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Old 09-19-2019, 12:08 PM
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Quantum entanglement and FTL information transfer.


While I understand that Alice can't send Bob a specific digital message because forcing a particle's spin state would distentangle the particles, what if the outcome of a coin flip was sufficient information? Let's say General Bob and General Alice were going to cooperatively invade Planet X and they were neutral as to who invaded the South pole of the planet and who invaded the North Pole but wanted to keep the information from Planet X so it wouldn't be able to create a counter to each General's strengths (yes I've made this unnecessarily complicated but now I'm committed). They are told to check the spin state of their entangled particles at a certain time and whoever had a down-spin invaded the South and vice versa. Since I know information, at least as defined by physicists in this context, can't travel faster than light, either I'm misunderstanding how entanglement works or physicists are using a qualified definition of information. Which is it? Thanks in advance.
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Old 09-19-2019, 12:13 PM
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I've come up with the same scenario when thinking about useful applications of this. I don't remember what the downside was myself.
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Old 09-19-2019, 12:21 PM
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I've come up with the same scenario when thinking about useful applications of this. I don't remember what the downside was myself.
Looks like Planet X is fucked. 😉
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Old 09-19-2019, 12:44 PM
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While I understand that Alice can't send Bob a specific digital message because forcing a particle's spin state would distentangle the particles, what if the outcome of a coin flip was sufficient information? Let's say General Bob and General Alice were going to cooperatively invade Planet X and they were neutral as to who invaded the South pole of the planet and who invaded the North Pole but wanted to keep the information from Planet X so it wouldn't be able to create a counter to each General's strengths (yes I've made this unnecessarily complicated but now I'm committed). They are told to check the spin state of their entangled particles at a certain time and whoever had a down-spin invaded the South and vice versa. Since I know information, at least as defined by physicists in this context, can't travel faster than light, either I'm misunderstanding how entanglement works or physicists are using a qualified definition of information. Which is it? Thanks in advance.
What information is being passed faster than the speed of light in this scenario?
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Old 09-19-2019, 12:48 PM
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What's the difference between this scenario, and one where Alice and Bob take the Ace of Spades and the Queen of Hearts, put a card into two sealed envelopes, and on D-Day they open the sealed envelopes. If Alice sees the Ace of Spades, she knows instantly that Bob has the Queen of Hearts, if she sees the Queen of Hearts, she knows instantly that Bob has the Ace of Spades.

No superluminal transfer of information has occurred in this scenario of sealed envelopes, right?

The only difference here is that the entangled particles haven't decided which state they are in until Alice looks, while the cards have. But it's the exact same amount of information.
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Old 09-19-2019, 01:20 PM
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What's the difference between this scenario, and one where Alice and Bob take the Ace of Spades and the Queen of Hearts, put a card into two sealed envelopes, and on D-Day they open the sealed envelopes. If Alice sees the Ace of Spades, she knows instantly that Bob has the Queen of Hearts, if she sees the Queen of Hearts, she knows instantly that Bob has the Ace of Spades.

No superluminal transfer of information has occurred in this scenario of sealed envelopes, right?

The only difference here is that the entangled particles haven't decided which state they are in until Alice looks, while the cards have. But it's the exact same amount of information.
Well like you said, in the case of the entangled particles, the information hasn't been "created" yet so there is no information to transmit. In the case of the playing cards, the information as to who will be invading where already existed and traveled, in some form, subluminally. I think your point is touching on where I'm wrong, I just don't quite see the whole picture.
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Old 09-19-2019, 02:43 PM
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While I understand that Alice can't send Bob a specific digital message because forcing a particle's spin state would distentangle the particles, what if the outcome of a coin flip was sufficient information?
Indeed, you can effect, for example, teleportation of a quantum state after transmitting classical bits of information, but not without doing so.
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Let's say General Bob and General Alice were going to cooperatively invade Planet X and they were neutral as to who invaded the South pole of the planet and who invaded the North Pole but wanted to keep the information from Planet X so it wouldn't be able to create a counter to each General's strengths (yes I've made this unnecessarily complicated but now I'm committed). They are told to check the spin state of their entangled particles at a certain time and whoever had a down-spin invaded the South and vice versa. Since I know information, at least as defined by physicists in this context, can't travel faster than light, either I'm misunderstanding how entanglement works or physicists are using a qualified definition of information. Which is it? Thanks in advance.
Measurements on entangled particles can be correlated; that's kind of the point. No information is transmitted, though, and the result of each measurement (disregarding the other one) is random, as expected.
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Old 09-19-2019, 02:51 PM
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Yes, the generals can coordinate their attacks in this way, and no, it doesn't violate any rules, and no, it doesn't even need quantum mechanics to get this effect.
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Old 09-19-2019, 03:51 PM
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Yes, the generals can coordinate their attacks in this way, and no, it doesn't violate any rules, and no, it doesn't even need quantum mechanics to get this effect.
But, as noted, it's not transferring any information.

