Why quantum entanglement doesn't allow faster-than-light communication (2016) • Hacker News Board

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#141

Post by taeric » Sat, Mar 30, 2024, 12:34 AM UTC

magicalhippo wrote: post #137

taeric wrote: post #134

Earlier quoted context omitted.

Apologies, I screwed up the names there. I was thinking down thread where I had A sending. So, A sends, B has a detector, C has a detector. Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. (And vice versa.) Now, I am assuming I saw an incomplete framing where this is only true if they communicate back to A? Stated differently, the framing I saw was that the…

> Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. That was the entire point of my initial post: there's no discernible difference in the actual individual measurement results regardless of detector settings. The quantum correlations only show up if someone compares both measurements pair by pair . And to do so, regular communication must happen. Many source…

Cool, thanks for sticking with me in this! I definitely took it to be that the individual detectors were replicable at the individual level. Guessing that is not claimed and was an assumption in my reading. Certainly fits intuition better.

I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected?

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#142

Post by magicalhippo » Sat, Mar 30, 2024, 2:33 AM UTC

taeric wrote: post #141

magicalhippo wrote: post #137

Earlier quoted context omitted.

> Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. That was the entire point of my initial post: there's no discernible difference in the actual individual measurement results regardless of detector settings. The quantum correlations only show up if someone compares both measurements pair by pair . And to do so, regular communication must happen. Many source…

Cool, thanks for sticking with me in this! I definitely took it to be that the individual detectors were replicable at the individual level. Guessing that is not claimed and was an assumption in my reading. Certainly fits intuition better. I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected?

> I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected?

Lets imagine electrons are objects in a program, then the "electron class" has a private field containing a seed value to a pseudo-random number generator (ie deterministic), and the two electrons are initialized with the same seed value.

Further imagine that performing a measurement of an electron amounts to taking the seed, generating a random number between 0 and 360 degrees (sample value), and then comparing that random number to the measurement angle. If the sample value and the measurement angle is closer than +/- 90 degrees we say the measurement result is up, otherwise down.

Alright, so, if we imagine that when Charlie prepared the electrons, he creates two "electron objects", and passes one to Alice and the other to Bob.

If Charlie prepares entangled electrons, he'll ensure both instances have the same seed value. If he wants to create regular non-entangled electrons, he'll make each have a random seed value.

For non-entangled electrons, Alice and Bob will not see any correlation if they later compare notes.

For entangled electrons, if Alice and Bob uses the same angle they must get the same result per definition[1]. And indeed one can find the correlation as a function of the difference in angle, and it's a linear function from perfect correlation if the angles are the same (zero difference) to zero correlation (perfect anti-correlation) when the angles are 180 degrees apart.

However on real, entangled electrons in the lab things are different. There you'll find that the correlation is higher than the linear function when the difference is smaller than 90 degrees, and less than the linear function when the difference is greater than 90 degrees[2].

Thus if we measure the entanglement at not just 0 and 90 degrees difference but also 45 degrees difference, we'll find that our lab measurements do not agree with our simulated measurements.

Hence we conclude that entangled electrons do not behave like small objects that were created with the same "hidden value", ie the seed value in my example.

That's the essence of Bell's theorem and the tests of it (at least according to my memory).

[1]: Note that measuring real entangled electrons Alice and Bob will get exactly the opposite result, they're perfectly anti-correlated, but this matters not for this explanation and not worrying about it will make the explanation easier.

[2]: IIRC it goes like cos(a/2)^2 or something along those lines, ie https://www.wolframalpha.com/input?i=plot+%7B1-abs%28x%2Fpi%...

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#143

Post by honeyed_coffee » Sat, Mar 30, 2024, 2:50 AM UTC

taeric wrote: post #136

honeyed_coffee wrote: post #133

Earlier quoted context omitted.

>The question is, how rapidly does the "spooky" distance change happen? I get that it would not be communication between A and B or C. I similarly get that you could not coordinate between B and C. But, from all of the framings I've seen so far, I don't understand why the change between B and C is not faster than speed of light. It's because measurements at B do not convey any information to C while the measurements…

I'm reading that as the distributions that are seen are not stable. It may be that you got 30% this time, but 40% next time. On the exact same setup. (Obviously making up numbers.) Yes, you know that the other side saw something, but that is obviously useless. Framing I saw was more of a truth table like where B/C have known states they can be in that each lead to a known distribution of outcomes. It was not clear th…

>I'm reading that as the distributions that are seen are not stable. It may be that you got 30% this time, but 40% next time. On the exact same setup. (Obviously making up numbers.) Yes, you know that the other side saw something, but that is obviously useless.

I'm not sure I'm understanding your percentages statement completely, but when the parties have entangled states, these inconsistencies will match on each side under the assumption that their measurement choice is the same.

> Framing I saw was more of a truth table like where B/C have known states they can be in that each lead to a known distribution of outcomes. It was not clear that the known distribution was only an observed distribution.

If the states are locally known to the parties, they'd still have to perform a statistical run of experiments as before. The choice of measurements would be key in distinguishing a classical from a quantum correlation

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#144

Post by ynniv » Sat, Mar 30, 2024, 3:03 AM UTC

amluto wrote: post #140

ynniv wrote: post #124

Earlier quoted context omitted.

> Print tickets such that, if one wins, the other is more likely to win, which you can do by having the number of winning spots per ticket be random and correlated. In order to refute this you need to get into the CHSH experiment's design, which doesn't use the raw detection rate but compares detections that should not have similar results. Imagine that both tickets have a prize on the right side, and both participan…

> In order to refute this Refute what? I'm pointing out that classical (non-quantum) scratch-off lottery tickets can have the property you described. This is only thematically related to the CHSH game, and I'm rather confused as to what you're trying to say. The Wikipedia articles about the CHSH inequality are IMO quite bad. The conclusion of CHSH has approximately nothing to do with measurement angles. Here's what C…

> I'm pointing out that classical (non-quantum) scratch-off lottery tickets can have the property you described.

