Helgoland: Making Sense of the Quantum Revolution

by Carlo Rovelli

The point I need to emphasize here is that having understood all this does not take away significance from notions such as “utility” and “relevance.” On the contrary, it clarifies their origin, the way they are rooted in the physical world: they are the characteristics of those natural systems that actually give rise to survival.

I see a rock falling toward me.124 If I move, I will survive. There is nothing mysterious about the fact that I move. It is explained by Darwin’s theory: those who did not move were crushed and killed; I am a descendant of those who move out of the way. But in order to be able to move, my body needs to know that the stone is heading for me. For it to know, there must be a physical correlation between a physical variable inside me and the physical state of the rock. This correlation is there, obviously, because the visual system does precisely this: it correlates the surrounding environment with neural processes in the brain. There are all sorts of correlation between internal and external, but this one has a particular characteristic: if it was not there, or if it was not well adjusted, I would be killed by the rock.

The existence of such relevant correlations reveals the physical foundation of the notion of meaning: relevant relative information. Relative information in the (physical) sense given by Shannon—which is relevant in the (biological, therefore ultimately also physical) sense clarified by Darwin.

If I look at a forest from afar, I see a dark green velvet. As I move toward it, the velvet breaks up into trunks, branches and leaves: the bark of the trunks, the moss, the insects, the teeming complexity. In every eye of every ladybug, there is an extremely elaborate structure of cells connected to neurons that guide and enable them to live. Every cell is a city, every protein a castle of atoms; in each atomic nucleus an inferno of quantum dynamics is stirring, quarks and gluons swirl, excitations of quantum fields. This is only a small wood on a small planet that revolves around a little star, among one hundred billion stars in one of the thousand billion galaxies constellated with dazzling cosmic events. In every corner of the universe we find vertiginous wells of layers of reality.

Nature follows its simple rules, but the complexity of things often renders the general laws irrelevant to us. Knowing that my girlfriend obeys Maxwell’s equations will not help me to make her happy. When learning how a motor functions, it is best to ignore the nuclear forces between its elementary particles.

There is an autonomy and independence of levels of understanding of the world that justifies the autonomy of the different areas of knowledge. In this sense, elementary physics is much less useful than physicists would like to think.

What happens is that the brain expects to see something, on the basis of what it knows and has previously occurred. The brain elaborates an image of what it predicts the eyes should see. This information is conveyed from the brain to the eyes, through intermediate stages. If a discrepancy is revealed between what the brain expects and the light arriving into the eyes, only then do the neural circuits send signals toward the brain. So images from around us do not travel from the eyes to the brain—only news of discrepancies regarding what the brain expects do.

The best description of reality that we have found is in terms of events that weave a web of interactions. “Entities” are nothing other than ephemeral nodes in this web. Their properties are not determined until the moment of these interactions; they exist only in relation to something else. Everything is what it is only with respect to something else.