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

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

#51

Post by gaze »

I’ve been working on quantum computing and quantum communication for 15 years now and what I really want people to know is that entanglement is “beyond classical correlation.”

Correlation which is not beyond classical is shaking up a shoebox with a pair of gloves in it, and having two people take the gloves far away from each other to then observe what hand they got. They then understand the hand the other has. There’s one bit of information here corresponding to which hand went to person A.

Entanglement is just this but “more.” You can’t communicate with a pair of gloves. Person A does not know when person B has realized person A knows what glove they have. Just the same, it does not matter who matters which of the two particles in a Bell pair first! Many quantum information theorists don’t even believe the wave function is “real” but just a mathematical tool for making predictions about measurement outcomes.

You should come at entanglement from this angle, because the main difference between the bell test and the pair of gloves is the state of the particles is undetermined before measurement, and the chosen measurements will result in different correlations between the measurement outcomes.

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

#52

Post by slfnflctd »

bowsamic wrote: post #7
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…

> I accept that this coin-hypothesis has been disproved by people who actually knows what they're talking about. The classical idea is more like as follows: you have a guy Charles who makes two letters each with a card in that has written on it either number 0 or 1, and gives one letter to Alice and another to Bob. Now, "entanglement" here is that if Charles writes 0 in Alice's letter he writes the number 1 in Bob's,…

I have a couple follow up questions.

> a guy Charles who makes two letters

So Charles at least knows the information on the cards. Are you essentially saying that there's no way for Alice or Bob to distinguish that information from 'background noise'? If so, that is not a great analogy other than to illustrate a historical & incorrect perspective. Which may have been what you were doing and if so is fine.

> measurement "chooses" one of the states in the measurement basis

Okay, say you 'do some entanglement of multiple particles stored in two separate tubes' where you know there is some kind of pattern in the entanglement. Is the issue that there is no way to know how to find that pattern without possessing both 'tubes'?

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

#53

Post by bowsamic »

slfnflctd wrote: post #52
bowsamic wrote: post #7

Earlier quoted context omitted.

> I accept that this coin-hypothesis has been disproved by people who actually knows what they're talking about. The classical idea is more like as follows: you have a guy Charles who makes two letters each with a card in that has written on it either number 0 or 1, and gives one letter to Alice and another to Bob. Now, "entanglement" here is that if Charles writes 0 in Alice's letter he writes the number 1 in Bob's,…

I have a couple follow up questions. > a guy Charles who makes two letters So Charles at least knows the information on the cards. Are you essentially saying that there's no way for Alice or Bob to distinguish that information from 'background noise'? If so, that is not a great analogy other than to illustrate a historical & incorrect perspective. Which may have been what you were doing and if so is fine. > measureme…

> If so, that is not a great analogy other than to illustrate a historical & incorrect perspective.

Read the next part, I was discussing the idea thought of by Einstein that was dismissed by Bell's theorem.

> Okay, say you 'do some entanglement of multiple particles stored in two separate tubes' where you know there is some kind of pattern in the entanglement. Is the issue that there is no way to know how to find that pattern without possessing both 'tubes'?

I'm not sure what you mean by this, can you elaborate?

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

#54

Post by slfnflctd »

bowsamic wrote: post #53
slfnflctd wrote: post #52

Earlier quoted context omitted.

I have a couple follow up questions. > a guy Charles who makes two letters So Charles at least knows the information on the cards. Are you essentially saying that there's no way for Alice or Bob to distinguish that information from 'background noise'? If so, that is not a great analogy other than to illustrate a historical & incorrect perspective. Which may have been what you were doing and if so is fine. > measureme…

> If so, that is not a great analogy other than to illustrate a historical & incorrect perspective. Read the next part, I was discussing the idea thought of by Einstein that was dismissed by Bell's theorem. > Okay, say you 'do some entanglement of multiple particles stored in two separate tubes' where you know there is some kind of pattern in the entanglement. Is the issue that there is no way to know how to find tha…

Well, we know entanglement happens because of experiments. In those experiments, researchers had to somehow independently verify that the entangled states produced predictable results, thereby demonstrating the entanglement.

If you set up the first part of experiment, wherein you know certain particles are entangled, would it be possible for someone on one side of that entanglement to read a pattern from that side alone (if they 'knew where to look') which matched a pattern on the other side?

At this point, if my question even makes sense, we're only talking about a very impractical method of data storage... I'm just curious whether even that is possible.

