Quantum Physics: non-locality (General)

by David Turell , Saturday, October 21, 2023, 22:06 (397 days ago) @ David Turell

It is connected everywhere:

https://quizwithit.com/start/1696811301328x301364883622124100

"This update of the wave-function is also sometimes called the collapse or reduction of the wave-function and it’s a key element of quantum mechanics. If you don’t update the wave-function, you will get wrong probabilities. If you want to know for example, what’s the probability of measuring the particle on the left given that it was measured on the right, the answer should be zero. But this only comes out correctly if you update the wave-function.

"The update of the wave-function is instantaneous. It happens at the same time everywhere and is the reason why quantum mechanics is non-local. This wave-function update is what Einstein called a “spooky action at a distance”.

***

'If quantum mechanics is fundamentally correct, then the world is non-local, period. But if there’s an underlying reality in which the outcome of a measurement was determined, we just didn’t know of it, then this reality could well be local. This is called a hidden variables model.

***

"The reason that quantum mechanics is non-local is a combination of (a) the observational fact that a measurement outcome in one place tells you something about another measurement outcome in another place. If you measure the particle here, you now know you won’t measure it there. Fact, not interpretation. And (b) the absence of other variables in the theory that could have carried the information locally. This is why quantum mechanics is non-local.

***

"The Many Worlds interpretation now is based on the idea that you can throw out the update of the wave-function by re-interpreting what happens in a measurement. According to this interpretation, all outcomes of a measurement happen, each in its own universe. But we can only ever see the result in one universe, so for us it *looks like the wave-function collapses.

"Instead of the measurement update, in many worlds, we have what is called a “branching” or “splitting” of worlds. This branching makes it impossible for one observer to see more than one outcome of a measurement. The major challenge for many worlds is to explain why the thing we call an observer does not itself branch with those worlds therefore sees all the outcomes, but somehow randomly only experiences one of those worlds.

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but then Many Worlds makes the same predictions and standard quantum mechanics.

"It also leaves you with the mind blowing idea that each time a quantum particle bounces off another one, which happens gazillions of times a second, our entire universe splits, and anything that can happen does happen. Had salad for lunch today? Well in some other universe you had pizza, with Elon Musk, on Mars. Whatever you can think of, so long as it respects the laws of nature, it’s real, in some parallel universe.

***

"No, the biggest problem with many worlds is that its supporters believe their interpretation is somehow better than the standard interpretation with the collapse when it’s really just as mediocre.

"Many worlds supporters often claim that their interpretation is simpler because it just does away with the collapse postulate. But as we saw earlier, you need the collapse postulate to calculate probabilities. You can’t just throw it out, that doesn’t work. And indeed, this is not how the many worlds interpretation works. It’s how Many Worlds supporters *say* that it works, but it’s not true.

"At this point things get a bit murky because there isn’t just one many worlds interpretation. There are two original ones, going back to Hugh Everett and Bryce DeWitt, but meanwhile there are dozens of slightly different versions.

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"I find it surprising how many physicists are confused by this. Lots of papers have been written about how many worlds can be made local. But of course, the only way to make it local would be to introduce some kind of hidden variable that transports information locally.

"This was exactly the point of the famous paper by Albert Einstein, Boris Podolsky and Nathan Rosen, now just known as the EPR paper. They said, if you want reality to be local, you need an element of reality that underlies quantum mechanics, therefore quantum mechanics is incomplete. The paper’s now almost 90 years old, but physicists still don’t get it, do they.

***

"As Einstein, Podolsky, and Rosen said, if you want to have a local theory, you need something to transport the information locally. The wave-function doesn’t do it, so you need something else. The many worlds interpretation doesn’t introduce anything new to get the job done, so of course it’s still non-local.

***

"In summary, the Many Worlds interpretation is neither wrong nor unscientific, but it’s exactly as problematic as standard quantum mechanics. Whether you believe that all those parallel universes exist is up to you. We can neither confirm them nor rule them out."

Comment: Hossenfelder is being polite. Many worlds theory is mental masturbation, totally unproven and unproveable. No matter how far apart particles are split each one knows what the other is doing.