/sci/ - Science & Math » Thread #11761164

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Anonymous Thu 04 Jun 12:33:55 2020 No.11761164 View Reply Original Report

Quoted By: >>11761174 >>11761426 >>11762394 >>11762451

i don't get what the big deal with quantum entanglement is.
imagine some game of snooker with perfectly elastic balls and no friction. one ball hits two, transfers all its energy and they go off in different directions. now measuring one of the moving balls tells you about the other, because they're linked by the initial impact and energy needs to be conserved.
i must be missing something because entanglement doesn't seem very spooky at all.

Anonymous

Anonymous Thu 04 Jun 2020 12:40:18 No.11761174 Report

Quoted By: >>11761178

>>11761164
>I don't understand quantum blank
>provides classical mechanics analogy

Anonymous

Anonymous Thu 04 Jun 2020 12:41:42 No.11761178 Report

Quoted By: >>11761195

>>11761174
that's why i'm saying i'm missing something - i don't understand why the classical mechanics analogy isn't sufficient. clearly it isn't or it'd be classical, not quantum. what am i missing?

Anonymous

Anonymous Thu 04 Jun 2020 12:44:05 No.11761184 Report

Quoted By: >>11761223

If you stopped one snooker ball the other one would keep going.

Anonymous

Anonymous Thu 04 Jun 2020 12:50:20 No.11761195 Report

Quoted By: >>11761223

>>11761178
It's a mixture of two things. 1. The quantum particles are photons in opposite directions. They are moving too fast to have any communication between them unlike the classical balls. 2. It is an inherent property that is known about one from the other. Your analogy would be mostly fitting for considering 4 momentum in a collision, but why are the inherent properties in agreement when they couldn't communicate? That's like saying that the que ball hit two balls such that the two balls go off at 45 degree angle relative to each other with some speed that is determined by collision mechanics. We don't know which two balls were hit, but because we looked at one ball afterwards and saw it was the 3, then the other must be the 7.

Anonymous

Anonymous Thu 04 Jun 2020 12:58:38 No.11761223 Report

Quoted By:

>>11761184
i'm not sure that satisfies, you're disturbing the system and effectively decohering them, so it makes sense they don't predict each other anymore.

>>11761195
the 3 and 7 idea is interesting, but when i read about entanglement examples it's always quantities that naturally determine each other because of conservation laws. like wikipedia:
"For instance, a spin-zero particle could decay into a pair of spin-½ particles. Since the total spin before and after this decay must be zero (conservation of angular momentum), whenever the first particle is measured to be spin up on some axis, the other, when measured on the same axis, is always found to be spin down."

that's just not spooky at all, of course it'd be the case when there's no interaction to external environments.

Anonymous

Anonymous Thu 04 Jun 2020 14:16:58 No.11761426 Report

Quoted By: >>11761448 >>11761469 >>11761503

>>11761164
Neither ball has a defined, say, spin until it is measured. When one is measured, the other particle immediately collapses into a certain correlated spin.

Anonymous

Anonymous Thu 04 Jun 2020 14:22:43 No.11761448 Report

Quoted By: >>11761504

>>11761426
doesnt that infer much moreso an issue with instrumentation than creating a magic formula meant to fit what we can currently observe?

Anonymous

Anonymous Thu 04 Jun 2020 14:29:50 No.11761469 Report

Quoted By: >>11761486

>>11761426
well that's like saying neither ball has defined momentum because we have to switch the lights off to make sure radiation pressure doesn't affect the results. we can't see it sure, but it's defined by the initial impact (momentum of which is known in this example). isn't this part of what entanglement means?

Anonymous

Anonymous Thu 04 Jun 2020 14:36:27 No.11761486 Report

Quoted By: >>11761492

>>11761469
No. It's in a superposition until we can observe it. And we can verify as part of our observation that it is in a superposition and not merely an unknown pure state.

Anonymous

Anonymous Thu 04 Jun 2020 14:38:33 No.11761492 Report

Quoted By:

>>11761486
i wasn't expecting to get into superposition with this question. is the main spooky part with entanglement that it allows for a system to continue to be in superposition? i.e. so non-classical effects can result?

Anonymous

Anonymous Thu 04 Jun 2020 14:44:31 No.11761499 Report

Quoted By:

"This thought experiment, which came to be known as the EPR paradox, hinges on the principle of locality. A common presentation of the paradox is as follows: two particles interact and fly off in opposite directions. Even when the particles are so far apart that any classical interaction would be impossible (see principle of locality), a measurement of one particle nonetheless determines the corresponding result of a measurement of the other. "
i'm still confused as shit. how does this count as interaction and why is it surprising in any way?

