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

by Carlo Rovelli

Aristotle’s studies of nature—of botany and zoology, for example—are extraordinary scientific works, grounded in meticulous observations of the natural world. The conceptual clarity, the attention to the variety of nature, the impressive intelligence and openness of mind of the great philosopher, made him an authority for centuries to come.

Aristotle presents it in a book entitled, precisely, Physics. The book didn’t take its title from the name of a discipline: it was the discipline that got its name from Aristotle’s book.

On Earth, on the other hand, it is necessary to distinguish between forced motion and natural motion. Forced motion is caused by a thrust and ends when the thrust ends. Natural motion is vertical—upward or downward—and depends both on the substance and the location.

Do not smile at this theory, or dismiss it, because it is very sound physics. It’s a good and correct description of the motion of bodies immersed in a fluid and subject to gravity and friction—namely, the real things we meet in our everyday experience. It’s not wrong physics, as is frequently said.* It’s an approximation.

the physics of Newton, too, is an approximation of general relativity. And probably everything that we know today, as well, is an approximation of something else that we don’t yet know.

The physics of Aristotle is still rough; it is not quantitative (we cannot compute with it), but it is coherent and rational, and it enables cor...

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 exercise begins with Eudoxus in Plato’s school and is pursued throughout the following centuries by astronomers such as Aristarchus and Hipparchus, bringing ancient astronomy onto an extremely high scientific level.

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.

From India, this knowledge returned to the West, thanks to learned Persian and Arab scientists who were able to understand and preserve it. But astronomy did not take any very significant step forward for more than a thousand years.

But the time is now ripe, and more than a thousand years after Ptolemy, Copernicus is able to make the leap forward that generations of Indian, Arab, and Persian astronomers had not been able to make: not simply learning, applying, and adding small ameliorations to the Ptolemaic system, 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.

Painstakingly analyzing new, precise observations, 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.

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.

This is the first mathematical law discovered for earthly bodies: the law of falling bodies.* Up until this point, only mathematical laws for the movements of the planets has been discovered.

If the effect is the same—a downward acceleration of 9.8 meters per second per second—the cause must be the same. And so: the force that causes the little moon to turn around its orbit must be the same as that which causes objects to fall to the ground on Earth.

So the universe, then, is a large space where bodies attract one another by means of forces; and there is a universal force, gravity: every body attracts every other body.

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

The power of the new Newtonian intellectual framework proves to be beyond all expectation. The entire technology of the nineteenth century and of our own modern world rests largely upon Newton’s formulas.

Newton knew that his equations do not describe all the forces that exist in nature. There are forces other than gravity that act upon bodies.

The first surprise is that almost all phenomena we see are governed by a single force, other than gravity: the force that today we call “electromagnetism.”

it’s always this force that creates the friction that stops a sliding object, that softens the landing of a parachutist, that turns electric motors and combustion engines,* or that allows us to turn on lights and listen to the radio.

The understanding of how electromagnetic force works was made by another Briton, or rather by two, science’s oddest couple: Michael Faraday and James Clerk Maxwell.

Without knowing mathematics, he writes one of the best books of physics ever written, virtually devoid of equations.

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 that is modified by electric and magnetic bodies and that, in turn, acts upon (pushes and pulls) the bodies. This entity, whose existence Faraday intuits, is today called the “field.”

Faraday sees it as formed by bundles of very thin lines (infinitely thin), which fill space: an invisible gigantic cobweb filling everything around us. He calls these lines “lines of force,” because in some way these lines “carry the force”: they transmit the electric and the magnetic forces from one body to another, as if they were cables pulling and pushing

An object with an electric charge (a rubbed glass rod, for instance) distorts the electric and magnetic fields (the lines) around itself, and in turn these fields produce a force on each charged object immersed in them. Thus two distant charged objects do not attract or repel each other directly, but only via the medium interposed between them.

He understands that behind the action at a distance of his theory, there must be something else, but he has no idea what, and leaves the question . . . “to the Consideration of my Readers”!

Newton’s theory worked so well, it turned out to be so useful, that for two centuries no one bothered any longer to question it. Until Faraday, the “reader” to whom Newton had bequeathed the unanswered question would find the key to understanding how bodies can attract and repel one another at a distance in a reasonable manner.

these same equations are needed to explain how atoms function (they are held together by electrical forces), and why the particles of the material that forms a stone adhere together, or how the sun works. They describe an amazing number and range of phenomena.