Maybe the missing point for some people is that a pair of entangled particles can only be created when the particles are in the same spot. Then, couriers would need to put the particles in separate boxes and carry the respective boxes to Alice and Bob. If they've got couriers who can do that, then the couriers could have just measured the spin before they left (or flipped a coin or whatever other random event they want) and used the result to put the Ace and Queen in one or the other box and brought that to the respective Generals.
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Old 09-19-2019, 04:20 PM
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But, as noted, it's not transferring any information.

Maybe the missing point for some people is that a pair of entangled particles can only be created when the particles are in the same spot. Then, couriers would need to put the particles in separate boxes and carry the respective boxes to Alice and Bob. If they've got couriers who can do that, then the couriers could have just measured the spin before they left (or flipped a coin or whatever other random event they want) and used the result to put the Ace and Queen in one or the other box and brought that to the respective Generals.
Well, that's the explanation I finally understand. And it's so obvious and easy, when you think about it.
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Old 09-19-2019, 04:49 PM
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Indeed, you can effect, for example, teleportation of a quantum state after transmitting classical bits of information, but not without doing so.


Measurements on entangled particles can be correlated; that's kind of the point. No information is transmitted, though, and the result of each measurement (disregarding the other one) is random, as expected.
As I guessed, my problem is either with my understanding of the definition of information or transmission as it's used in this context. From a colloquial sense, it seems like information is being transmitted FTL because Bob is doing something that answers the question "Where should I invade?" for Alice nearly instantaneously. Why isn't the result of a coin flip information?
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Old 09-19-2019, 05:21 PM
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But, as noted, it's not transferring any information.

Maybe the missing point for some people is that a pair of entangled particles can only be created when the particles are in the same spot. Then, couriers would need to put the particles in separate boxes and carry the respective boxes to Alice and Bob. If they've got couriers who can do that, then the couriers could have just measured the spin before they left (or flipped a coin or whatever other random event they want) and used the result to put the Ace and Queen in one or the other box and brought that to the respective Generals.
I do see your point. But there is a difference though in the sense that the couriers' information would be different than if Bob had measured it at Place Y, Time X. It would be a different coin flip. While the particles are entangled, there is no coin flip result. The information about who is invading whom simply doesn't exist. I see your point but do you see mine? I know I'm wrong, I guess I'm looking for how information is specifically defined in this context.
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Old 09-19-2019, 08:37 PM
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I do see your point. But there is a difference though in the sense that the couriers' information would be different than if Bob had measured it at Place Y, Time X. It would be a different coin flip. While the particles are entangled, there is no coin flip result. The information about who is invading whom simply doesn't exist. I see your point but do you see mine? I know I'm wrong, I guess I'm looking for how information is specifically defined in this context.
You're not wrong about the result that entangled particles can generate, and the fact that it could it could be strategically useful. You could randomly pick which general attacks which target just before the attack begins, so that the defenders cannot possibly plan ahead; yet ensure that one general attacks south and one attacks north. It's notable, however, that you cannot use this technique to set up a scenario where both generals attack the same target, even if the target is chosen randomly.

And you're also not wrong that the two particles must be able to communicate certain information between one another instantaneously at arbitrary distance. That's the "spooky action at a distance" that is so difficult to make intuitive sense of in QM. And observers can passively read the correlated states of the two particles instantaneously.

What cannot happen is for one observer to actively pass information to another distant observer; observers cannot "choose" what information is exchanged instantaneously between the particles. There is no way for one general to tell the other general "attack south". What you're trying to pin down is the No-Communication Theorem.

https://en.wikipedia.org/wiki/No-communication_theorem

Last edited by Riemann; 09-19-2019 at 08:41 PM.
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Old 09-19-2019, 08:47 PM
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While I understand that Alice can't send Bob a specific digital message because forcing a particle's spin state would distentangle the particles, what if the outcome of a coin flip was sufficient information?

Maybe I'm missing something. If you could send the binary outcome of a coin flip, why couldn't you send multiple binary outcomes to form a digital message?
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Old 09-19-2019, 08:58 PM
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Maybe itís more like a coin toss where a third party (who is deaf, dumb, and blind) does the toss and has no way of communicating with A and B. So A and B could see the coin toss and see the result at any given moment, but with no way of inputting a pattern themselves. Honestly, it seems like no better than having synchronized watches and both being able to read the time simultaneously, even a light-year apart. Yes, you know what the other guy or gal's watch says because theyíre synchronized, but you canít then move the minute hand back on yours and expect the other's to do the same.
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Old 09-19-2019, 09:04 PM
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Maybe I'm missing something. If you could send the binary outcome of a coin flip, why couldn't you send multiple binary outcomes to form a digital message?
How would you do that, when you have no control over the random outcomes of the coin flips?
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Old 09-19-2019, 09:09 PM
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...It's notable, however, that you cannot use this technique to set up a scenario where both generals attack the same target, even if the target is chosen randomly...
Oops, on rereading this sentence is completely wrong, obviously you could get them to attack the same target if it's chosen randomly and you don't care which.
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Old 09-19-2019, 09:14 PM
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Yes, the generals can coordinate their attacks in this way, and no, it doesn't violate any rules, and no, it doesn't even need quantum mechanics to get this effect.
How does it not require QM, if a requirement is that the coordinated action be chosen truly at random at the last moment before the attack - i.e. the information cannot be intercepted by the enemy ahead of time, even in principle? This is equivalent to saying "no hidden variables".