Anonymous

Anonymous Thu 04 Jun 2020 14:44:53 No.11761503 Report

Quoted By: >>11761516

>>11761426
>When one is measured, the other particle immediately collapses into a certain correlated spin.
But you can't detect in the other particle when this collapse happens. All you can do is measure its spin also, and you'll find that it agrees (sums to zero, whatever) with the measurement at the first particle.
All you have is
>entangle A and B
>measure spin of A
>measure spin of B
>they agree
I don't see any spooky action at a distance here. The whole confusion arises because of how people explain "wave function collapse" as if the measurement at A causes, instantaneously (ftl), an event at B. Can we experimentally verify that a collapse happens at B at the same time the measurement happens at A?

Anonymous

Anonymous Thu 04 Jun 2020 14:45:38 No.11761504 Report

Quoted By: >>11761516

>>11761448
Say you have a device that produces two electrons at a time, entangled so that they have opposite spin. Now, classically, these spins should have definite directions the moment the electrons exit the device. Statistically, we expect a distribution of possible spins: some of the pairs should be up and down, some should be left and right, some should be 45 degrees and 225 degrees, etc (really, any 3d orientation).
Now, it's experimentally known that, if you measure the spin of an electron off-axis, you can get either result along that axis. For instance, if you measure a spin-up electron along the 45-225 degree axis, the spin could either end up 45 or 225. Crucially, through this 45-225 degree detector, you could send a spin up followed by a spin down electron, and it's possible that both would be 45.
The weird thing about entanglement is that, no matter what axis you choose for measuring spin, the electrons will always give opposite measurements. Even though, statistically, there should be many pairs whose spins are off-axis from our measuring axis, and we would therefore expect that some pairs would give the same result along our measurement axis.
There is an analogous effect for photon polarization.

Anonymous

Anonymous Thu 04 Jun 2020 14:50:41 No.11761516 Report

Quoted By: >>11761518 >>11761525

>>11761503
this is what i've been trying to say

>>11761504
if the electrons are generated in pairs by some process that demands they have opposite spins (seemingly all entanglement scenarios require this) why is it surprising?
i think i will have to learn more about spin and the measurement axis to get this.

Anonymous

Anonymous Thu 04 Jun 2020 14:52:41 No.11761518 Report

Quoted By: >>11761544

>>11761516
Yeah, look into how spin works. This is the main point:
>Crucially, through this 45-225 degree detector, you could send a spin up followed by a spin down electron, and it's possible that both would be 45.
But with entangled pairs, this never happens.

Anonymous

Anonymous Thu 04 Jun 2020 14:56:31 No.11761525 Report

Quoted By: >>11761544

>>11761516
Because the generated spins have no physical reason to all be in pure states in your basis. A spin 0 degrees electron and a spin 180 degrees electron giving opposite results is not surprising, it's required. A spin 45 degree and a spin 135 degree electron collapsing into spin 0 and spin 180 respectively is also not surprising, it's fairly likely. A thousand superposition particles of this type all collapsing in a perfectly correlated manner is so unlikely as to be impossible without action at a distance.

Anonymous

Anonymous Thu 04 Jun 2020 15:06:47 No.11761531 Report

Quoted By: >>11761544 >>11761550 >>11762238

So if I'm understanding it right the point is specifically about the axis along which you measure the spins? I.e. you measure the spin of A along 0-180 axis (axis x let's say), and you will find B has the opposite spin along that same axis. But if you had measured along 45-225 axis (axis y), you would have found that they both have opposite spins along that axis, too. BUT (this is the spoopiness right) a positive spin along axis x is not really the opposite of a negative spin along axis y, so by choosing the axis of measurement, you determine along what axis the "oppositeness" happens.
Is that about right?

Anonymous

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Anonymous Thu 04 Jun 2020 15:13:36 No.11761544 Report

Quoted By:

>>11761518
>>11761525
now i'm really confused, but i think i'm confused about the correct thing now.

>>11761531
this is how i'm interpreting it.

Anonymous

Anonymous Thu 04 Jun 2020 15:17:25 No.11761550 Report

Quoted By: >>11761586

>>11761531
Yeah, that sounds about right.

