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Comment by mikk14 | Hacker News Reader

I have visited it in the Easter week, with my 4 year old. It's nice, but I experienced a major disappointment when visiting the immersive experience about the quantum world: they explain quantum entanglement in the wrong way!

The voice-over _clearly_ says: "when two particles are entangled _they can influence each other_ no matter the distance", which is super wrong. If you have two spin entangled particles and you change the spin of one, the spin of the other does not change, there is no influence, only correlation.

Maybe they intended something about the wave function collapse, but they didn't say it. If even explanations coming from CERN get this wrong, I despair for the status of science communication in general...

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> If you have two spin entangled particles and you change > the spin of one, the spin of the other does not change

It really irks me we don't use a simple analogy for entanglement: put two balls of different colors in two boxes. Randomize the boxes. Take one of the boxes to the other side of the room then open it.

You now know the colour of the ball in the other box.

Only you can't implement faster-than-light communication with that so we instead mislead people..

What? That's simple deduction. Is it really that trivial? Am I missing something?

No, that’s not quite right. You’re taking two wavefunctions that have not collapsed yet.

See https://en.m.wikipedia.org/wiki/Bell_test

This ball box analogy is not the same as quantum entanglement … outcome of the boxes are predetermined but not for two entanglement spins …

That's not the correct analogy. In classical case, the color of the ball is fixed even before you open the box (you just don't know it). But in quantum entanglement case, the spin is not fixed until it is measured (because the wave function hasn't collapsed). But as soon as you measure the spin you get either up or down value. If you get up, then the other entangled particle will necessarily have down value when measured, If you get the spin down, the other particle will necessarily have the spin up. Now you may say that, the spin of the first particle was fixed all along and we just didn't know it. This argument is called "hidden variables theory". But it is proven by Bell's inequality that such a theory cannot exist, so the spin of particle 1 is indeed a random outcome. What's "spooky" is that in spite of it being random, it instantaneously fixes the spin of the other particle.

Yeah, but I didn't "get" (to the extent that I can without grokking maths) quantum entanglement until I had it explained with this analogy, and then the "but that's not exactly what's happening here" real explanation. Leading with the complicated (albeit correct) wave form collapse explanation spun my head around and got me nowhere.

Good pedagogy (like, you know, science itself) starts simple and adds complexity as you dive deeper into the subject.

> But it is proven by Bell's inequality that such a theory cannot exist, so the spin of particle 1 is indeed a random outcome.

It's still allowed so long as a) the theory is nonlocal b) the outcome was not actually independent of the observer or c) superdeterminism.

No. There are no “hidden local variables” (color in your example), and it really is “spooky action from a distance”.

See Bell’s Theorem, this has been mathematically and experimentally proved: https://en.m.wikipedia.org/wiki/Bell's_theorem

This thing that might be confusing you is the action you are taking is collapsing the wave function of a pair of entangled particles (by measuring one of them), so they go from a superposition of up|down to each having one definite value. You can’t repeatedly twiddle the bit here.

Other people are pointing out how its not actually like that, but I'm gonna buck the trend and say, for the purposes of explaining to the general public the basics of quantum mechanics, its perfectly acceptable.

Quantum mechanics is so counterintuitive that you have to re-calibrate people's intuition before you can really pick at the confusing parts.

So picking the nit re: wave function collapse is the right thing to do, but it needs to be done in the context of "...but its weirder than just we don't know what the colors are until we open the box. It turns out that...", rather than just immediately "correcting" the partially, arguably incorrect information.

As a challenge to the folks correcting the OP over neglecting wave function collapse, can any of you describe what is wrong with the infinite square well very-first-mathematical-example-of-quantum-mechanics? Aside from the "infinite" part, I mean.

> The voice-over _clearly_ says: "when two particles are entangled _they can influence each other_ no matter the distance", which is super wrong. If you have two spin entangled particles and you change the spin of one, the spin of the other does not change, there is no influence, only correlation.

Sounds like you subscribe to the Copenhagen interpretation, whereas they’re using a Bohm interpretation.

Granted that I'm not a physicist and I may be wrong about this, but I don't think what you say is correct. This is not a matter of interpretation. In no interpretation changing the spin of an entangled particle will change the spin of the other at a distance.

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