Helgoland

by Carlo Rovelli

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.

Mach does not think, of course, that there is nothing outside our mind. On the contrary, he is interested precisely in what is outside our minds (whatever the ‘mind’ is): nature, in all its complexity, of which we are a part. Nature presents itself as a set of phenomena, and Mach recommends the study of those phenomena in order to build syntheses and conceptual structures that make sense of them, rather than to postulate a priori underlying realities.

Whereas previously we thought that the properties of every object could be determined even if we overlooked the interactions occurring between this object and others, quantum physics demonstrates that the interaction is an inseparable part of phenomena. The unambiguous description of any phenomenon requires the inclusion of all the objects involved in the interaction in which the phenomenon manifests itself.

The mind does not enter into the equation. Special ‘observers’ have no real role to play in the theory. The central point is simpler: we cannot separate the properties of the objects from the objects interacting with them in order for these properties to be manifested in the first place. 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. An isolated object, taken in itself, independent of every interaction, has no particular state. At most we can attribute to it a kind of probabilistic disposition107 to manifest itself in one way or another. But even this is only an anticipation of future phenomena, a reflection of phenomena past, and only and always relative to another object. 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.108 This, I believe, is what we have discovered about the world with quanta.