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it's a long distance relationship (lemmy.world)

submitted 2 months ago by kali_fornication@lemmy.world to c/science_memes@mander.xyz

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[-] Ephera@lemmy.ml 39 points 2 months ago

The analogy that makes most sense to me so far, is this:
You rip a photograph in half and put both halves into envelopes. Now you send one of the envelopes to your friend in Australia. You open the other envelope. Boom! Instantaneous knowledge of what's in the envelope in Australia. Faster than light!!!

In quantum terms, you "rip a photograph in half" by somehow producing two quanta, which are known to have correlated properties. For example, you can produce two quanta, where one has a positive spin and the other a negative spin, and you know those to be equally strong. If you now measure the spin of the first quantum, you know that the other has the opposite spin.

[-] lemonskate@lemmy.world 23 points 2 months ago

The important distinction here (and I get it, analogies are always imperfect) is that the photograph analogy has "hidden variables". That is, each half is fixed at the moment of their separation and you just don't know what's in the envelopes until you open one. That's not how entangled particles work though, and which "half" is which is not determined until the instant of measurement, at which point the state of both are known and fixed.

[-] Ephera@lemmy.ml 5 points 2 months ago

I'm open for counterarguments, but I always felt this was a silly way of looking at things. You cannot measure stuff at the quantum level without significantly altering what you measured. (You can never measure without altering what you measured, since we typically blast stuff with photons from a light source to be able to look at it, but for stuff that's significantly larger than photons, the photons are rather insignificant.)

As such, you can look at measuring quanta in two ways:

Either the quantum had the state that you end up measuring all along. It is only "undetermined", because strictly nothing can measure it before you do that first measurement.
Or you can declare it to have some magical "superposition", from which it jumps into an actual state in the instant that you do the measurement.

Well, and isn't quantum entanglement evidence for 1.? You entangle these quanta, then you measure one of them. At this point, you already know what the other one will give as a result for its measurement, even though you have not measured/altered it yet.
You can do the measurement quite a bit later and still get the result that you deduced from measuring the entangled quantum. (So long as nothing else altered the property you want to measure, of course...)

[-] lemonskate@lemmy.world 21 points 2 months ago* (last edited 2 months ago)

This is pretty conclusively addressed by the Bell Inequalities and empirically tested. It's absolutely counter-intuitive and feels "wrong" but it is definitely how our universe operates.

https://m.youtube.com/watch?v=9OM0jSTeeBg A relatively short but decent explainer for Bell's Theorem and the Nobel prize winning experiment to successfully test it.