Helgoland by Rovelli, Erica (ebook reader below 3000 .txt) 📗

influence the probability of events that occur in relation to others.* The “quantum state” ψ is always a relative state.57

The world is the network of relative facts: relations realized when physical entities interact. A stone collides with another stone. The light from the sun reaches my skin. You read these lines.

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 quantum world is more tenuous than the one imagined by the old physics; it is made up of happenings, discontinuous events, without permanence. It is a world with a fine texture, intricate and fragile as Venetian lace. Every interaction is an event, and it is these light and ephemeral events that weave reality, not the heavy objects charged with absolute properties that our philosophy posited in support of these events. “There are fewer things in heaven and earth, Horatio, Than are dreamt of in your philosophy . . .”

The life of an electron is not a line in space: it is a dotted manifestation of events, one here and another there. Events are punctiform, discontinuous, probabilistic, relative.

In Cosmological Koans, Anthony Aguirre describes this disconcerting conclusion in the following way:

An electron is a particular type of regularity that appears among measurements and observations that we make. It is more pattern than a substance. It is order . . . Thus we arrive at a strange place. We break things down into smaller and smaller pieces, but then the pieces, when examined, are not there. Just the arrangements of them are. What then, are things, like the boat, or its sails, or your fingernails? What are they? If things are forms of forms of forms of forms, and if forms are order, and order is defined by us . . . they exist, it would appear, only as created by, and in relation to, us and the Universe. They are, the Buddha might say, emptiness.58

The solidity of the world to which we have become accustomed in our daily lives does not reflect the actual grain of reality: it is the result of our macroscopic vision. A lightbulb does not emit continuous light, it emits a hail of evanescent photons. At small scale, there is no continuity, or fixity, in the real world: there are discrete events, interactions, gapped and discrete.

Schrödinger had fought tooth and nail against quantum discontinuity, against Bohr’s quantum leaps, against Heisenberg’s world of matrices: he wanted to defend the image of continuous reality provided by classical intuition. But in the end even he capitulated, decades after the clashes of the 1920s, and admitted defeat. Schrödinger’s words after the ones mentioned earlier (“There was a moment when the creators of wave mechanics nurtured the illusion of having eliminated the discontinuities in quantum theory”) are clear and definitive:

It is better to consider a particle not as a permanent entity but rather as an instantaneous event. Sometimes these events form chains that give the illusion of being permanent, but only in particular circumstances and only for an extremely brief period of time in each individual case.59

The Many Worlds and Hidden Variables interpretations sketched in the previous chapter seek to “fill” the world with additional realities beyond what we see, to recover the “plenitude” of the classical world, to exorcise the indeterminacy of quanta. The cost of these approaches is to postulate a world full of invisible things. The relational perspective takes the theory as it is—it is the best theory that we have—with its sketchy description of the world, and accepts indeterminacy,* as QBism does. But while QBism is about the information of a subject, the relational understanding of quantum theory is about the structure of the world.

To understand quantum theory, we need to modify the grammar of our understanding of reality, as when Anaximander understood that the true shape of the Earth changed the grammar of notions of what is “up” and “down.”60 Objects are described by variables that assume value when interacting, and this value is determined in relation to the objects in the interaction, not to others. An entity is one, no one and one hundred thousand.

The world fractures into a play of points of view that do not admit of a univocal, global vision. It is a world of perspectives, of manifestations, not of entities with definite properties or unique facts. Properties do not reside in objects, they are bridges between objects. Objects are such only with respect to other objects, they are nodes where bridges meet. The world is a perspectival game, a play of mirrors that exist only as reflections of and in each other.

This phantasmal world of quanta is our world.

IV

THE WEB OF RELATIONS THAT WEAVES REALITY

In which I speak about how things speak to one another.

ENTANGLEMENT

There is a subtle, beguiling quantum phenomenon that embodies the radical interdependence of things. The most enchanted and dreamy of the quantum phenomena. Entanglement.

Entanglement is the strangest of all strange quantum phenomena, the one that takes us furthest away from our old understanding of the world. But it is also something general, which in a sense weaves the very structure of reality.

It is the phenomenon by which two distant objects maintain a kind of weird connection, as if they continued to speak to each other from afar. They remain, as we say, “entangled,” linked