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it's a long distance relationship (lemmy.world)
submitted 9 months ago by to c/science_memes@mander.xyz
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[–] 53 points 9 months ago (17 children)

To be fair, the "change one" part is wrong. Two particles that are quantum entangled maintain the same quantum state when separated. But if you change the quantum state of one it doesn't propogate. They are just in sync.

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  • [–] 37 points 9 months ago (16 children)

    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.

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  • [–] 22 points 9 months ago (14 children)

    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.

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  • [–] 4 points 9 months ago

    The photograph example has local hidden variables. The quantum version doesn't, but nothing has ruled out non-local hidden variables. Locality is a nice property we want the universe to have, but the universe doesn't have to obey our desires.

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  • [–] 3 points 9 months ago (12 children)

    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:

    1. 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.
    2. 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...)

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  • [–] 13 points 9 months ago (8 children)

    Something something Bell's Theorem. I don't really understand it but that one was supposed to be counterevidence to hidden variables.

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  • [–] 2 points 9 months ago (7 children)

    "it can't be hidden variables because they're not as even as this math says they should be!" really just seems to be the whole QM field agreeing to stop arguing about spooky action at a distance.

    The distinction between wave-functions as real things that collapse at superluminal speed and the same as mere mathematical placeholders for deterministic local effects which occur without subjective time seems to be a semantic and philosophical one, similar to the "multiple realities" explanation of quantum uncertainty or the "11 dimensions" explanation for why gravity is weaker.

    As a practical matter, the only thing that students and non-physicts should remember is that wavefunction collapse allows superluminal coordination but not superluminal communication.

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  • [–] 4 points 9 months ago* (6 children)

    okay so if i understand this right, if i take half of schroedingers box and open it up, by observing the half of the cat i have i will instantly know if the half of the box the other guy's got has got half of an alive cat in it? and i'll be able to tell if his half of an alive cat is purring and void or garfield and shit is my stupid analogy right?

    but i cannot pet my half of a cat and make it purr and thus make their half of a cat purr. because cats do not work that way.

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  • [–] 5 points 9 months ago (1 child)

    You want to cut Schrödinger's box in half? This kills the cat, unless the box is big enough for the cat to avoid the blade, in which case you've opened the box and the cat is probably going to need some convincing to get out from under whatever furniture it can find.

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  • [–] 2 points 9 months ago (1 child)

    schroedinger's cat is an intentionally absurd metaphor from when QM dorks were still arguing about spooky action at a distance.

    Both the cat, the box, the vial of poison, and the cesium atom itself are all observers as far as a real QM wavefunction would care. But as i understand it, getting any utility out of the idea of real collapsing wave-functions requires treating at least the atom as if it wasn't, and once we start including atomic scale things we might as well just include everything up to and including the cat.

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  • [–] 8 points 9 months ago* (1 child)

    The whole idea is that the quantum particle can't have had the state you're measuring all along. If it did, then measuring a particular set of outcomes would be improbable. If you run an experiment millions of times, you have a choice in how you do the final measurement each time. What you find with quantum particles is that the measurements of the two different particles are more correlated than they should be able to be if they had determined an answer (state) in advance.

    You can resolve this 3 ways:

    1: you got extremely unlucky with your choice of measurement in each experiment lining up with the hidden/fixed state of each particle in such a way as to screw with your results. If you do the experiment millions of times, the probability of this happening randomly can be made arbitrarily small. So then, the universe must be colluding to give you a non uniform distribution of hidden states that perfectly mess with your currently chosen experiment

    2: the particles transfer information to each other faster than the speed of light

    3: there is no hidden state that the particle has that determines how it will be measured in any particular experiment

    See https://www.quantamagazine.org/how-bells-theorem-proved-spooky-action-at-a-distance-is-real-20210720/ for a short explanation of what 'more correlated than expected' means

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  • [–] 9 points 9 months ago

    My personal example is identical twins. If they've had the same experiences, then knowing what one looks like tells you what the other looks like, but ripping the arm off of one doesn't magically rip the arm off the other.

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