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Old 09-20-2019, 12:00 AM
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How does it not require QM, if a requirement is that the coordinated action be chosen truly at random at the last moment before the attack - i.e. the information cannot be intercepted by the enemy ahead of time, even in principle? This is equivalent to saying "no hidden variables".
In the scenario as given, it indeed doesn't require QM---the same sort of behavior could be obtained by local hidden variables, i. e. each box containing a particle with a definite, but up until measurement simply unknown, spin value. That is, you could replace the scenario with one in which the information about which pole to attack is sent from the source of both entangled particles to their eventual measurement, and there'd be no way to know the difference.

In particular, if the enemy intercepted the particles, and measured them, they'd know which pole would be attacked beforehand, and a re-measurement in the same basis of both generals at the point of attack would tell them the same information. In this sense, no information is being transferred between the generals at the moment of measurement.

A crucial part of this setup is that it has to be decided beforehand in which basis a measurement must be undertaken, as both generals must measure in the same basis (e. g. spin in X-direction vs. Y- or Z-direction) in order to observe appropriately (anti-)correlated outcomes. If an enemy interceptor then were to measure in the wrong basis, both generals would no longer be guaranteed to attack opposing poles, but of course, there'd be no way for them to know that before getting together and comparing notes, so also in this case, they could not detect enemy meddling.

There would be a way to be certain that nobody intercepted each particle, and hence, to ensure that the information is 'created' only at the point of measurement, but in order to do so, they'd have to meet and compare notes at some point. Essentially, the idea is that you use a sufficiently large number of entangled particle pairs, and using some (random) part to verify the violation of a Bell inequality (which would not be verified if an enemy had interfered). Essentially, this is the so-called Ekert protocol for quantum cryptography.

However, in order to do so, both generals need to exchange information about their measurement results, and the bases they used; at which point, of course, you don't have any FTL information transfer anymore.

So the reason there's no FTL information transfer in the original case is that there's no way to experimentally verify whether there's actually an entangled particle pair distributed between both generals and still coordinate the attack without further communication. If both generals just measure the particles and attack, then they may just reveal information contained in the particles from the source; and if they were to verify that the particles are not secretly in some definite, non-entangled state (i. e. the information is actually not in there before measurement), they must communicate. So each case is in fact consistent with information transfer only ever happening with less than light speed.

Now, of course, it may be that information is transferred FTL, and nature just conspires against us such that we can never actually verify that fact---indeed, that's sorta what happens in Bohmian mechanics. But a difference you can't observe doesn't really make a difference (and hence, fails to be information ).

By the way, it also wouldn't work to coordinate the attack using many pairs of particles, and meeting up later to see whether a Bell inequality had been violated: to coordinate, both parties need to know what basis the other measures in, but to violate a Bell inequality, they need to randomly measure different bases at random; if they agree on a recipe to measure in different bases, then we can again use a local hidden variable theory to explain the observed violation, and have no FTL information transfer.
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Old 09-20-2019, 03:10 AM
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How would you do that, when you have no control over the random outcomes of the coin flips?
I think I get it. So would it be accurate to say that both parties can see that there is a coin flipping at random, but as soon as they try to specifically set the coin to heads or tails, it goes away?
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Old 09-20-2019, 06:17 AM
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I think I get it. So would it be accurate to say that both parties can see that there is a coin flipping at random, but as soon as they try to specifically set the coin to heads or tails, it goes away?
That is correct. As soon as one party attempts to force their outcome to be heads or tails, the particles become distentangled, and the second party measures an entirely separate coin flip.
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Old 09-20-2019, 07:41 AM
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You're not wrong about the result that entangled particles can generate, and the fact that it could it could be strategically useful. You could randomly pick which general attacks which target just before the attack begins, so that the defenders cannot possibly plan ahead; yet ensure that one general attacks south and one attacks north. It's notable, however, that you cannot use this technique to set up a scenario where both generals attack the same target, even if the target is chosen randomly.

And you're also not wrong that the two particles must be able to communicate certain information between one another instantaneously at arbitrary distance. That's the "spooky action at a distance" that is so difficult to make intuitive sense of in QM. And observers can passively read the correlated states of the two particles instantaneously.

