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

by Carlo Rovelli

An immense scaffolding in which objects run in straight lines, until a force causes them to curve. But what is this “space,” which contains the world, made

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

what if the gravitational field turned out actually to be Newton’s mysterious space?

What if Newton’s space was nothing more than the gravitational field?

This extremely simple, beautiful, brilliant idea is the theory of...

The world is not made up of space + particles + electromagnetic field + gravitational field. The world is made up of par...

But unlike Newton’s space, which is flat and fixed, the gravitational field, by virtue of being a field, is something that moves and undulates, subject to equations—like Maxwell’s field, like Faraday’s lines.

The sun bends space around itself, and Earth does not circle around it drawn by a mysterious distant force but runs straight in a space that inclines, like a bead that rolls in a funnel: there are no mysterious forces generated by the center of the funnel;

Planets circle around the sun, and things fall, because space around them is curved

Figure 3.6 Earth turns around the sun because spacetime around the sun is curved, rather like a bead that rolls...

A little more precisely, what curves is not space ...

Einstein himself had shown to be a structured whole rather than a su...

Riemann’s result was that the properties of a curved space (or spacetime) in any dimension are described by a particular mathematical object, which we now call “Riemann curvature,” and indicate with the letter R.

Using Riemann’s theory, it is possible to describe the shape of curved spaces in three or four dimensions.

spacetime curves more where there is matter. That is it. The equation is the analogue of the Maxwell equations, but for gravity rather than electricity.

But within this equation there is a teeming universe. And here the magical richness of the theory opens up into a phantasmagorical succession of predictions that resemble the delirious ravings of a madman, but which have all turned out to be true.

To begin with, Einstein recalculates the effect of a mass like the sun on the curvature of the space that surrounds it, and the effect of this curvature on the movements of the planets.

planets as predicted by Kepler’s and Newton’s equations, but not exactly: in the vicinity of the sun, the effect of the curvature of space is stronger than the effect of Newton’s force.

Einstein’s equation, then, describes how space curves very close to a star. Due to this curvature, light deviates. Einstein predicts that the sun causes light to curve around it. In 1919, the measurement is achieved; a deviation of light is measured that turns out to be exactly

But it is not only space that curves: time does too. Einstein predicts that time on Earth passes more quickly at higher altitude, and more slowly at lower altitude.

Place a watch on the floor and another on a table: the one on the floor registers less passing of time than the one on the table.

Because time is not universal and fixed; it is something that expands and shrinks, according to the vicinity of masses:

route taken by an airplane flying from Rome to New York, it does not seem to be straight: the airplane makes an arc toward the north. Why? Because, Earth being curved, crossing northward is shorter than keeping to the same parallel. The distances between meridians are shorter the more northerly you are; therefore it is better to head northward, to shorten the route

Well: believe it or not, a ball thrown upward falls downward for the same reason: it “gains time” moving higher up, because time passes at a different speed up there. In both cases, airplane and ball follow a straight trajectory in a space (or spacetime) that is curved

When this happens to a large enough star, the weight is so strong that matter is squashed down to an enormous degree, and space curves so intensely as to plunge down into an actual hole. A black hole.

Further still, the theory predicts that space ripples like the surface of the sea, and that these ripples are waves similar to the electromagnetic ones that make television possible. The effects of these “gravitational waves” can be observed in the sky on binary stars: they radiate such waves, losing energy and slowly falling toward each other.*

This rich and complex range of phenomena—bending of rays of light, modification of Newton’s force, slowing down of clocks, black holes, gravitational waves, expansion of the universe, the Big Bang—all of this follows from understanding that space is not a dull, fixed container but possesses its own dynamic, its own “physics,” just like the matter and the other fields it contains.

Einstein writes the equations that describe Democritus’s void and finds for its “certain physics” a colorful 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. .