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

by Carlo Rovelli

The discovery of the Higgs boson is a rock-hard confirmation of the validity of the ideas behind the standard model of elementary particles, based on quantum mechanics.

The three results, obtained with strenuous technological efforts and extensive collaborations between hundreds of scientists, do nothing other than reinforce the understanding that we already had of the structure of the universe. No real surprises.

Traces of the great primordial heat must also be in the gravitational field itself. The gravitational field, too, that is to say, space itself, must be tremulous like the surface of the sea. Therefore, a cosmic gravitational background radiation must also exist – older even than the electromagnetic one, because the gravitational waves are disturbed less by matter than the electromagnetic ones and were able to travel undisturbed even when the universe was too dense to let the electromagnetic waves pass.

If LISA is launched, it should be able to see not only the gravitational waves produced by stars and black holes but also the diffuse background of primordial gravitational waves generated at a time close to the Big Bang. These waves should tell us about the quantum bounce.

on the one hand, taking quantum mechanics into account resolves the problems generated by the infinities of Einstein’s theory of gravity, that is to say, the singularities. On the other, taking gravity into account solves the problems generated by quantum field theory, that is to say, the divergences. Far from being contradictory, as they at first seemed, the two theories each offer the solution to the problems posed by the other!

Special relativity may be summarized as the discovery that there exists a maximum velocity for all physical systems. Quantum mechanics can be summarized as the discovery that there exists a maximum of information for each physical system.

It suggests that what we call infinite often is nothing more than something which we have not yet counted, or understood. I think this is true in general. ‘Infinite’, ultimately, is the name that we give to what we do not yet know. Nature appears to be telling us that there is nothing truly infinite.

The point at stake here is not the presumption of knowing everything. It is the opposite: an awareness that yesterday’s ignorance may have light shed on it today, and that today’s might be illuminated tomorrow.

The sophisticated playfulness of The Sand Reckoner is perhaps not only about an audacious mathematical construction, or the virtuosity of one of the most extraordinary minds of antiquity. It is also a defiant cry of reason, which recognizes its own ignorance but refuses to delegate to others the source of knowledge. It is a small, reserved and powerfully intelligent manifesto against infinity – against obscurantism. Quantum gravity is one of the many lines that continue the quest of The Sand Reckoner. We are counting the grains of space of which the cosmos is made. A vast cosmos, but a finite one. The only truly infinite thing is our ignorance.

concrete applications of quantum gravity: the description of what happened to the universe around the time of the Big Bang; the description of the properties of the heat of black holes and the suppression of infinity.

information is the measure of the number of possible alternatives for something.

information. It carries the capacity to distinguish between alternatives.

Why is the notion of information useful, perhaps even fundamental, to understanding the world? For a subtle reason: because it measures the ability of one physical system to communicate with another physical system.

The world isn’t, then, just a network of colliding atoms: it is also a network of correlations between sets of atoms, a network of real reciprocal information between physical systems.

The tea can’t warm itself up, because information cannot increase by itself.

Entropy is ‘missing information’, that is, information with a minus sign. The total amount of entropy can only increase, because information can only diminish.fn50

Quantum mechanics can be understood as the discovery that information in nature is always finite.

A physical system manifests itself only by interacting with another.

The description of a physical system, then, is always given in relation to another physical system, the one with which it interacts. Any description of a system is therefore always a description of the information which a system has about another system, that is to say, the correlation between the two systems. The mysteries of quantum mechanics become less dense if interpreted in this way, as the description of the information that physical systems have about one another.

The entire formal structure of quantum mechanics can in large measure be expressed in two simple postulates:1 The relevant information in any physical system is finite. You can always obtain new information on a physical system.

The first postulate characterizes the granularity of quantum mechanics: the fact that a finite number of possibilities exists.

Therefore, the theory lends itself in a surprising way to being expressed in terms of information.

Wheeler coined the phrase ‘It from bit’ to express this idea, meaning that ‘everything is information’.