What cannot happen is for one observer to actively pass information to another distant observer; observers cannot "choose" what information is exchanged instantaneously between the particles. There is no way for one general to tell the other general "attack south". What you're trying to pin down is the No-Communication Theorem.

https://en.wikipedia.org/wiki/No-communication_theorem
Choice does appear to be the distinguishing factor. In a paper by Weaver on "The Recent Contributions to The Mathematical Theory of Communications" he writes:


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A simple definition of information, provided by Weaver, is that "information is a measure of one's freedom of choice when one selects a message"; or more correctly, the logarithm of that freedom of choice. Information is thus more clearly understood as a the number of combinations of component parts that are available to be chosen arbitrarily.
After searching for a half hour, that's one of the two sources (both in obscure places) I can find that define information that way. In other sources I see information described as "the minimum number of bits necessary to describe the state of a system.". The first definition would preclude the coin toss as information, while the second one doesn't. In any case it's strange how hard it is to find the definition of information in the context we're discussing here.

Even the Wikipedia entry on No-Communication Theory doesn't make it clear. They define the issue as "the theorem states that, given some initial state, prepared in some way, there is no action that Alice can take that would be detectable by Bob." Thats an awfully specific use of the term "information.". Anyway, I guess I can accept it now, though I will be holding a grudge for some time against physicists for not making clear their precise definition (at least in their explanations to lay persons on the internet.). 😉
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Old 09-20-2019, 07:45 AM
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Anyway, I guess I can accept it now, though I will be holding a grudge for some time against physicists for not making clear their precise definition (at least in their explanations to lay persons on the internet.). 😉
Note that this jibe was not directed at those answering here. It's to the ten or so physicists who wrote articles I've read about this issue.
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Old 09-20-2019, 07:50 AM
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Maybe itís more like a coin toss where a third party (who is deaf, dumb, and blind) does the toss and has no way of communicating with A and B. So A and B could see the coin toss and see the result at any given moment, but with no way of inputting a pattern themselves. Honestly, it seems like no better than having synchronized watches and both being able to read the time simultaneously, even a light-year apart. Yes, you know what the other guy or gal's watch says because theyíre synchronized, but you canít then move the minute hand back on yours and expect the other's to do the same.
If you are a light year apart there is no objective now or synchronized clocks at all. The illusion of a universal clock only holds if you are local...but letís go back to the idealized thought experiment.

A way to think of the OP is that you don't know if you were the first to look and just randomly got a result or if the other side forced your particle into that state. It's only after you compare notes that you know that the two particles were entangled and that the measurement of one depended on the other. Without that communication you cannot tell if either side was dealing with an already-set particle.

Comparing notes is the transfer of information and is not FTL.
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Old 09-20-2019, 08:15 AM
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They are told to check the spin state of their entangled particles at a certain time and whoever had a down-spin invaded the South and vice versa. Since I know information, at least as defined by physicists in this context, can't travel faster than light, either I'm misunderstanding how entanglement works or physicists are using a qualified definition of information. Which is it? Thanks in advance.
I can't tell if this question has now been clearly answered (Half Man Half Wit has the answer).

The thing is, this scenario does not involve anything strange dealing with quantum mechanics or FTL or paradoxes. You are using particle measurements in the same way as the playing-cards-in-sealed-envelopes example mentioned in this thread. Nothing strange is happening here. The generals get perfect anticorrelation just like they get perfectly different playing cards in that example.

If you try to change the scenario such that "strangeness" was actually happening it would be completely obvious that no information is being passed.

I think the clearest explanation is the wavy graph in the "Overview" section on the Wiki about Bell's Theorem

That is, the generals would have to measure their particles in slightly different directions, preferably 45 degrees, to get the strangest discrepancy.

(The following percentages are rough illustrations, too lazy to calculate these correctly right now)

So, their results would be only partially correlated. They would attack the same target in 20% of cases (70% anticorrelation), if they repeated this experiment many many times (conquering one planet after another).

So after a quarter century of this, attacking a new planet every month, they'd retire and reminisce about how they messed up in 20% of their conquests.

THEN they'd realize that something strange was going on. They should've messed up in 25% of cases (50% anticorrelation)! How is it possible that they only went wrong in 20% of conquests?? How strange! But quantum mechanics has the explanation!

This makes it clear that you only notice the strangeness afterwards!!

Anybody feel free to calculate the correct percentages!

Last edited by Frankenstein Monster; 09-20-2019 at 08:15 AM.
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Old 09-20-2019, 08:38 AM
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That's a bit simplified BTW, I could complicate the scenario a bit further to make it "perfectly strange"
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Old 09-20-2019, 08:51 AM
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I can't tell if this question has now been clearly answered (Half Man Half Wit has the answer).

The thing is, this scenario does not involve anything strange dealing with quantum mechanics or FTL or paradoxes. You are using particle measurements in the same way as the playing-cards-in-sealed-envelopes example mentioned in this thread. Nothing strange is happening here. The generals get perfect anticorrelation just like they get perfectly different playing cards in that example.

If you try to change the scenario such that "strangeness" was actually happening it would be completely obvious that no information is being passed.

I think the clearest explanation is the wavy graph in the "Overview" section on the Wiki about Bell's Theorem

That is, the generals would have to measure their particles in slightly different directions, preferably 45 degrees, to get the strangest discrepancy.

