cancels out on average, leaving only fluctuations too minute for us to perceive in everyday life.

The notions of fields and particles, separated by Faraday and Maxwell, end up merging in quantum mechanics.

Photons are the quanta of the electromagnetic field.