Helgoland: Making Sense of the Quantum Revolution

by Carlo Rovelli

A reality that is more subtle than the simplistic materialism of particles in space. A reality made up of relations rather than objects.

the “relational” interpretation of quantum theory.

We are asked to accept that reality may be profoundly other than we had imagined: to look into the abyss, without fear of sinking into the unfathomable.

But above all, these formulas had about them something that was truly absurd. They assumed, for no good reason, that the electrons in atoms orbited around the nucleus only on certain precise orbits, at certain precise distances from the nucleus, with certain precise energies—before magically “leaping” from one orbit to another.

Let’s change, instead, our way of thinking about the electron. Let’s give up describing its movement. Let’s describe only what we can observe: the light it emits. Let’s base everything on quantities that are observable. This is the idea.

he is fascinated by Asian thought, in particular Vedanta Hinduism, and passionate (as Einstein was) about the philosophy of Arthur Schopenhauer, which interprets the world as “representation.”

What Born understands is that the value of Schrödinger’s ψ wave at a point in space is related to the probability of observing the electron at this point.

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. It is like the weather forecasts telling us what could happen tomorrow.

Quantum theory, just as much in Heisenberg’s version as in Schrödinger’s, predicts probability, and not certainty.

Granularity is the third idea of quantum theory, next to probability and observations

It is the interference that we see, not the superposition

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.

The price to be paid for taking this theory seriously is to accept the idea that an entire physical reality exists that is in principle inaccessible to us. Its sole purpose, when it comes down to it, is merely to comfort us with regards to what the theory does not tell us.

Interpretations of the theory that do not take the ψ waves so seriously are called “epistemic,” because they interpret ψ only as a summary of our knowledge (ἐπιστήμη) of what happens.

The weakness of QBism, in my opinion—and this is the turning point in this whole discussion—is that QBism anchors reality to a subject of knowledge, an “I” that knows, as if it stood outside nature. Instead of seeing the observer as a part of the world, QBism sees the world reflected in the observer. In so doing, it leaves behind naive materialism but ends up falling into an implicit form of idealism.51 The crucial point that QBism disregards, I believe, is that the observer himself can be observed. We have no reason to doubt that every real observer is himself described by quantum theory.

the main concept of this book: relations

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. And this is the best description of nature that we have.

The properties of an object are the way in which it acts upon other objects; reality is this web of interactions. 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.

Facts that are real with respect to an object are not necessarily so with respect to another.* A

Physical variables do not describe things: they describe the way in which things manifest themselves to each other. There is no sense in attributing a value to them if it is not in the course of an interaction.

The quantum world is more tenuous than the one imagined by the old physics; it is made up of happenings, discontinuous events, without permanence.

This is a solution to the puzzle, but it comes at a cost: no universal set of facts exists.

example of the relativity of reality. The joint properties of two objects exist only in relation to a third.

Everything that manifests itself does so in relation to something. A correlation between two objects is a property of the two objects—like all properties, it exists only in relation to a further, third object.

From an external perspective, any manifestation of one object to another, which is to say any property, is a correlation; it is an entanglement between an object and another. Entanglement, in sum, is none other than the external perspective on the very relations that weave reality: the manifestation of one object to another, in the course of an interaction, in which the properties of the objects become actual.

This is the meaning of Planck’s constant. It is the limit up to which we can determine physical variables.

This reads: “Delta X times Delta P is always greater than or equal to h-bar divided by two.” This general property of reality is called “Heisenberg’s uncertainty principle.” It applies to everything.

“Noncommutative” means: such that their order cannot be changed freely.

At our scale, the world is like the wave-agitated surface of the ocean seen from the moon: the smooth surface of a blue marble.

The solidity of the classical vision of the world is nothing other than our own myopia. The certainties of classical physics are just probabilities. The well-defined and solid picture of the world given by the old physics is an illusion.

“Empiriocriticism” was the name Ernst Mach had associated to his own ideas. Ernst Mach—remember him?—was the source of philosophical inspiration for both Einstein and Heisenberg.

Science must be freed from all metaphysical assumptions: knowledge should be based, that is, only upon what is “observable.”

“It is the aim of this work to lay the foundation for a theory of quantum mechanics based solely on relations between quantities that are in principle observable.” Almost a quotation from Mach.

For Mach, knowledge is the science of nature, but its perspective is not far from the historicism of dialectical materialism. The consonance between Mach’s ideas and those of Engels and Marx is developed by Bogdanov and gains currency in prerevolutionary Russia.

His most radical suggestion is to stop thinking of phenomena as manifestations of objects and to think, instead, of objects as nodes between phenomena.

the elements useful for thinking the world are manifestations of physical systems to each other, not absolute properties belonging to each system.

The “anti-metaphysical” spirit that Mach promoted is an attitude of openness: We should not seek to teach the world how it should be. Let’s listen to the world instead, in order to learn from it how to think about it.

“The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.”

believe that we need to adapt our philosophy to our science, and not our science to our philosophy.

of the fact that all nature is quantum, and that there is nothing special about a physics laboratory containing measuring apparatus.

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.

The conclusion is revolutionary. It leaps beyond the idea that the world is made up of a substance that has attributes, and forces us to think about everything in terms of relations.103 This, I believe, is what we have discovered about the world with quanta.

What really interests us about ancient texts is not what the author initially intended to say: it is how the work can speak to us now, and what it can suggest today.

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.

ultimate reality, the essence, is absence, is vacuity. It does not exist.

Even emptiness is devoid of essence: it is conventional. No metaphysics survives. Emptiness is empty.

Reality, including our selves, is nothing but a thin and fragile veil, beyond which . . . there is nothing.

A technical term for “referring to something” in our mental processes (promoted by the German philosopher and psychologist Franz Brentano) is “intentionality.”

the information theory of Claude Shannon, information is only counting the number of possible states of something.

living beings survive because these structures are there. We do not love in order to live: we live because we love.