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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

#31

Post by martincmartin »

martincmartin wrote: post #25
willyg123 wrote: post #4

What is the prevailing theory to explain quantum entanglement? Must there be another dimension we cannot access or measure that is not subject to the laws of relativity? (I understand the laws of relativity break down at the quantum level but please ELI5)

Quantum entanglement falls out of the quantum mechanics, so in some sense, the prevailing theory to explain quantum entanglement is quantum mechanics. Of course, it's unintuitive and unsettling, so you could generate other theories about other dimensions if you like. But as far as predicting the results of any experiments we can do, QM is all you need. Also, there are two very different theories of relativity, the sp…

It's easy to explore QM and SR, because it's easy to accelerate fundamental particles to near the speed of light. Here's a video from 1962 where electrons were accelerated, they measure the time between passing two points (to get speed), and heat energy deposited on a target (to get kinetic energy) to show how SR works. Nothing QM specific, but shows how easy it is to get quantum particles moving that fast, so you can do experiments on them: https://www.youtube.com/watch?v=B0BOpiMQXQA

Combining gravity, which needs great mass, with QM, which needs small space scales, is "hard" to do in a lab.

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

#32

Post by misiti3780 »

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?

This book elucidates the concepts well:

https://en.wikipedia.org/wiki/Something_Deeply_Hidden

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

#33

Post by tylerhou »

kolinko wrote: post #24
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…

The analogy you mentioned is exactly the wrong one - it suggest that it’s just a matter of a hidden variable. A proper (but less elegant) would be: you have two balls with the same color or a pattern. You take one out. If you check the color first, you will find the other’s color the same, but the pattern sometimes different. If you check the pattern first, you will find the pattern the same, but the color sometimes…

> it suggest that it’s just a matter of a hidden variable.

I disagree. Suppose that I create a machine that chooses which ball to place in each box. This machine makes the choice based on some measurement of a quantum particle (electron spin). Then the colors of the ball are entangled with the state of the quantum particle, which cannot be described by some local hidden variable.

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

#34

Post by drcode »

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?

Imagine you have a pouch with a red and a blue marble in it, then take out a marble without looking at it and hand the pouch to a friend. Later, if you look at your marble, you instantly have information about the other marble at a speed greater than the speed of light... but you couldn't use that fact to send a message.

The only difference in quantum physics is that there are actually two parallel universes: One in which you took out the red marble & one in which you took the blue one. You don't know what universe you're in until you look at the marble, but still it doesn't help you to transmit a message to your friend.

(This is assuming the "multiple universes" interpretation- In the other interpretations there is "spooky action at a distance", but this action happens in EXACTLY THE RIGHT WAY to prevent you from transmitting a message to your friend)

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

#35

Post by cma »

SV_BubbleTime wrote: post #14
someguyorother wrote: post #8

Earlier quoted context omitted.

It's not really immutable as you can change the parameters of an entangled pair. You just can't communicate any information by doing so, because you need a classical signal to make sure you don't read one of the particles the wrong way.

I could be WAY off, but if locality isn’t entirely true, and the “read success” is 33-67%, doesn’t that still leave quite a bit of wiggle room for communicating information in some fault tolerant method?

Correlations only but no useable communication. You can both make a decision on the same random info that isn't determined until later when you are apart, but can't know anything other than that if they followed the plan they made their choice based on the same later-determined random info, correlated with your random info.

If they didn't follow the plan and measured orhogonal/same (can't remember which) spins, then your results are uncorrelated but you can't know until you meet back up (maybe barring superdeterminism that is also accessible to the individual).

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

#36

Post by snissn »

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?

Two balls are a box. Neither are spinning. The box gets “shaken up” and the balls hit each other. We know that one ball is spinning clockwise and the other is counter clockwise because angular momentum spin is conserved. The balls launch far away from each other. We know the spin is entangled in that one is clock wise the other is counter clockwise but we don’t know which is which until we measure. How do we use that to communicate?

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

#37

Post by stevenjgarner »

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?

