Helgoland: Making Sense of the Quantum Revolution

by Carlo Rovelli

Schrödinger’s ψ is therefore not a representation of a real entity: it is an instrument of calculation that gives the probability that something real will occur.

“Are the laws of nature really not deterministic?” As we shall see, a hundred years after Heisenberg and Schrödinger’s bickering, this question is still open.

X P – P X = i ħ That’s it. The letter X indicates the position of a particle, the letter P indicates its speed multiplied by its mass (what we call “momentum”). The letter i is the mathematical symbol of the square root of –1 and, as we have seen, ħ is Planck’s constant divided by 2π. In a sense, Heisenberg and company have added to physics only this simple equation: everything else follows from it—from the quantum computer to the atomic bomb.

(I always sniff books before buying them: the smell of a book is decisive.)

we never see a quantum superposition. What we see are consequences of the superposition. These consequences are called “quantum interference.” It is the interference that we see, not the superposition.

It seems that you need only to observe what is happening for it to change! Note the absurdity: if I don’t look for where the photon passes, it always finishes below. But if I look at where it passes, it can end up above. The astonishing thing is that a photon can end up above even if I haven’t seen it. That is to say, the photon changes trajectory due to the fact that I was waiting for it at the gate, on the side where it hasn’t passed. Even if I haven’t actually seen it!

Try yourself to think of a sensible explanation of this behavior. For a century now, we’ve all been trying. If you find all this confusing, if you cannot make head or tail of it, you are not alone. It is why Richard Feynman wrote that nobody understands quanta.

The idea is brilliant: the phenomena of interference are determined by the ψ wave that guides objects; but those objects themselves are not in a quantum superposition.

This provides a good explanation of the Zeilinger experiment described above. Why, when I block one of the two paths, does my hand influence the movement of the photons passing along the other path? Because the electron passes along one path only, but its wave passes along both. My hand alters the wave that then guides the electron in a way that is different to how it would behave if my hand had not intervened.

As it happens, we cannot ever know the wave, because we never see it: we only see the electron.45 Hence the behavior of the electron is determined by variables (the wave) that for us remain hidden. The variables are hidden in principle: we can never determine them. This is how the theory gets the name Hidden Variables.

There is a third way of considering the ψ wave to be real that avoids both Many Worlds and Hidden Variables: by thinking of the predictions of quantum mechanics as approximations that overlook something capable of rendering everything more coherent.

QBism gets its name from “Quantum-Bayesianism.” (Thomas Bayes was an eighteenth-century Presbyterian minister who studied probability.) But the word “QBism” alludes also to the Cubism of artists such as Georges Braque and Pablo Picasso—the

QBism abandons a realistic image of the world, beyond what we can see or measure. The theory gives us the probability that we will see something, and this is all that it is legitimate to say.

If I observe an observer, I see things that the observer does not. I deduce, by reasonable analogy, that therefore there are also things that I, as an observer, do not see. I want a theory of physics that accounts for the structure of the universe, that clarifies what it is to be an observer in the universe, not a theory that makes the universe depend on me observing it.

The discovery of quantum theory, I believe, is the discovery that the properties of any entity are nothing other than the way in which that entity influences others. It exists only through its interactions. Quantum theory is the theory of how things influence each other.

Instead of seeing the physical world as a collection of objects with definite properties, quantum theory invites us to see the physical world as a net of relations. Objects are its nodes.

talking about something that has no meaning, for there are no properties outside of interactions.54 This is the significance of Heisenberg’s original intuition: to ask what the orbit of an electron is when it is not interacting with anything is an empty question. The electron does not follow an orbit because its physical properties are only those that determine how it affects something else, for instance, the light that it emits when it is interacting. If the electron is not interacting, there are no properties.

The world that emerges from these considerations is a rarefied one. A world in which, rather than independent entities with definite properties, there are entities that have properties and characteristics only with regard to others, and only when they interact. A stone does not have a position in itself: it only has a position in relation to another stone with which it collides. The sky does not in itself have any color: it has color with respect to my eyes when they look at it. A star does not shine in the sky as an autonomous entity: it is a node in the network of interactions that forms the galaxy in which it resides.

The joint properties of two objects exist only in relation to a third. To say that two objects are correlated means to articulate something with regard to a third object: the correlation manifests itself when the two correlated objects both interact with this third object, which can check.

When Einstein objected to quantum mechanics by remarking that “God does not play dice,” Bohr responded by admonishing him, “Stop telling God what to do.” Which means: Nature is richer than our metaphysical prejudices. It has more imagination than we do.

Special “observers” have no real role to play in the theory. The central point is simpler: the properties of an object become manifest when this object interacts with others. We cannot separate the properties from these other objects. We cannot attribute them just to a single object. All of the (variable) properties of an object, in the final analysis, are such and exist only with respect to other objects. “Contextuality” is the technical name that denotes this central aspect of quantum physics: things exist in a context.

If nothing exists in itself, everything exists only through dependence on something else, in relation to something else. The technical term used by Nāgārjuna to describe the absence of independent existence is “emptiness” (śūnyatā): things are “empty” in the sense of having no autonomous existence. They exist thanks to, as a function of, with respect to, in the perspective of, something else.

I believe that one of the greatest mistakes made by human beings is to want certainties when trying to understand something. The search for knowledge is not nourished by certainty: it is nourished by a radical absence of certainty. Thanks to the acute awareness of our ignorance, we are open to doubt and can continue to learn and to learn better. This has always been the strength of scientific thinking—thinking born of curiosity, revolt, change. There is no cardinal or final fixed point, philosophical or methodological, with which to anchor the adventure of knowledge.

We are a delicate and complex embroidery in the web of relations of which, as far as we currently understand it, reality is constituted.

Knowing that my girlfriend obeys Maxwell’s equations will not help me to make her happy.