How Bell’s Theorem proved ‘spooky action at a distance’ is real • Hacker News Board

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#261

Post by dreamcompiler » Wed, Jul 21, 2021, 1:45 AM UTC

pontus wrote: post #232

jussij wrote: post #219

Couldn't that 'hidden variable' be just another dimension? So while those particles might have been separated in space by a large distance, on that fifth dimension they haven't moved and are still sitting side by side.

The distance between two points is basically just the Pythagorean theorem: x^2 + y^2 + z^2. If you have extra dimensions, you just add more terms to this. Now, if you know that e.g. x is very large then that puts a lower bound on the distance: Distance^2 = x^2 + y^2 + z^2 + ... > x^2. In other words, if x is large there's no way that the two particles are still somehow close together.

This is only true for cartesian spaces. If the low-dimensional manifold is "folded" or "crinkly" inside a higher-dimensional space then you can indeed have particles that look far apart on the manifold but which are actually close together in the higher-dimensional space.

(My terminology here is largely wrong; this is not my field. Perhaps an expert can correct me.)

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#262

Post by fungiblecog » Wed, Jul 21, 2021, 2:03 AM UTC

It did not prove that. It proved that at least one of three assumptions about the universe is violated: Statistical Independence, Locality or Determinism. The usual assumption is that Locality has to go, but that is not necessarily true. It is possible that Statistical Independence is not true for the experimental systems that have been studied.

See for example: https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#263

Post by tines » Wed, Jul 21, 2021, 2:10 AM UTC

mannerheim wrote: post #247

tines wrote: post #235

Earlier quoted context omitted.

As someone who doesn't know any better than the explanation you have a beef with, I would love if you could explain in layman's terms why it's wrong and what a more accurate understanding is. I always thought that analogy was exactly how it worked, but it seems I have been misled unawares.

It presupposes that position and momentum have definite states that are just uncertain to us, while in most interpretations of quantum mechanics (e.g. Copenhagen and many worlds) the particle exists as a wavefunction, lacking a specific position or momentum, but instead existing as a probability density function in this space. The uncertainty principle here then relates to how much this probability density function '…

So is it true that there are multiple properties of a particle---such as location, position, and maybe its spin---that are all described as wave functions, and therefore they can all be entangled? Can anything that is described by a wave function be entangled?

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#264

Post by jiggawatts » Wed, Jul 21, 2021, 2:16 AM UTC

fungiblecog wrote: post #233

georgeecollins wrote: post #228

Earlier quoted context omitted.

It is also strange, but I think the many worlds hypothesis makes the most sense to me. Then there is no spooky action at a distance, the observer is just always in a particular version of reality that all states, even distant ones, conform to. I am not a physicist!

But there is still spooky splitting of the universe... Many worlds just replaces one mystery with another

Yes, but not action at a distance. There's no violation of relativity in MWI.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#265

Post by jiggawatts » Wed, Jul 21, 2021, 2:23 AM UTC

tbabb wrote: post #225

danbruc wrote: post #213

Earlier quoted context omitted.

I think it is not as simple as counting the number of postulates. The ultimate arbiter is physical reality and that decides whether removing or adding postulates is what brings you into agreement. The fact that the quantum state in some equation before wave function collapse looks like many worlds say nothing whether you should take this at face value or whether you are missing an additional postulate that gives you…

You said: > Yes, it gets rid of the collapse postulate, but no, it actually introduces many worlds. [...] but in the end many worlds makes a metaphysical claim that two cats exist, [...] while Copenhagen claims only one cat exists in the end. If you are saying that many worlds makes a greater number of claims than Copenhagen, that's incorrect, as explained above. The claims made by many worlds are a strict subset of…

I've noticed that some people are simply allergic to the MWI view, likely because of a religious background. Strangely, my physics classes were about 80% fundamental, bible-carrying Christians, as were many of the "Fathers of Quantum Mechanics"!

Many mathematically and physically theories that are perfectly reasonable are rejected by such people out of hand because it doesn't mesh well with their preconceptions of "The Earth is Special", "I have a unique soul that is me", "Jesus came to us, here, specifically", etc...

I know I'll probably get voted down for this, but these are the literal arguments that I was given once I pressed some of my fellow students hard enough on why they reject MWI.

It's not because they investigated the logic of the situation, like you have. They just "feel" like MWI makes them less special and unique in Creation.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#266

Post by mannerheim » Wed, Jul 21, 2021, 2:37 AM UTC

tines wrote: post #263

mannerheim wrote: post #247

Earlier quoted context omitted.

It presupposes that position and momentum have definite states that are just uncertain to us, while in most interpretations of quantum mechanics (e.g. Copenhagen and many worlds) the particle exists as a wavefunction, lacking a specific position or momentum, but instead existing as a probability density function in this space. The uncertainty principle here then relates to how much this probability density function '…

So is it true that there are multiple properties of a particle---such as location, position, and maybe its spin---that are all described as wave functions, and therefore they can all be entangled? Can anything that is described by a wave function be entangled?

