More on Superdeterminism
Entry posted by Silly Druid in Physics
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The goal of this entry is to explain my thoughts about superdeterminism in a more detailed way.
We should start with the double slit experiment: there is a source of particles, two slits, and a screen. The particles can go through either of the slits, and even if we emit a single particle at a time, after many particles there is an interference pattern at the screen, showing that the particle seems to go through both slits at the same time. But if we make a measurement to determine which slit the particle used, it always uses only one of them and there is no interference pattern. So what actually happens here?
The most obvious explanation would be a simple hidden variables theory, stating that the particle already carries all information needed to determine the results of all possible measurements. So if we don't measure it, it goes through both slits, but if we measure it, it already "knows" which slit it should use. But this explanation is wrong. The Bell Inequality experiments prove it. It's simply not possible.
Another explanation is the standard quantum mechanics (Copenhagen interpretation). As far as I understand it, it states that if we don't measure it, it's truly undefined which slit the particle (or rather its wave function) uses. It moves through both slits with the probability of 50% each, and it interferes with itself. But when we try to determine which slit was used, the wave function "collapses", so it becomes 0% in one slit and 100% in the other. The problem with this interpretation is that the "collapse" doesn't make much sense as a physical process, because it happens at the same time in all the space occupied by the wave function: if we detect the particle in one slit, the part of the wave function in the other slit instantly becomes 0. This is Einstein's "spooky action at a distance". And we don't even know what "instantly" means, because in special relativity the concept of "at the same time" depends on the frame of reference we use.
So how does superdeterminism fix these problems? By assuming that the following two things are correlated: the path of the particle (let's call it A), and the way we measure it (B).
Before watching Sabine Hossenfelder's video, I didn't believe in superdeterminism. Not because the free will nonsense, but because in the sources I've seen it was presented as if the correlation meant there is a common cause (C), that causes both A and B. This makes no sense, because there is no way a physical process would determine both the path of the particle, and our decision how to measure it.
But now I realized there are other options as well. A correlation can also mean that A causes B, or B causes A.
"A causes B" makes no sense either, because again there is no way a physical process would make the path of the particle cause our decision how to measure it.
But "B causes A" is an interesting possibility. If the way we measure a particle affects its path, then we can explain the "problem of measurement" without the need of "wave function collapse". Instead of collapsing, it's already created in a way that depends on what we measure (or don't measure). So if we don't detect which slit the particle uses, the wave function goes through both slits and interferes with itself, and if we do detect it, it goes through one slit. There is no collapse, and it's also consistent with the Bell Inequality results.
But the measurement is made after we emitted the particle and it has "chosen" its path. Which means backwards causality: the effect (particle choosing its path) happens before the cause (the measurement). The way I see it is when we try to do something with a particle (which means trying to emit it, absorb it, measure it etc.), it creates a "vertex" on the grid of all possible particle interactions, and these vertices have a probability to be connected with each other by "particle paths". I think it's something similar to the "transactional interpretation" of quantum mechanics.
[EDIT] An important thing is why this backwards causality doesn't cause time paradoxes. It's because it affects only the wave function, which is immeasurable. We can't actually see what happens with the wave function before we measure it. So you can argue that it simply doesn't exist at all. But it should exist, because it's what the Schrödinger equation describes, in other words it's what makes the entire quantum mechanics work. Without it (or some equivalent of it) we don't understand what happens at all. So my understanding of superdeterminism implies backwards causality, but in a way that doesn't allow you to actually send any information back in time.
As for the tachyons, for those who think they are forbidden in special relativity, it's not true. If we assume a particle has an imaginary mass, it must move faster than light. But now I'm not so sure they are what really happens here, maybe it's something different, like wormholes. (Yes, there is a conjecture, called "ER = EPR", which states that quantum entanglement creates a wormhole between the particles.) What I mean here is that if we successfully combine two theories, it usually predicts something new. For example, when Dirac combined quantum mechanics and special relativity, it predicted the existence of anti-matter, which was experimentally confirmed later. So if we combine superdeterministic quantum mechanics with general relativity, who knows what we will find?
(Sorry if this entry is too long and messy for you, you can ask questions in the comments so hopefully I can explain some things better.)
Edited by PawelS
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