Almost everything that we witness taking place, with the exception of gravity and little else besides, is well described by Maxwell’s equations.

Maxwell realizes that his equations predict that Faraday’s lines can tremble and undulate, just like the waves of the sea. He computes the speed at which the undulations of Faraday’s lines move, and the result turns out to be . . . the same as for light! Why? Maxwell understands: because light is nothing other than this rapid trembling of Faraday’s lines!

Color as we perceive it is our psychophysical reaction of the nerve signal generated by the receptors of our eyes, which distinguish electromagnetic waves of different frequencies.

I wonder how Maxwell felt when he realized that his equations—written to describe bobbins, small cages, and little needles in Faraday’s lab—turned out to explain the nature of light and color.

It isn’t true that we “do not see” Faraday lines. We only see vibrating Faraday lines. To “see” is to perceive light, and light is the movement of the Faraday lines.

Maxwell recognizes that the equations foresee that Faraday’s lines can also vibrate at much lower frequencies, that is to say slower than light. Therefore there must be other waves that nobody had ever yet seen, produced by the movement of electrical charges, and that in turn move electrical charges.

All modern communications technology—radio, television, telephones, computers, satellites, WI-FI, the Internet, and the like—is an application of Maxwell’s prediction; the Maxwell equations are the basis for all calculations of telecommunications engineers.

Our entire current technology is founded on the use of a physical thing—electromagnetic waves—that was not discovered empirically: it was predicted by Maxwell, simply by searching for the mathematical description accounting for the intuition Faraday got from bobbins and needles.

In fact there are two theories of relativity. The envelope sent by the twenty-five-year-old Einstein contained the exposition of the first of these, the theory known today as “special relativity.” This is an important clarification of the structure of space and time, which I illustrate here before turning to the other, and the most important of Einstein’s theories, general relativity.

The theories of Newton and of Maxwell appear to contradict each other in a subtle way. Maxwell’s equations determine a velocity: the velocity of light. But Newton’s mechanics are not compatible with the existence of a fundamental velocity, because what enters Newton’s equations is acceleration, not velocity. In Newton’s physics, velocity can only be velocity of something with respect to something else.

Velocity, we say, is a relative concept. That is, there is no meaning to the velocity of an object by itself: the only velocity that exists is the velocity of an object with respect to another object.

But if this is so, then the speed of light determined by Maxwell’s equations is velocity with respect to what?

The experimental attempts to measure the speed of the Earth with respect to this hypothetical substratum, tried at the end of the nineteenth century, all failed.

Between the past and the future of an event (for example, between the past and the future for you, where you are, and in the precise moment in which you are reading), there exists an “intermediate zone,” an “extended present”; a zone that is neither past nor future. This is the discovery made with special relativity.

the greater the distance of the event from you, the longer the duration of the extended present. At a distance of a few meters from your nose, the duration of what for you is the “intermediate zone,” neither past nor future, is no more than a few nanoseconds: next to nothing (the number of nanoseconds in a second is the same as the number of seconds in thirty years).

On the other side of the ocean, the duration of this “intermediate zone” is a thousandth of a second, still well below the threshold of our perception of time, the minimum amount of time we perceive with our senses, which is somewhere on the order of a tenth of a second.

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 of events during which things occur that are neither in our past nor in our future.

Say I am on Mars and you are here; I ask you a question and you reply as soon as you’ve heard what I said; your reply reaches me a quarter of an hour after I had posed the question. This quarter of an hour is time that is neither past nor future to the moment you’ve replied to me.

The key fact that Einstein understood is that this quarter of an hour is inevitable: there is no way of reducing it. It is woven into the texture of the events of space and of time: we cannot abbreviate it, any more than we can send a letter to the past.

In technical terms, we say that Einstein has understood that “absolute simultaneity” does not exist: there is no collection of events in the universe that exist “now.” The collection of all the events in the universe cannot be described as a succession of “nows,” of presents, one following the other;

The figure describes that which in physics is called “spacetime”: the set of all past and future events, but also those that are “neither-past-nor-future”; these do not form a single instant: they have a duration.