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

#55

Post by Jensson »

slfnflctd wrote: post #47
mrkeen wrote: post #40

Earlier quoted context omitted.

I don't dispute this, but it's indistinguishable from the 'no spooky action' hypothesis. I can invent a story that the coin in my pocket has two sides, each of which is in a superposition of heads and tails. If you disprove that by repeatedly looking at both sides and noting that they always end up different, then I can invent the story that the observed side tells the unobserved side what to become. If you disprove…

I've never quite grasped this stuff to my satisfaction, and I strongly suspect part of why is because of some really bad popular examples (metaphors, allegories, thought experiments, whatever) which contain factually or logically incorrect language. This comment chain is a prime example of the ways quantum mechanics are (mis)understood and how many of us struggle to fully reason out the implications of each piece of…

Fact is that the position isn't "unknown", it is in both positions at once. The double-slit experiment is strong evidence for this, we can see that the particle went through both slits at once by measuring a lot of particles and seeing where they end up.

If you block one slit and then shoot particles, then block the other slit and shoot particles, you end up with a completely different pattern than if you have both open at once.

This isn't difficult to understand, the only difficult part is people not wanting to believe it is true and trying to come up with classical explanations for quantum mechanics, even though we have hard proof that classical physics can't explain this.

The difficult part is that even though the particle went through both slits, it only ever ends up in a single spot at the end, that is the "wave function collapse", it was a wave that went through both slits, and then it collapses to a single point when we measure it at the end. That is still not well understood by anyone today.

https://en.wikipedia.org/wiki/Double-slit_experiment

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

#56

Post by mrkeen »

yetihehe wrote: post #25
elcritch wrote: post #11

Earlier quoted context omitted.

> 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. The quantum erasure experiment is to my mind the best example of the spooky action at a distanc…

Calling it "action" may be a mistake. Action implies something changes (there is a state called "before" and state called "after"). But nothing changes "over there", we just now know what actually happened (there is no state called "before" or it's value is "unknown", only state called "after").

This is the simplest explanation, but it's also the null hypothesis.

I believe that "nothing changes over there" implies not only no spooky action, but also no entanglement.

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

#57

Post by yetihehe »

mrkeen wrote: post #56
yetihehe wrote: post #25

Earlier quoted context omitted.

Calling it "action" may be a mistake. Action implies something changes (there is a state called "before" and state called "after"). But nothing changes "over there", we just now know what actually happened (there is no state called "before" or it's value is "unknown", only state called "after").

This is the simplest explanation, but it's also the null hypothesis. I believe that "nothing changes over there" implies not only no spooky action, but also no entanglement.

This is about word used for description of that thing. Just using entanglement is ok, but using "action" suggests that something changes. Words have some meaning and incorrect usage causes misunderstandings.

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

#58

Post by rssoconnor »

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…

While you don't get action at a distance, you do get something like "correlation without causation". This ability does let you "cheat" at particular games of chance, which is spooky in its own right.

https://r6.ca/blog/20150816T185621Z.html

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

#59

Post by TheDudeMan »

snapcaster wrote: post #22
euroderf wrote: post #5

Earlier quoted context omitted.

That precisely is so much of the appeal of The Expanse ! For getting around our solar system, take plausible, foreseeable tech and add just a bit of secret sauce (the Epstein drive). So,.. the evolutionary path of society as depicted in the series is plausible.

I wanted to like it so badly but the show just didn't hook me. Need to give it another chance sometime

You gotta get through the political stuff in the first episode or so. Just push through. No need to follow it in detail.

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

#60

Post by stephc_int13 »

Filligree wrote: post #43
stephc_int13 wrote: post #33

Most people don't understand enough about the fundamental properties of modern Physics. What we usually call the speed of light is in fact the maximum speed of information propagation. It is not about light, and you should think about quantum entanglement as a hidden variable, there is no magic there.

I was under the impression that local hidden variable theories had been disproven? It could be nonlocal of course, but that would imply information moving faster than light again. Unless you count the entirety of MWI as "hidden variables", I guess?

Yeah, I am aware of Bell's theorem.

What I am saying is that the hidden variable hypothesis is nonetheless a good mental model as a first approximation to reason about QE, say as a non-professional physicist.

People get confused, especially sci-fi authors, adding to the general confusion about anything touching Quantum Physics being almost magical/limitless or unexplainable. This is a lot more pedestrian in practice.

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