Or even better than instantaneously, let's get messages sent to us from the future using a Ronald Lawrence Mallett time machine based on a ring laser's properties, such that at sufficient energies, the circulating laser might produce not just frame-dragging but also closed timelike curves (CTC), allowing time travel into the past. I cannot believe that Ronald Mallett's biggest challenge is getting funding for a feasibility test. Isn't it the greatest venture capital opportunity of all time?

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

#38

Post by gpsx »

willyg123 wrote: post #4

What is the prevailing theory to explain quantum entanglement? Must there be another dimension we cannot access or measure that is not subject to the laws of relativity? (I understand the laws of relativity break down at the quantum level but please ELI5)

I think people get confused when they think that each object has a wave function. This is not correct. The universe has one wave function. The wave function consists of a bunch of possible states along with the coefficient for each state. You can think of each state as being a distinct snapshot of what the universe might look like - including for example the position and spin of each particle. In the example of two electrons shown here, the wave function has non-zero coefficients only for states where the two electron spins are in opposite directions.

When we make a measurement, the state of the universe appears to collapse, meaning any state that is not consistent with that measurement disappears. This means the other electron is left in the opposite spin state. (Important aside here, some people believe the wave function collapses, "Copenhagen interpretation" and some people believe the wave function doesn't change but the the brain of the observer correlates/entangles with the electron, "Many Worlds Interpretation". Either way there is an operational collapse of the wave function.)

A special case for a wave function is when the coefficients are arranged so that state of one particle, say particle 1 spin, is symmetric no matter what the state of another particle, particle 2, is. This special case is when particles are NOT entangled.

(Edit: added paragraph on measurement)

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

#39

Post by tsimionescu »

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…

The problem with your classical analogy for entanglement is that it doesn't match the data. Or rather, it only matches the data for quantum properties that are similarly blue or red.

The non-classical properties of entanglement start appearing once you start measuring combinations of the redness and blueness of those balls.

Let's say that instead of looking at the balls, you pass them through some machine that will let a red ball pass through with some probability P that you control; if the ball is blue, the machine will let it pass with probability 1-P. Let's say further that you have three such machines. You set the first machine to P=1. You pass each ball falling from this machine through a second machine, which has P = 0. You will never see a ball pass through to the end - if it were red, it would pass the first machine, but not the second; if it were blue, it would not pass the first machine at all.

But, let's say you now put a third machine between the other two, and you set P = 0.5. With classical balls, nothing changes - a blue ball doesn't make it past the first machine, while a red ball goes through the first, may or may not pass the second, and never makes it through the third regardless.

However, a quantum ball actually has a chance to pass through the 3 machines if you set it up this way. In fact, that chance is pretty large - more than half of the balls will start passing once you add the middle filter machine.

Still, this is easy to explain if we assume that the middle machine actually paints the ball instead of just detecting its color. This is where the entanglement experiment comes in: if you pass the pair of balls through the three machines, with ball 1 passing through machines P=1 and P=0.5, and ball 2 passing through P=1, you will find that sometimes both balls make it through, even though both balls can't be red at the same time, and they can't communicate about passing through the P=0.5 machine (you can repeat the experiment with the balls being taken arbitrarily far away before passing through the filters).

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

#40

Post by lisper »

tylerhou wrote: post #33
kolinko wrote: post #24

Earlier quoted context omitted.

The analogy you mentioned is exactly the wrong one - it suggest that it’s just a matter of a hidden variable. A proper (but less elegant) would be: you have two balls with the same color or a pattern. You take one out. If you check the color first, you will find the other’s color the same, but the pattern sometimes different. If you check the pattern first, you will find the pattern the same, but the color sometimes…

> it suggest that it’s just a matter of a hidden variable. I disagree. Suppose that I create a machine that chooses which ball to place in each box. This machine makes the choice based on some measurement of a quantum particle (electron spin). Then the colors of the ball are entangled with the state of the quantum particle, which cannot be described by some local hidden variable.

Only if you can completely isolate the balls so their states don't decohere. That is not practically possible to achieve, particularly since in your scenario you reach into the bag and touch the balls. As soon as you interact with the balls in any way, you become entangled with them and the behavior of the system becomes classical.
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