External Reality – By Ignacio Cirac

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How is the notion of reality put into question in Quantum Physics?

Quantum Physics does not question the existence of an independent reality. What it does say is that the properties of objects beyond us don’t have to be defined. And that’s what collides with classical principles. If one works with classical Physics, every object has many properties: it has movement, position, velocity, color, mass… and even when we’re not observing it, those properties are well defined. An object, even if it’s not being observed, is at some place. However, Quantum Physics says that’s not true: that objects exist but, when we’re not observing them, their properties start to become indefinite and they only become definite after an observation. And that’s something Quantum Physics claims and which, along with another property called non-locality, has been experimentally proven. It’s not only an interpretation of Quantum Physics: it’s a precise fact, the fact that objects’ properties don’t have to be defined when they’re not being observed. And, if one also believes that actions cannot be propagated at infinite velocity, those things give rise to experiments which prove that Nature does not define properties. What I mean is that my position is based on experiments, that is, on objective facts. Later, one may attempt to explain what’s going on in reality. One may even go further an ask: “if they are not defined, what is happening? Do we have many universes? Or do we have something else?” And those are the things that have a certain interpretation within Quantum Physics. There’s no interpretation I like more than any other, precisely because of what I said before about not having any evidence pointing to one or another, because they predict the same experimental outcomes.

Mathematics seems to play an essential role in the description of the universe. Why do you think that happens? Could it be some other way?

I don’t think so. I think a universe must have certain laws. The question is: how many? Maybe there are as many laws as possible phenomena, so we would have a description in terms of laws but it would be completely useless, since it wouldn’t be predictive. For every thing that happened we would need to have a different description. It’s pretty logical to think that the laws have to be consequent with each other and, therefore, that many phenomena should be deduced from some fundamental laws. And that’s what Mathematics does: to transcribe those laws, in terms of formulas -a different language- and using logics in order to draw consequences from the postulates, which gives rise to the description of Nature we have. And I have no problem with that. Of course, there are very interesting details, like the fact that Mathematics is not complete, in the sense that not everything that’s true in a certain theory can be proven. All of that gives rise to other very interesting questions, but it seems natural to me that the laws the universe follows be mathematical, which after all they are nothing but logical deductions.

So Galileo was right when he said Nature is a book which is written in mathematical characters.

In principle, yes. But there are two small details Galileo didn’t know about: one of them is Quantum Physics, and Quantum Physics breaks away from determinism. One would think that, if everything is written, then we are totally deterministic. If one knows the positions and velocities of every particle at a given moment, that completely determines what will happen later. But Quantum Physics, after introducing measurement, stops being deterministic. That gives rise to the existence of intrinsic probabilities, which implies that history is not written. That, on one hand. And on the other hand we have what I was saying before about Gödel: that, even if there were some fundamental principles, it would very hard or impossible for us to know them all, in the first place and, in the second place, even if we did know them all, it may be impossible to guess what’s going to happen next, to predict the future, because of what I said before that Mathematics is not complete, something we know through Gödel.

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