space, time, and all other physical quantities only relational?"
What do we actually know about the physical world after the scientific revolution of the last century? Before the XXth century, the picture of the physical world was simple: matter formed by particles (and fields) moving in time over the stage of space, pushed and pulled by forces, according to deterministic equations, which we could write down. That's it.. But the 20th Century has changed all that in depth. Matter has quantum properties: particles can be delocalized -as if they were clouds- although they manifest themselves always as a single point when interacting with us. Space and time are not just curved: they are dynamical entities very much like the electric and magnetic field. Is there a new consistent picture of the physical world, that takes all this new knowledge into account?
The most remarkable aspect of quantum theory is its relational character: elementary quantum events (such as a certain quantum particle being "here") only happen in interactions, and, in a precise sense they are only "real" with respect to, or in relation with, another system. Indeed, I can see the particle "here", but at the same time the particle and I can be in a quantum superposition in which the particle has no precise localization. Thus, a quantum particle is not just "here", but only "here for me".
On the other hand, the most remarkable property of general relativity is that localization in space and time are not defined. Things are only localized with respect to other things. In fact, the spacetime coordinates have no meaning in general relativity, and only quantities that are independent of these coordinates (such as relative localizations) have physical meaning.
Now the question is: are the quantum relationalism (quantum systems have definite properties only in nteracting with other systems) and the general relativistic relationalism (position is only relative) connected to each other? Are they indeed two aspects of the same relationalism?
There is clearly some deep connection. In order to interact quantum mechanically, two systems must be close in space and time, and, viceversa, spacetime contiguity can only be checked via a quantum interaction. So, is perhaps spacetime just the geography of the net of the quantum interactions? Is the world just made of relations?
We are far from understanding all this, and the current highly speculative physical theories haven't even started addressing this kind of questions. But until we address these questions -which are the interesting ones in physics for me- the great revolution of the 20th century is not over. We have lost the old picture of the physical world, but we haven't a new credible one yet.
Carlo Rovelli is a theoretical physicist at the Centre de Physique Theorique in Marseille, France.