Thus we can associate a quantity of missing information, that is to say, entropy, with a black hole.

When information enters into a black hole, it is no longer recoverable from outside. But the information which enters the black hole carries with it the energy by which the black hole becomes larger and increases its area. Viewed from outside, the information lost in the black hole now appears as entropy associated with the area of the hole.

I showed that it is not necessary to use the notion of time to describe physics.

Time plays no role at the fundamental level of physics. Once we have understood this, it is easier to write the equations of quantum gravity.

What does ‘the passage of time’ mean, if time plays no part in the fundamental description of the world?

The answer is simple. The origin of time may be similar to that of heat: it comes from averages of many microscopic variables. Let’s look at this in detail.

If you think about it, all phenomena where we detect the passage of time are co-involved with temperature.

At the precise moment when heat is produced, the process is irreversible: the past differs from the future. It is always heat and only heat that distinguishes the past from the future.

But heat is our way to name averages over many variables.

The idea of thermal time reverses this observation. That is to say, instead of enquiring how time produces dissipation in heat, it asks how heat produces time.

The idea of thermal time is that the notion of time, too, comes from the fact that we interact only with averages of many variables.fn53

As long as we have a complete description of a system, all the variables of the system are on the same footing; none of them acts as a time variable. That is to say: none is correlated to irreversible phenomena. But as soon as we describe the system by means of averages of many variables, we have a preferred variable that functions like common time. A time along which heat is dissipated. The time of our everyday experience.

Hence time is not a fundamental constituent of the world, but it appears because the world is immense, and we are small systems within the world, interacting only with macroscopic variables that av...

Averages behave like averages: they disperse heat and, intrinsically, generate time.

The difficulty of grasping this idea comes from the fact that it is hard for us to think of a world without time, and of time emerging in an approximate manner. We are too used to thinking of reality as existing in time. We are beings who live in time: we dwell in time, and are nourished by it. We are an effect of this temporality, produced by average values of microscopic variables. But the limitations of our intuitions should not mislead us. Understanding the world better often entails going against intuition. If this were not the case, understanding would be easy. Time is an effect of our overlooking of the physical microstates of things. Time is information we don’t have. Time is our ignorance.

I believe that in order to understand reality we have to keep in mind that reality is this network of relations, of reciprocal information, which weaves the world.

We slice up the reality surrounding us into objects. But reality is not made up of discrete objects.

For this reason, DNA exists, together with immune systems, sense organs, nervous systems, complex brains, languages, books, the library of Alexandria, computers and Wikipedia: they maximize the efficiency of information management – the management of correlations favouring survival.

This acute awareness of our ignorance is the heart of scientific thinking. It is thanks to this awareness of the limits of our knowledge that we have learned so much. We are not certain of all which we suspect, just as Socrates was not sure of the spherical nature of the Earth. We are exploring at the borders of our knowledge.

Science is not reliable because it provides certainty. It is reliable because it provides us with the best answers we have at present. Science is the most we know so far about the problems confronting us. It is precisely its openness, the fact that it constantly calls current knowledge into question, which guarantees that the answers it offers are the best so far available: if you find better answers, these new answers become science. When Einstein found better answers than Newton, he didn’t question the capacity of science to give the best possible answers – on the contrary, he confirmed it. The answers given by science, then, are not reliable because they are definitive. They are reliable because they are not definitive. They are reliable because they are the best available today. And they are the best we have because we don’t consider them to be definitive, but see them as open to improvement. It’s the awareness of our ignorance that gives science its reliability. And it is reliability that we need, not certainty.

The search for knowledge is not nourished by certainty: it is nourished by a radical distrust in certainty.

Ignorance can be scary. Out of fear, we can tell ourselves calming stories:

The world revealed by quantum gravity is a new and strange one – still full of mystery, but coherent with its simple and clear beauty. It is a world which does not exist in space and does not develop in time. A world made up solely of interacting quantum fields the swarming of which generates – through a dense network of reciprocal interactions – space, time, particles, waves and light