It's worth pointing out that there isn't really a single, clear physical interpretation of how spin "should" work in the classical case. It's most closely analogous to measuring one component of the angular momentum of a spinning object - but the component you measure always has the exact same magnitude, so that kind of goes out the window.
Some would make the argument that, classically, entangled electrons SHOULD always have opposite spins. This is why Bell's theorem ends up being important to really solidify the strangeness of entanglement. With Bell's theorem, you can abstract away all the physical interpretation of how spin "should" work and just focus on binary values of detector measurements.

Anonymous

Anonymous Thu 04 Jun 2020 15:39:43 No.11761586 Report

Quoted By: >>11761596

>>11761550
so you can measure x and z axis and they may be opposite. spin is quantized, so you wouldn't expect a smooth transition if you then measure between those two axes.
unlike earth for instance, where if you take the temperature at two points and all the points inbetween you'll see a transition.
so at what point does the spin "switch"? if you started at x and rotated through to z, taking measurements as you go, what do you see? is this why measuring on different axes in succession seems to force the spins to redistribute randomly?

Anonymous

Anonymous Thu 04 Jun 2020 15:44:27 No.11761596 Report

Quoted By: >>11761610

>>11761586
It could potentially 'switch' at any arbitrarily small nonzero angle. The probability is just smaller and smaller.

Anonymous

Anonymous Thu 04 Jun 2020 15:49:27 No.11761610 Report

Quoted By:

>>11761596
ok, i think i get it. i was thinking of spin as something the particle has all the time and doesn't change, but it seems it's more of a statistical property that emerges at measurement time. hence the "forgetting".

Anonymous

Anonymous Thu 04 Jun 2020 19:18:33 No.11762238 Report

Quoted By: >>11762256

>>11761531
Now going on from here, I don't get why local hidden variables are ruled out in QM. Why can't there be a function that takes as input the axis of measurement and as output a spin, that is "fixed" at the time of entanglement, and gives opposite results for two entangled particles for every axis?
I guess this is what Bell's theorem is all about but I just don't get it.

Anonymous

Anonymous Thu 04 Jun 2020 19:28:56 No.11762256 Report

Quoted By:

>>11762238
Okay never mind, I shouldn't believe everything I read on wikipedia. Bell's theorem only rules out certain types of local hidden variable theories, not all of them.
See https://en.wikipedia.org/wiki/Talk:Hidden-variable_theory#Local_Hidden_Variables
Also I found this https://arxiv.org/abs/quant-ph/0309111
but it's way over my head obviously. Does seem to claim a local hidden variable theory in some limited sense:
>In the light of these results, we argue that the violation of Bell-type inequalities in the quantumcase cannot be a valid argument in the discussion on locality or non-locality of quantum interactions

Anonymous

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Anonymous Thu 04 Jun 2020 20:20:17 No.11762394 Report

Quoted By:

>>11761164
if you measure one, you know what the other one had to be after the entanglement
literally physicists intentionally obfuscating things to keep scamming funding from the government

Anonymous

Anonymous Thu 04 Jun 2020 20:41:26 No.11762451 Report

Quoted By:

>>11761164
The main idea is that qua tum objects shouldn't be able to contain enough information to explain all the entanglement phenomena with "they were made that way". Also, try the delayed-choice quantum eraser to really drive home the point

Anonymous

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Anonymous Thu 04 Jun 2020 23:38:20 No.11762999 Report

Quoted By: >>11763827

Can someone explain to a brainlet why we can't use quantum entanglement for FTL communication?

Anonymous

Anonymous Fri 05 Jun 2020 04:35:41 No.11763827 Report

Quoted By:

>>11762999
Sure. Let's say there are two entangled particles, one with you and one with your partner. You affect your particle in some way, and in so doing, you find that your particle had spin down. You now know that the other particle has spin up.
Once measured, the entanglement is broken for you. Nothing you do to your particle can change the other particle from being spin up. The spins of the particles are no longer correlated. You couldn't force which measurement outcome you would get, and you can't affect the other particle once you know.
If the other person is far away, your measurement may not have yet occurred in their causal past. Therefore, the entanglement is still active for them, and they don't know what the spins will be when they measure them. Even though you know their particle is spin up, for them their measurement will be randomly decided. When you and your partner later compare the measurements, they will be consistent, even though you disagree about whether the partner's measurement was random or determined.

Anonymous

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Anonymous Fri 05 Jun 2020 09:28:19 No.11764221 Report

Quoted By:

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1968727
Here's a nice paper that clears up a lot of the misunderstanding surrounding double slit, quantum erasers etc.. I'm not at that part yet but it deals with the delayed-choice quantum eraser at the very end.

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