(The following percentages are rough illustrations, too lazy to calculate these correctly right now)

So, their results would be only partially correlated. They would attack the same target in 20% of cases (70% anticorrelation), if they repeated this experiment many many times (conquering one planet after another).

So after a quarter century of this, attacking a new planet every month, they'd retire and reminisce about how they messed up in 20% of their conquests.

THEN they'd realize that something strange was going on. They should've messed up in 25% of cases (50% anticorrelation)! How is it possible that they only went wrong in 20% of conquests?? How strange! But quantum mechanics has the explanation!

This makes it clear that you only notice the strangeness afterwards!!

Anybody feel free to calculate the correct percentages!
The issue isn't so much whether QM can be exploited in some way to send a specific bit, but rather why the result of a coin flip communicated via distant entangled particles isn't information. As I explained in a previous post, the playing cards and two entangled particles are qualitatively different. The explanation lies, I think, in my last post that defines information as requiring a choice on the part of the sender.

Last edited by KidCharlemagne; 09-20-2019 at 08:52 AM.
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Old 09-20-2019, 09:36 AM
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As I explained in a previous post, the playing cards and two entangled particles are qualitatively different.
I assume you are referring to this?

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Well like you said, in the case of the entangled particles, the information hasn't been "created" yet so there is no information to transmit.
It is true entangled particles are qualitatively different from playing cards. It is NOT TRUE that they are qualitatively different in the specific way you use entangled particles in your example.

You are using entangled particles to send information entirely classically and normally.

(In fact I am still doing the same in my scenario, remember I simplified that a bit too much, maybe I should fix it to show how entangled particles are REALLY qualitatively different.)

It just seems to me that it should matter HOW entangled particles are qualitatively different, I'm not seeing where that (accurately/clearly) enters into your question.
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Old 09-20-2019, 09:46 AM
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If you are a light year apart there is no objective now or synchronized clocks at all.
Why can't you have synchronized clocks if you are a light year apart?
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Old 09-20-2019, 11:36 AM
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The issue isn't so much whether QM can be exploited in some way to send a specific bit, but rather why the result of a coin flip communicated via distant entangled particles isn't information. As I explained in a previous post, the playing cards and two entangled particles are qualitatively different. The explanation lies, I think, in my last post that defines information as requiring a choice on the part of the sender.
The choice there is, I think, among the number of possible messages still consistent with a signal. Suppose you have some fixed number of possible messages you could send, such as 'the British attack by land' and 'the British attack by sea'. As long as you haven't received any signal, you have no information about the attack; as soon as you get one bit of information, say by counting the number of lamps hung in some bell-tower, you know exactly what the message is---that is, that information reduced your uncertainty to zero.

But there are cases where a given signal only partially reveals the message; then, each element of the signal---think of something like a series of light pulses, or a stream of letters, or what have you---will further reduce the number of possible messages you could be receiving, thus decreasing your uncertainty about what message is being sent to you, and reducing the amount of choice between possible messages consistent with the signal. A highly unlikely, thus unexpected signal-element will then serve to constrain the message more, reduce your uncertainty about the message by a greater quantity, and highly limit your choice---and thus, carry lots of information. (This is often called the 'surprisal' of the signal.)

The information content of the whole message is then the number of bits to specify it fully, to completely eliminate choice, and thus, the freedom of choice you had before. In the case of the British attack, you had a choice between two alternatives, and hence, the message has one bit of information.

As for the information transfer in your scenario, perhaps it helps to think about it in the many-worlds scenario: there, the creation of the entangled pair creates two 'worlds', one in which A has spin up, and B has spin down, and the other in which things are the other way around. Upon making a measurement, each general then just learns in which universe they are; thus, no information is exchanged between them, and all information that is transmitted is transmitted from the source of the particles.

Last edited by Half Man Half Wit; 09-20-2019 at 11:37 AM.
  #31  
Old 09-20-2019, 02:33 PM
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If you are a light year apart there is no objective now or synchronized clocks at all. The illusion of a universal clock only holds if you are local...but letís go back to the idealized thought experiment.
Sorry, not buying it. There is (or can be) as much a concept of "now" between two individuals separated by arm's width as two individuals separated by a mile, the diameter of the Earth, the distance from Earth to the moon, a light-year, and beyond. Being local has nothing to do with it. How accurate you can be about that shared moment of "now" may have something to do with how local you are, but even at a light-year or more an understanding of the physics involved with how you got there can allow you to account for things like time dilation and delays in communication. Two people can synchronize watches, move apart, and take whatever means necessary to be reasonably confident (as confident as they need to be, to within whatever margin of error they deem acceptable for the experiment) that their watches remain synchronized, and then be "reading" the clocks at the same time, coming up with the same time (each having access to the same information) but without being able to communicate. Thatís the point Iím getting at. Itís an analogy. If you get hung up on how certain they could be of a shared moment of "now," it could just as easily be the measurement of time with respect to the orbit of a planet around a star placed exactly midway between them instead of a watch as the "synchronizer." Itís not meant to be a perfect analogy, only an example of how two people without the ability to communicate can nevertheless have access to the same information at the same moment in time and then make a decision off of it at the same instantócould even know what decision the other will make based off previous communicationóbut still not be "communicating" at faster than the speed of light, much less instantaneously.
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Old 09-20-2019, 02:36 PM
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ASL, I'm not sure I'm following you, but are you arguing that two events can be said to be simultaneous for two arbitrary inertial observers? That statement contradicts Special Relativity if the observers are moving with respect to each other. Here is Wikipedia's explanation.

