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

there are interference effects: the cat is in one state, but in the other state there is a part of its wave that generates interference.

This provides a good explanation of the Zeilinger experiment described above. Why, when I block one of the two paths, does my hand influence the movement of the photons passing along the other path? Because the electron passes along one path only, but its wave passes along both. My hand alters the wave that then guides the electron in a way that is different to how it would behave if my hand had not intervened. In this way, my hand alters the future behavior of the electron, even if the electron passes at a distance from my hand. It’s a very good explanation.

The Hidden Variables interpretation brings quantum physics back into the same logical realm as classical physics: everything is deterministic and predictable. If we knew the position of the electron and the value of the wave, we could predict everything.

But it’s not really as simple as this. As it happens, we cannot ever know the wave, because we never see it: we only see the electron.45 Hence the behavior of the electron is determined by variables (the wave) that for us remain hidden. The variables are hidden in principle: we can never determine them. This is how the theory gets the name Hidden Variables.46

The price to be paid for taking this theory seriously is to accept the idea that an entire physical reality exists that is in principle inaccessible to us. Its sole purpose, when it comes down to it, is merely to comfort us with regards to what the theory does not tell us. Is it worth assuming the existence of an unobservable world, with no effect not already foreseen by quantum theory, only to assuage our fear of indeterminacy?

There are other difficulties as well. Bohm’s interpretation is favored by some philosophers because it offers a conceptually clear framework. But it is liked less by physicists, because as soon as you try to apply it to something more complicated than a single particle, problems accumulate. The ψ wave of more particles, for example, is not the sum of the single particles: it is a wave that does not move in physical space, but in an abstract mathematical space.47 The intuitive and clear image of reality that Bohm’s theory provides in the case of a single particle is lost.

Even more serious problems occur when relativity is taken into account. The hidden variables of the theory violate relativity brutally: they determine a privileged (unobservable) reference system. The price of thinking that the world is made of variables that are always determined, as in classical physics, is not just accepting that these variables are forever hidden, but also that they contradict everything that we have learned about the world, precisely through classical physics. Can it really be worth such a cost?

Physical Collapse

There is a third way of considering the ψ wave to be real that avoids both Many Worlds and Hidden Variables: by thinking of the predictions of quantum mechanics as approximations that overlook something capable of rendering everything more coherent.

There could be a real physical process, independent of our observations, that happens spontaneously every so often and prevents the wave from scattering. This hypothetical mechanism, never directly observed to date, is called the “physical collapse” of the wave function. The “collapse of the wave function” would not happen because we observe it; it happens spontaneously. The more macroscopic the objects in question, the more rapidly it would occur.

In the case of the cat, the ψ would quickly leap by itself to one of the two configurations, and the cat would rapidly be asleep or awake. If so, regular quantum mechanics no longer applies for macroscopic entities such as cats.48 Hence this type of theory gives predictions that deviate from those of usual quantum theory.

Various laboratories around the world have tried and are continuing to try to check these predictions in order to see who’s right. For now, it is quantum theory that has always turned out to be right. Many physicists, including your humble author, would bet on quantum theory continuing to be right for a while yet.

ACCEPTING INDETERMINACY

The interpretations of quantum theory discussed so far avoid indeterminacy by taking ψ to be a real entity.49 The cost is to add to reality things such as multiple worlds, inaccessible variables, or never-observed processes. But there is no reason to take the ψ wave so seriously.

ψ is not a real entity: it is a calculation tool. It is like weather forecasts, the profit predictions of a company, like horseracing odds.50 Real events in the world happen in a probabilistic way, and the quantity ψ is our way of calculating the probability of them occurring.

Interpretations of the theory that do not take the ψ waves so seriously are called “epistemic,” because they interpret ψ only as a summary of our knowledge (ἐπιστήμη) of what happens.

An example of this way of thinking is QBism. QBism takes quantum theory as it finds it, without seeking to “complete” the world: without hypothesizing other worlds, hidden variables or processes for which we have no evidence.

The idea is that ψ is only the information that we ourselves have about the world. It describes “that which we know about the world.” The information I have grows when I make an observation. That is why ψ changes when we observe: not because something happens in the external world, but just because the information that we have changes. Our forecast of the weather changes if we look at a barometer: not because the weather promptly changes the moment that we consult the barometer, but because we have suddenly learned something that we did not know before.

QBism gets its name from “Quantum-Bayesianism.” (Thomas Bayes was an eighteenth-century Presbyterian minister who studied probability.) But the word “QBism” alludes also to the Cubism of artists such as Georges Braque and Pablo