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

by Carlo Rovelli

I think I like physics because it opens a window through which we can see further. It gives me the sense of fresh air entering the house.

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

Science is a continual exploration of ways of thinking.

This adventure rests upon the entirety of past knowledge, but at its heart is change.

The incompleteness and the uncertainty of our knowledge, our precariousness, suspended over the abyss of the immensity of what we don’t know, does not render life meaningless: it makes it interesting and precious.

Science is about reading the world from a gradually widening point of view.

Through discussion, it is possible to reach the best decisions for the community; through discussion, it is possible to understand the world. This is the immense legacy of Miletus, cradle of philosophy, of the natural sciences, and of geographical and historical studies.

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

And there is a serene acceptance of the inevitability of death, which cancels every evil, and about which there is nothing to fear. For Lucretius, religion is ignorance: reason is the torch that enlightens.

“To a wise man, the whole earth is open, because the true country of a virtuous soul is the entire universe.”

But 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.

space and time in relativity; matter and energy in quantum theory.

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;

“neither-past-nor-future”; these do not form a single instant: they have a duration.

energy and mass are two facets of the same entity, just as the electric and magnetic fields are two facets of the same field, and as space and time are two facets of the one thing, spacetime.

that saying “here and now” makes sense, but that saying “now” to designate events “happening now” throughout the universe makes no sense.

There isn’t an “up” and a “down” in the universe. Similarly, there isn’t always a “before” and an “after” between two events in the universe.

The world is made up of particles + fields, and nothing else;

what curves is not space but spacetime—the spacetime that, ten years previously, Einstein himself had shown to be a structured whole rather than a succession of instants.

R is proportional to the energy of matter. In words: spacetime curves more where there is matter.

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

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 (figure 3.10).*

Each ball “surrounds” and is surrounded by the other. Einstein’s idea is that space could be a 3-sphere: something with a finite volume (the sum of the volume of the two balls), but without borders.*

“surrounds and is at the same time surrounded by” the sphere of our universe!

for someone who has learned from his teacher that the form of the surface of our planet is such that by walking always in a straight line we return to the point we started from, it is perhaps not so difficult to take the next obvious step and imagine that the form of the entire universe is such that, flying always in a straight line, we return to the same point of departure: a 3-sphere is a space in which “two winged knights who could fly in opposite directions would meet up on the other side.” In technical terms: the description of the geometry of Earth offered by Brunetto Latini in Li tresor is given in terms of intrinsic geometry (seen from the inside) rather than extrinsic (seen from the outside), and this is exactly the one that is suitable to generalize the notion of “sphere” from two dimensions to three. The best way of describing a 3-sphere is not to try to “see it from the outside” but rather to describe what happens when moving within it.

Einstein’s spacetime is not curved in the sense that it curves “in an external space.” It is curved in the sense that its intrinsic geometry—that is to say, the web of distances between its points, which can be observed by staying within it—is not the geometry of a flat space. It is a space where Pythagoras’s theorem is not valid, just as Pythagoras’s theorem is not valid on the surface of Earth.*

Our culture is foolish to keep science and poetry separated: they are two tools to open our eyes to the complexity and beauty of the world.

the three aspects of reality it has unveiled: granularity, indeterminism, and relationality.

it happens or doesn’t happen depending on the color of light (the frequency) rather than its intensity (energy).

the energy of each grain is determined by frequency, the phenomenon will occur only if frequency is sufficiently high, that is to say if the individual grains of energy are sufficiently large, independently from the total amount of energy that’s around.

Only someone in his twenties can take such delirious propositions seriously. You have to be a twentysomething to believe that they can be turned into a theory of the world. And perhaps you have to be this young to understand better than anyone else, for the first time, the deep structure of nature. Just as Einstein was in his twenties when he realized that time does not pass in the same way for everyone, so too was Heisenberg on that Copenhagen night. Perhaps, it is no longer a good idea to trust your intuitions after thirty. . .

the relational aspect of things. Electrons don’t always exist. They exist when they interact. They materialize in a place when they collide with something else. The “quantum leaps” from one orbit to another constitute their way of being real: an electron is a combination of leaps from one interaction to another. When nothing disturbs it, an electron does not exist in any place.

His physics has the pristine clarity of a song. For him the world is not made of things; it’s constituted of an abstract mathematical structure that 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 space,* 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 only acquire reality when it collides—interacts—with another object. It is not just its position that is undefined, as Heisenberg had recognized: no variable of the object is defined between one interaction and the next. The relational aspect of the theory becomes universal.

This is a radical change from Newton’s theory, where it is possible, in principle, to predict the future with certainty. Quantum mechanics brings probability to the heart of the evolution of things. This indeterminacy is the third cornerstone of quantum mechanics: the discovery that chance operates at the atomic level.

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.

things. First, to calculate which values a physical variable may assume.

The second thing that Dirac’s quantum mechanics allows us to do is to compute the probability that this or that value of a variable appears at next interaction.

What happens between one interaction and the next is not mentioned in the theory. It does not exist.

Particles are quanta of a field, just as photons are quanta of light.

There are no longer particles that move in space with the passage of time, but quantum fields whose elementary events happen in spacetime. The world is strange but simple

the first meaning of quantum mechanics is the existence of a limit to the information that can exist within a system: a limit to the number of distinguishable states in which a system can be.

Due to this indeterminacy, in the world described by quantum mechanics, things are constantly subject to random change. All the variables “fluctuate” continually, as if, at the smallest scale, everything was constantly vibrating. We do not see these omnipresent fluctuations only because of their small scale; they cannot be observed at large scale,

Quantum mechanics reveals to us that the more we look at the detail of the world, the less constant it is. The world is not made up of tiny pebbles. It is a world of vibrations, a continuous fluctuation, a microscopic swarming of fleeting microevents.

It is as if the electron, in order to go from A to B, passed “through all possible trajectories,” or, in other words, unfurled into a cloud in order to then converge mysteriously on point B, where it collides again with something else

like a life.

In order to move from A to B, an electron behaves as if passing through all possible trajectories.

The theory does not describe things as they “are”: it describes how things “occur,” and how they “interact with each other.” It doesn’t describe where there is a particle but how the particle shows itself to others. The world of existent things is reduced to a realm of possible interactions. Reality is reduced to interaction. Reality is reduced to relation.

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 quantum mechanics is not a world of objects: it is a world of events.

We, like waves and like all objects, are a flux of events; we are processes, for a brief time monotonous. . . .

I think that the obscurity of the theory is not the fault of quantum mechanics but is rather due to the limited capacity of our imagination.