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Old 09-20-2019, 02:53 PM
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ASL, I'm not sure I'm following you, but are you arguing that two events can be said to be simultaneous for two arbitrary inertial observers? That statement contradicts Special Relativity if the observers are moving with respect to each other. Here is Wikipedia's explanation.
I don’t think so. Take points A, B, and, C. Put B midway between A and C. Flip a light on at B. A and C both receive news that the light has turned on at B at the same time—or merely "about the same time" if you prefer. Either way, A and C both can make a pre-planned decision on what to do when the light turns on—and "know" that the other will also make a decision, and what that decision will be—and both A and C "know" what decision the other will make (assuming they go off what was previously agreed upon) just based off the light from B. Nothing has been communicated from A to C, and yet both A and C "know," merely in the time it takes for light to travel half the distance between them what the other will do.

And of course when I use "know" here, I mean to extent that we can predict/anticipate what anyone will do based off a previous agreement (a previous communication, if you will), not in the sense of metaphysical certainty. Because, again, they can’t communicate FTL, they can however receive information from an intermediate source at "about the same time" and make a pre-planned decision off it.

Does that break physics?

ETA: And if it does break physics, than how does two people being "local"to one another (at arm’s reach) break physics any less?

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  #34  
Old 09-20-2019, 03:14 PM
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I don’t think so. Take points A, B, and, C. Put B midway between A and C. Flip a light on at B. A and C both receive news that the light has turned on at B at the same time—or merely "about the same time" if you prefer.
Do you want these events to be exactly simultaneous with each other, or merely 'about the same time'? The second option is achievable, but the first one is not.

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  #35  
Old 09-20-2019, 03:20 PM
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The second option is achievable, but the first one is not.
The second will suffice, just so long as A and C can infer each other's decisions faster than if C were to wait for a signal from A or vice versa.

And, yes, I know how trivial that all is. At least I hope it’s trivial. The idea was to give an example of making a decision under circumstances similar to the OP that seemingly involves faster than light information sharing, but really is just one party inferring what the other will do based off a pre-arranged signal with no means of transmitting new information FTL.

Last edited by ASL v2.0; 09-20-2019 at 03:21 PM. Reason: Replying with quote
  #36  
Old 09-20-2019, 04:14 PM
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I donít think so. Take points A, B, and, C. Put B midway between A and C. Flip a light on at B. A and C both receive news that the light has turned on at B at the same timeóor merely "about the same time" if you prefer.
This is just the Einstein's Train thought experiment. Some observers will see A and C receive the signal at the same time and some won't. It's described in the wikipedia article.

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Does that break physics?

ETA: And if it does break physics, than how does two people being "local"to one another (at armís reach) break physics any less?
Yes you broke physics. Please put it back together before you leave.

Local observers have the same issue but the amount of discrepancy is vanishingly small if the observers are not at cosmological distances.
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Old 09-20-2019, 04:32 PM
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The information content of the whole message is then the number of bits to specify it fully, to completely eliminate choice, and thus, the freedom of choice you had before. In the case of the British attack, you had a choice between two alternatives, and hence, the message has one bit of information.

As for the information transfer in your scenario, perhaps it helps to think about it in the many-worlds scenario: there, the creation of the entangled pair creates two 'worlds', one in which A has spin up, and B has spin down, and the other in which things are the other way around. Upon making a measurement, each general then just learns in which universe they are; thus, no information is exchanged between them, and all information that is transmitted is transmitted from the source of the particles.
So it seems like you're equating, at least in some sense, a reduction in certainty with "choice." And while a conscious choice between two alternatives reduces uncertainty, so does the result of a coin flip. The information entropy of a coin flip and a conscious choice of heads or tails are both 1 bit. We know Alice receives one bit of data upon measuring her particle's spin and that her uncertainty drops from .5 to 0. That is definitely information, which leaves the only other possible alternative - which is that it didn't transfer FTL.