No, if A always scratches the left, and B alternates between left and right, your classical trick of having some tickets with two winners and some with two losers won’t produce the correct win distributions for B’s choices.

The CHSH game is an attempt at explaining the experiment. The core of CHSH is a formula describing an expected distribution: `S = E(a, b) - E(a, b') + E(a', b) + E (a', b')`, where classical explanations can achieve an S with magnitude approaching 2. Experiments arriving at an S > 2 are said to support the quantum theory.

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#145

Post by MagicMoonlight » Sat, Mar 30, 2024, 3:50 AM UTC

Is entanglement real or is it just that two things happen to be aligned the same way and measuring them reveals how they always were?

Like if I gave two people red balls without telling them what the colour is. One takes it out and sees it is red, the other now instantly has a red ball. But they were always red. So there is no actual interaction between the balls, they were just set up in a pre-defined matching state.

To our observations the balls are in a quantum state (the balls are wrapped in paper you can’t see through) but they always had a specific state. We just weren’t capable of determining that state because the interior is invisible to us. The interaction would always have produced that outcome, so it appears like they are communicating when in reality it would happen regardless.

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#146

Post by taeric » Sat, Mar 30, 2024, 4:56 AM UTC

magicalhippo wrote: post #142

taeric wrote: post #141

Earlier quoted context omitted.

Cool, thanks for sticking with me in this! I definitely took it to be that the individual detectors were replicable at the individual level. Guessing that is not claimed and was an assumption in my reading. Certainly fits intuition better. I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected?

> I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected? Lets imagine electrons are objects in a program, then the "electron class" has a private field containing a seed value to a pseudo-random number generator (ie deterministic), and the two electrons are initialized with the same seed value. Further imagine that performing a measurement of an electron amounts to tak…

I thought the hidden variable idea was proven not to hold, though? Like, I thought that was the point? That the behavior observed can only be explained using the state of the remote measure as part of the explanation?

I have not looked back at the book I read. I definitely remember it had examples that were not paired off. I'm assumingy memory is simply off.

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#147

Post by RamiAwar » Sat, Mar 30, 2024, 7:45 AM UTC

But what if we figure out a way to make a particle generator that is entangled to another particle generator?

Then we could generate particles from each that can be potentially entangled to the original system as well. Can it work that way?

If we keep looking at one bit there's probably not much we can do. But when we start looking at entanglement of systems then maybe some we learn about some phenomenon that makes this possible.

Excuse my ignorance, I'm not even a physics major so I probably spewed meaninglessness but I'd love to learn more from replies!

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#148

Post by Jensson » Sat, Mar 30, 2024, 8:04 AM UTC

MagicMoonlight wrote: post #145

Is entanglement real or is it just that two things happen to be aligned the same way and measuring them reveals how they always were? Like if I gave two people red balls without telling them what the colour is. One takes it out and sees it is red, the other now instantly has a red ball. But they were always red. So there is no actual interaction between the balls, they were just set up in a pre-defined matching state…

What you are talking about is hidden variable theory, Bells theorem shows that it is likely not the case, so quantum entanglement is likely real. Bells theorem has been tested experimentally so we know the world works as it says, the question is just how we interpret it.

https://en.wikipedia.org/wiki/Bell%27s_theorem

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#149

Post by Sai_ » Sat, Mar 30, 2024, 9:42 AM UTC

mrkeen wrote: post #3

The gap I still have in my understanding: One hypothesis is that there is no spooky action. One particle was 'always' going to resolve one way, likewise with the other. Like inspecting 'heads' on one side of a coin 'forces' 'tails' onto the other side. I accept that this coin-hypothesis has been disproved by people who actually know what they're talking about. But to me, this implies that you should build your spooky…

> You got yourself a one-way single-use FTL byte.

But you don’t.

If I have the entangled version of the MEASURED byte and measure, only I know I’ve measured it. The corresponding MEASURED particles on your side still appear fuzzy to you because you haven’t measured them. If you measure them, they will reveal their state which correlates to mine BUT that doesn’t tell you if I have already measured my side of byte or not.

In effect, the heads/tail coin on your side is still spinning even after I have grabbed my side of the tail/head coin.

You may get HEAD as your result but that only tells you that I will get TAIL when I measure on my side, not that I have measured it already.

If you and I agree to measure at the same time, then I will know your state by determining mine but this is the same as knowing your state because I know how a two sided coin works, not FTL.

Re: Why quantum entanglement doesn't allow faster-than-light communication (2016)

#150

Post by a_gnostic » Sat, Mar 30, 2024, 10:24 AM UTC

Jensson wrote: post #118

d_tr wrote: post #113

Earlier quoted context omitted.

From the perspective of the MWI there is no spooky action. If you measure one of the particles and look at the result, you then know what term of the quantum state you ended up in, so you know the probability distribution of the other particle's results. Nature does not need to work according to what our brains consider reasonable, but the MWI getting rid of several very weird notions makes it very appealing, I won't…

> From the perspective of the MWI there is no spooky action. There is a spooky world split that separates the entire universe instead of just having a spooky thing having to the particle. Some prefers having a small spooky thing over a large spooky thing.

The universe is not split, though. You just perceive it that way. Think of it as a wave function, where some of the time, some waves cancel each other out, and are not observed, and some of the time they are amplified and easily observable.