Since position and momentum (assuming that's what you meant, since you said location and position which are synonyms) have this sort of dual relationship, I don't think it makes sense to talk about entanglement with respect to them - they intrinsically have to be related to each other, and the position state (i.e., function) a particle is in fully determines its momentum state.

But it is possible to imagine usually unrelated properties of a particle being entangled, e.g. a two-peaked position function, spin up if it's over here and spin down if it's over there. So that's possible. Usually when discussing entanglement, though, we're talking about 'distinct'* particles. Electron A's spin entangled with electron B's spin. Not that it has to be spin, of course. But that's a common case because of how naturally this sort of entanglement occurs, for example, in atoms where electrons have to form spin pairs.

* This is complicated by QFT where particles are not exactly distinct, but exist as excitations in a particle field. E.g. there aren't two electrons but the electron field is excited by two quanta. At least, that's my understanding; I never went to grad school for physics, so I'm limited to undergraduate knowledge and some extracurricular reading.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#267

Post by seph-reed » Wed, Jul 21, 2021, 2:59 AM UTC

crystalmeph wrote: post #259

nsxwolf wrote: post #27

I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the explanations why it can't be used to do so. I don't understand how you can be sure about the state of the other particle, what if someone already measured it and then did something to it?

Total layman, I’m sure I’ll get corrected if I’m wrong here: Bell's Theorem is just a model showing how two sets of measurements of certain properties of entangled particles would differ in a “Quantum Physics” regime where there is spooky action at a distance changing the measured properties versus a “common sense” regime where both particles leave the entanglement site with those properties already set. [0] is a fai…

Why can't you control the outcome on your end? Or can we just not do it yet?

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#268

Post by IncRnd » Wed, Jul 21, 2021, 3:10 AM UTC

joe_the_user wrote: post #254

IncRnd wrote: post #244

Earlier quoted context omitted.

> The generally means that adding unneeded hypotheses should generally be avoided. This is the logical fallacy with what you wrote. You don't know what is needed or unneeded.

None of my arguments here have been classical syllogisms and so none of my arguments can be "logical fallacies". Your not responding to the meaning of my informal statements in a rather transparently bad faith manner.

Please stop trying to win a debate. You haven't actually addressed what I or anyone else wrote. If you want people to discuss with you, understand they expect you to do the same.

A logical fallacy is an error in reasoning that is based on poor or faulty logic.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#269

Post by chii » Wed, Jul 21, 2021, 3:14 AM UTC

kgwgk wrote: post #251

tylerhou wrote: post #171

Earlier quoted context omitted.

> By saying ahead of time, "one ball is red, the other is blue", you're describing a hidden-variables theory of entanglement. No, consider the case of neutral pion decay, which emits one spin up electron and one spin down electron. We can clearly say ahead of time one electron will be spin up, and the other will be spin down. But there is no hidden variable that determines which. If there were a hidden variable, then…

> No, consider the case of neutral pion decay, which emits one spin up electron and one spin down electron. No, it does emit two electrons with total spin zero which is not the same thing. > We can clearly say ahead of time one electron will be spin up, and the other will be spin down. Let’s imagine that one was really up and the other was down. But you decide to measure instead the spins along a perpendicular axis.…

> The red and blue balls analogy is very misleading and has nothing to do with entanglement.

this is exactly why classical analogies should not be used to describe quantum entanglement - it gives the layperson a wrong impression. Those analogies makes it easy for the layperson to imagine the hidden variable hypothesis, which is proven to be wrong.

Re: How Bell’s Theorem proved ‘spooky action at a distance’ is real

#270

Post by thisrod » Wed, Jul 21, 2021, 3:18 AM UTC

lisper wrote: post #256

thisrod wrote: post #236

Earlier quoted context omitted.

I agree with this. But it's worth pointing out that lots of good physicists don't. It's a statement of the Everett/relative state/many worlds interpretation, which is simply too weird for many people to accept. That's why there are about a dozen other interpretations of quantum measurement theory, which are all weird in other ways that I can't accept. The weird part is precisely this: "you can get back to a state whe…

> It's a statement of the Everett/relative state/many worlds interpretation No, it isn't. I've said nothing about many-worlds, only reversibility. And on that point everyone agrees. > Now it's dinner time, you don't believe that You really need to read the link above. It goes into all that in great detail. But the TL;DR here is that if it's dinner time, you haven't actually reversed the measurement, notwithstanding y…

> You really need to read the link above

I don't like claims to authority, but maybe you should read the papers I've published about Bell inequalities too? :-p

It sounds like you're saying that, in principle, physical processes are all reversible. (Although that is often thermodynamically impossible in practice.) You're also saying that it's impossible in principle for someone to learn the result of a measurement in the morning, then unlearn it when the measurement is reversed during the afternoon. I don't see how there could be a self-consistent interpretation of quantum measurement where both those things are true.