I can begrudgingly accept that since the particles required colocation to be entangled, the ability to communicate information over that distance didn't happen FTL. One of the reasons I didn't accept it at first is partly because other posters maintained that information wasn't transferred, rather than that it wasn't transferred FTL. When physicists, at least internet popularizers of physics, make the argument for why information can't transfer FTL, they never mention the colocation issue - they only mention the disentanglement issue. Please correct me if I'm wrong on any of the above.
  #38  
Old 09-20-2019, 06:12 PM
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This is just the Einstein's Train thought experiment. Some observers will see A and C receive the signal at the same time and some won't. It's described in the wikipedia article.
This is not the train thought experiment. I donít care at what time observers D/E/F or even A and C see someone receive the signal. I guess, in a sense, you could think of this as a single observer at B sending a signal to A and C at the same time (whatever A might think of C and C might think of A), not personally observing the arrival at either, and yet nodding with satisfaction at the knowledge that both A and C will act on the signal according to pre-arranged plans. A and C both act off a signal from B in a way that suggests communication between A and C, but does not actually involve transfer of information from A to C, only the illusion of it due to the pre-arranged plan (which did require communication between A and C at some point, but was not done faster than light).
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Old 09-20-2019, 08:19 PM
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Okay, so I donít get accused of just going "nuh uh!" in response to criticism (and I wouldnít blame you if thatís what you were thinking), Iíve got my definitive scenario to try and distinguish what Iíve been driving at about apparent (but not actual) "simultaneous" or FTL communication, and why, in spite of the (apparently) deficient explanations Iíve offered so far, itís not a violation of special relativity:

Scenario: Johnny Rocket will be the first man on Mars. Three women have already been to Mars before him (and returned, to great acclaim), but thatís beside the point. Anyway, prior to leaving for Mars, he got together with his buddy, Jane Newscaster, and his other buddy, Shannon Physicist. The three of them have determined that on the date and time Johnny is to land on Mars, it will take five minutes for light to travel from Earth to Mars. Based on a pre-arranged script, they have worked out the following sequence of events and communication, with t representing the time in minutes.

t=-270000 Johnny, Jane, and Shannon come up with a script and ensure Johnny has a copy of the script packed along with all the other stuff for his EVA
t=-259200 Johnny blasts off in a rocket bound for Mars
t=-1000 Jane and Shannon, in anticipation of Johnny's landing, start a timer on their watches. Itís a count up timer.
t=-995 Jane and Shannon hit pause on their timers (so itís paused at five minutes)
t=0 Johnny lands on Mars, rattles off his motivational speech, and starts his count up timer at the conclusion of the phrase "And a Happy New Year!"
t=1.5 Johnny pulls out the script (with time annotations) he was given prior to his departure and says "Yes, like a billion dollars landing in my bank account."
t=2.5 Johnny continues reading from the script, "I'm going to Disney World!"
t=5 Jane learns Johnny lands on Mars and hears the phrase "And a Happy New Year!" and resumes her count up timer. Being a newscaster, she has of course inserted her script on the teleprompter.
t=5.5 Jane says "Wow, Johnny, weíre all so glad you made it. Now please standby for our science advisor, Doctor Shannon Physicist."
t=6 Doctor Physicist (who, being a Physicist has memorized the full script) says, "Johnny, is the ansible that we worked on and patented together with Jane working?"
t=6.5 You, sitting in the studio and listening with Jane, Shannon, and half the world hear Johnny say "Yes, like a billion dollars landing in my bank account."
t=7 Jane says "That's great, Johnny, what are you going to do next?"
t=7.5 As if that werenít enough, you, above the uproarious cheers of the studio, hear Johnny say "Iím going to Disney World!"
t=8 The CEO of Disney puts in a call to his or her on-call corporate lawyer to determine whether they should sue for trademark infringement, or just let it slip and think of it as free advertising. "Why not both?" the lawyer replies.
t=10.5 Johnny hears "Wow, Johnny, weíre all so glad you made it. Now please standby for our science advisor, Doctor Shannon Physicist."
t=11 Johnny hears "Johnny, is the ansible that we worked on and patented together with Jane working?"
t=12 Johnny hears "That's great, Johnny, what are you going to do next?"
t=12.1 Johnny jumps for joy in the diminished Martian gravity, though weighed down by his suit.
t=? After a long, drawn out legal battle, Johnny is broken and penniless as Disney has taken all his assets and it still isnít enough to feed the Mouse. Back on Earth, looking up at Mars through his child's telescope, just before dawn and waiting for the corner pawnshop to open, he wonders, was it worth it?

/Scenario

Did Johnny Rocket and Doctor Physicist just communicate at faster than the speed of light? No, of course not. Does the above scenario contradict special relativity? No, though it helps that you, sitting in the studio, heard Johnny respond to Jane and Shannon at almost the same time they did, the precise simultaneity of the event is not important. Obviously you heard it some fraction of a second after they did, even ignoring relativistic effects, just due to the speed of sound being less than infinity. Even if you werenít in the studio, but you were in a colony on the Moon and heard it some seconds after they did (and heard them going out some time later too) it wouldnít matter. The point is, Jane, Shannon, and Johnny, through prior communication at equal to or less than the speed of light, created the illusion of faster than light communication (not even the illusion of simultaneous communication, just faster than light) based on following a pre-arranged script. It didnít have to be watches or time pieces or Johnny's initial radio communication that set the script in motion, it could have been the orbit of the four largest moons of Jupiter or myriad other (relatively) mutually observable phenomena that set the plan in motion, and they didnít even have to see those phenomena occur at the same time, they could have just done the math and accounted for the delay in light traveling from Jupiter to the Earth vs Jupiter to Mars and it still would have worked, just been a little trickier.

