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

quantamagazine.org

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

#251

Post by kgwgk »

tylerhou wrote: post #171
sawalk4 wrote: post #140

Earlier quoted context omitted.

Isn't this distinction exactly what the article is about? By saying ahead of time, "one ball is red, the other is blue", you're describing a hidden-variables theory of entanglement. It may be unknowable (before measurement) which color the ball in your left hand is, but it has a color. But Bell's theorem provides a very measureable counterexample to this type of explanation of entanglement. Sure, in the article they…

> 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. You would expect to find no correlation between them.

However, what you actually see is that if you measure both spins along any (common) axis they will point in opposite directions.

It doesn’t make any sense to say that before any measurement one was up and the other down. The red and blue balls analogy is very misleading and has nothing to do with entanglement.

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

#252

Post by bawana »

tylerhou wrote: post #20
lisper wrote: post #13

Earlier quoted context omitted.

That's not a bad analogy, but you have to be very careful here because no classical analogy can be a perfect fit for entanglement. The wave function is deeply and fundamentally different than our classical reality, and there is no way to reproduce its behavior classically. Among the fundamental differences is the fact that classical information can be copied but quantum states cannot be cloned. This is IMHO the singl…

A classical analogy for entanglement: suppose I have two balls in a bag. They are identical in every way, except one is red and the other is blue. I randomly grab one in each hand and show my hands closed. Now the states of the ball are entangled: as soon as you see the color of one ball, that "determines" the color of the other. (Not claiming that this is a perfect analogy, but I don't see where it diverges from how…

agree. The issue that is confusing in explanations of entanglement is the use of a statistical measurement to a model of an individual event. Entanglement is a state made after the observations of many events. Before there is a red ball and a blue ball, there is a precursor, purple ball. That purple ball splits and two balls wrapped in paper are created. They can travel great distances over long times to separate locations, A and B. When some humans start to unwrap them, they are amazed at the correlation-one blue is always matched by one red. So until the balls are unwrapped, how are the humans to describe them? They use the word superposition-unfortunately, many interpret that as a SIMULTANEOUS existence of both states in each ball. Whereas in reality there are many events - the humans at location A see both colors as do the humans at location B. That does not mean that individual particles assume both states. A similar confounding statistic was taught in grade school. The average family has 2.5 children. Yet half a live child was never born.

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

#253

Post by sooheon »

state_less wrote: post #229
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.

From what I can gather, in a single world view, it seems that thinking of reality in an information oriented way results in more accuracy. So you might have interactions that aren't dependent on distance, like with entangled particles. Taking on this view is a bit of a mind bender from a normal spatial view of the world though.

Information sharing without physical interaction is "spooky action at a distance"

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

#254

Post by joe_the_user »

IncRnd wrote: post #244
joe_the_user wrote: post #201

Earlier quoted context omitted.

You are fundamentally misunderstanding Occam's Razor. It is not a law - Occam's Razor is a preference for how to view the world, not a law that was violated. Yes, Occam's Razor isn't a law but a method of understanding reality. My point is that if you throw out Occam's Razor in total, not in one or another situations, you're left with nothing to understand the world with. The "God wants it that ways" and "because it'…

> 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.

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

#255

Post by sooheon »

bawana wrote: post #252
tylerhou wrote: post #20

Earlier quoted context omitted.

A classical analogy for entanglement: suppose I have two balls in a bag. They are identical in every way, except one is red and the other is blue. I randomly grab one in each hand and show my hands closed. Now the states of the ball are entangled: as soon as you see the color of one ball, that "determines" the color of the other. (Not claiming that this is a perfect analogy, but I don't see where it diverges from how…

agree. The issue that is confusing in explanations of entanglement is the use of a statistical measurement to a model of an individual event. Entanglement is a state made after the observations of many events. Before there is a red ball and a blue ball, there is a precursor, purple ball. That purple ball splits and two balls wrapped in paper are created. They can travel great distances over long times to separate loc…

> Whereas in reality there are many events - the humans at location A see both colors as do the humans at location B

This is one of many interpretations, I don't think it's fair to label conjecture as "reality" yet.

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

#256

Post by lisper »

thisrod wrote: post #236
lisper wrote: post #212

Earlier quoted context omitted.

> I don't get why reflection isn't an observation/interaction there either. I know this comment is going to get lost in the noise, but that is a really excellent point, one of the best that has been raised here so far. This is a point that is often glossed over, but it is actually really important, and quite challenging to explain without getting deep into the weeds. The answer is that passing through a half-silvered…

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 your current mental state with respect to the photon.

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

#257

Post by lisper »

tylerhou wrote: post #128
tsimionescu wrote: post #86

Earlier quoted context omitted.

I'm not sure what you mean by classical wave functions - I've only seen the term 'wave function' used for quantum mechanics. Are you referring to classical wave equations? I'm not sure how the concept of entanglement is supposed to apply to classical waves though.

I'm saying that you can represent probability distributions of classical objects as a "wave function."

No, you can't, at least not one that behaves like the quantum wave function does. Classical probabilities are real numbers between zero and one. The wave function takes on complex values, which allows you to add two wave-function values with non-zero magnitude and get zero, i.e. produce destructive interference. Classical probabilities can't do that.

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

#258

Post by mcbits »

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.

Put two iron pellets on a very thin, flexible fabric. Underneath the fabric, put a magnet so each pellet is stuck to one pole of the magnet through the fabric. Tug the magnet "down" in the third dimension, which will fold the fabric and move the pellets so that from their perspective (stuck to the 2D surface of the fabric) they're moving far away from each other. They also maintain a constant and very short distance in the third dimension, but they can't take advantage of it because I forgot to mention the fabric is impervious to them.

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

#259

Post by crystalmeph »

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 fairly easy to understand graph of the correlation of the measurements at the two sites in both regimes, even if the math that generates those charts is beyond me.

Given that, just knowing what measurements you got only gives you an idea of what the other guy across the universe would see were he to measure your particle’s entangled partner - he has no more control over what those measurements actually are than you do, you just know that there’s a certain correlation between what you saw and what he saw. That’s why you can’t use Bell’s Theorem to communicate, neither one of you is actually controlling the measured property, there’s just a certain correlation between the measurements both of you got.

It seems to this layman that in order to communicate FTL using QM, you’d need a way to determine that the property being measured has already been “collapsed” (if that’s the right word) by spooky action at a distance, e.g if the other guy had already measured it. Bell’s Theorem gives us no way to determine if the other guy has actually made the measurements, it assumes that both you and he have both made those measurements.

[0]: https://en.wikipedia.org/wiki/Bell's_theorem#/media/File:Bel...

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

#260

Post by unholiness »

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.

Bell's theorem is strong: No local hidden variables can explain quantum entanglement.

Your proposed fifth dimension is exactly such a variable – there is no set of values it can take on that explains the quantum phenomena we can easily and reliably observe.

The fact that you think of it as a distance itself isn't important to Bell's theorem. Such a theory is cannot even obey relativity in our existing 3 dimensions (obeying the speed of light in your extra dimension would be even stricter!)

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