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

by Carlo Rovelli

The third discovery about the world articulated by quantum mechanics is the most profound and difficult—and one that was not anticipated by the atomism of antiquity.

The theory does not describe things as they “are”: it describes how things “occur,” and how they...

It doesn’t describe where there is a particle but how the particle shows itself to others.

As a 1 or a 0... A positive or negative charge, does it attract (magnetism) or repel (electrical)? Is the electromagnetic spectrum the frst duality in the sequencing of consciousnes?

The world of existent things is reduced to a realm of possible interactions. Reality is reduced to interaction. Reality is reduced to relation.3

Speed is not a property of an object on its own: it is the property of the motion of an object with respect to another object.

Quantum mechanics extends this relativity in a radical way: all variable aspects of an object exist only in relation to other objects.

It is only in interactions that nature draws the world.

In the world described by quantum mechanics, there is no reality except in the relations between physical systems. It isn’t things that enter into relations, but rather relations that ground to the notion of thing. The world of quantu...

wave is not an object, in the sense that it is not made of matter that travels with it. The atoms of our body, as well, flow in and away from us. We, like waves and like all objects, are a flux of events; we are processes, for a brief time monotonous.

Quantum mechanics does not describe objects: it describes processes and events that are junction points between processes.

This is Shunyata... Form is emptiness; emptiness is form... It is flow of energy... Entropy... Our personal entropy; our breath, the initial exchange of H2O for CO2? Is what allows our soul to have a human existence?

To summarize, quantum mechanics is the discovery of three features of the world: — Granularity (figure 4.7).

The information in the state of a system is finite, and limited ...

Indeterminacy. The future is not determined unequivocally by the past. Even the more rigid regularities we see are ultimately statistical. — Relationality.

The events of nature are always interactions. All events of a system occur in relation to another system.

A process is the passage from one interaction to another. The properties of “things” manifest themselves in a granular manner only in the moment of interaction—that is to say, at the edges of the processes—and are such only in relation to other things.

And yet . . . are you sure, dear reader, that you have fully understood what quantum mechanics reveals to us?

An electron is nowhere when it is not interacting . . . mmm . . . things only exist by jumping from one interaction to another . . . well . . . Does it all seem a little absurd?

A century has passed, and we are at the same point. Richard Feynman, who more than anyone has known how to juggle with the theory, has written: “I think I can state that nobody really understands quantum mechanics.”

What is quantum theory, a century after its birth?

An extraordinary dive deep into the nature of reality?

A blunder that works, ...

Part of an incomplet...

Or a clue to something profound regarding the structure of the world, which we ha...

Quantum mechanics is only a physics theory: perhaps tomorrow it will be corrected by an understanding of the world that is different and even more profound.

Some scientists today try to iron it out a bit, to render it more in keeping with our intuition. In my opinion, its dramatic empirical success should compel us to take it seriously, and to ask ourselves not what there is to change in the theory, but rather what is limited about our intuition that makes it seem so strange to us.

This would require a much stronger confidence in the things we cannot see; it would require a trust in our "felt sense" of realty.

The moon is too large to be sensitive to minute quantum granularity; so we can forget the quanta when describing its movements.

On the other hand, an atom is too light to curve space to a significant degree, and when we describe it we can forget the curvature of space.

But there are situations where both curvature of space and quantum granularity matter, and for these we do not yet have an established physical theory that works...

Another is what happened to the universe duri...

In all these instances, today’s theories become confused and no longer tell us anything reasonable: quantum mechanics cannot deal with the curvature of spacetime, and general relativity cannot accou...

Newton discovered universal gravity precisely by combining Galileo’s physics of how things move on Earth with Kepler’s physics of the heavens.

Maxwell and Faraday found the equations of electromagnetism by bringing together what was known about electricity and magnetism.

Einstein found special relativity in order to resolve the apparent conflict between Newton’s mechanics and Maxwell’s electromagnetism—and then general relativity in order to resolve the resulting conflict be...

Now let us take Einstein’s theory into account. Energy makes space curve.

A lot of energy means that space will curve a great deal.

But if a particle plummets into a black hole, I can no longer see it. I can no longer use it as a reference point for a region of space. I can’t manage to measure arbitrarily small regions of space, because if I try to do this, these regions disappear inside a black hole.

This argument can be made more precise with a little mathematics. The result is general: quantum mechanics and general relativity, taken together, imply that there is a limit to the divisibility of space.

Dirac dedicated the final years of his life to the problem, opening avenues and introducing ideas and techniques on which a good part of current work on quantum gravity is based.

Feynman tried, attempting to adapt the techniques he had developed for electrons and photons to the context of general relativity, but without success: electrons and photons are quanta in space; quantum gravity is something else: it isn’t enough to describe “gravitons” moving in space; space itself that is “quantized.”

He imagined it as a cloud of superimposed geometries, just as we can think of a quantum electron as a cloud of positions.

Imagine that you are looking at the sea from a great height: you perceive a vast expanse of it, a flat cerulean table. Now you descend and look at it more closely. You begin to make out the great waves swollen by the wind. You descend farther, and you see that the waves break up, and that the surface of the sea is a turbulent frothing. This is what space is like, as imagined by Wheeler.

If we try to use it to do calculations, we soon obtain results that are infinite, which makes no sense.

Hmmm maybe infinity is a thing

Among its disconcerting aspects is the fact that it no longer contains the time variable.

Isn't This infinity... No time... It is the primordial from which all dualities come from. What if infinity is heaven?

The solutions had a curious peculiarity: they depended on closed lines in space. A closed line is a “loop.” Smolin and Jacobson could write a solution to the Wheeler-DeWitt equation for every loop: for every line closed on itself.

The first works of what will later become known as “loop quantum gravity” emerge from these discussions, as the meaning of these solutions of the Wheeler-DeWitt equation gradually clarify.

The second new aspect, the crucial one, is that we are speaking of gravity and therefore, as Einstein understood, we are not speaking of fields immersed in space, but of the very structure of space itself. Faraday’s lines of the quantum gravitational field are the threads with which space is woven.

it became clear that the key to understanding the physics of these solutions lies in the points where these lines intersect. These points are called “nodes,” and the lines between nodes are called “links.” A set of intersecting lines forms what is called a “graph,” that is to say, a combination of nodes connected by links, as in figure 6.3

A calculation, in fact, demonstrates that without nodes, physical space has no volume. In other words, it is in the nodes of the graph, not in the lines, that the volume of space “resides.”

But the gravitational field is a physical quantity and, like all physical quantities, is subject to the laws of quantum mechanics.

Dirac provided us with the formula with which to compute the spectrum of every variable. The calculation took time, first to formulate and then to complete, and made us suffer. It was completed in the mid-1990s,