And if all that strikes you as trivial, albeit contrived and more than a little convoluted... QED
  #40  
Old 09-21-2019, 02:53 AM
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So it seems like you're equating, at least in some sense, a reduction in certainty with "choice." And while a conscious choice between two alternatives reduces uncertainty, so does the result of a coin flip.
I'm not saying anything about 'conscious' choice or the like. Think of it more in terms of options: before receiving any information, say you have four options---
  1. The British attack by land, at night
  2. The British attack by land, during the day
  3. The British attack by sea, at night
  4. The British attack by sea, during the day

You have four options---four choices. To single out one of them requires log2(4) = 2 bits of information. Hence, the choices you have determine the information content of the message.

Before you receive any bit of information, you're maximally uncertain about the message (your uncertainty is, in this sense, equal to two bits). If you receive the first bit of the message, your uncertainty is reduced---say, you know that the British attack by land, but not when. You still have a choice between two options, so still an uncertainty of one bit, so a message needs to carry one bit of information to eliminate any residual uncertainty.

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I can begrudgingly accept that since the particles required colocation to be entangled, the ability to communicate information over that distance didn't happen FTL. One of the reasons I didn't accept it at first is partly because other posters maintained that information wasn't transferred, rather than that it wasn't transferred FTL.
This is exactly the sense in which your scenario is equivalent to that with the two cards in sealed envelopes---there exists a story according to which the information was just present from the beginning, i. e. from the creation of the entangled particle pair, and there's no way to operationally distinguish whether that's the true story without resorting to classical communication at sub-light speeds, which makes the point moot.

It should be noted that two particles need never be in the same place in order to entangle them, since you can swap entanglement between different particles. I. e. you might create an entangled pair of photons, then perform an operation on one of the photons and, say, an atom, which leaves the atom and the other photon entangled, but disentangles the first photon. But of course, this can only be done with particles in the forward light cont of the photon creation event (which only means that photons can only be transported at the speed of light).

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Originally Posted by ASL v2.0 View Post
Okay, so I donít get accused of just going "nuh uh!" in response to criticism (and I wouldnít blame you if thatís what you were thinking), Iíve got my definitive scenario to try and distinguish what Iíve been driving at about apparent (but not actual) "simultaneous" or FTL communication, and why, in spite of the (apparently) deficient explanations Iíve offered so far, itís not a violation of special relativity:
It's not, but it also doesn't really do what you want it to do (if I understand you correctly). The scenario only works out that way in a specific reference frame. It's not a problem to synchronize clocks within one reference frame, but once you get to relatively moving frames, synchronization won't be given anymore.

Take this animation: each row of clocks are synchronized with respect to a co-moving observer; but, viewed from the frame of the 'bottom' clocks, the clocks of the 'moving' reference frame won't be synchronized. The scenario is symmetrical: from the point of view of the 'moving' reference frame, i. e. from that of a co-moving observer, the clocks of the 'stationary' (with respect to you) reference frame will fail to be synchronized.

If that doesn't help, I think it would be more effective if you opened up a new thread to discuss these issues.
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Old 09-21-2019, 09:05 AM
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I'm not saying anything about 'conscious' choice or the like. Think of it more in terms of options: before receiving any information, say you have four options---
  1. The British attack by land, at night
  2. The British attack by land, during the day
  3. The British attack by sea, at night
  4. The British attack by sea, during the day

You have four options---four choices. To single out one of them requires log2(4) = 2 bits of information. Hence, the choices you have determine the information content of the message.

Before you receive any bit of information, you're maximally uncertain about the message (your uncertainty is, in this sense, equal to two bits). If you receive the first bit of the message, your uncertainty is reduced---say, you know that the British attack by land, but not when. You still have a choice between two options, so still an uncertainty of one bit, so a message needs to carry one bit of information to eliminate any residual uncertainty.
I get the concept of information entropy, and that, for example, a "z" in a telegraphed message in English reduces the entropy of the message by more than an "e." My misunderstanding came from various vague articles (and some posts) that left me thinking that my scenario was not FTL information transfer because information wasn't being transferred, rather than because it wasn't being transfered FTL. Then when I see a term like "choice" rather than your more instructive and entropy- reducing term "uncertainty," I started to conflate the notion of "choice" with "can't be the result of a coin flip." Thanks for your help.

Last edited by KidCharlemagne; 09-21-2019 at 09:08 AM.
  #42  
Old 09-21-2019, 01:29 PM
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If that doesn't help, I think it would be more effective if you opened up a new thread to discuss these issues.
I understand. I have understood. The point of the analogy was not to give another example of things being "entangled," but the other half of this thread, which is how a variety of scenarios can be concocted that give the illusion of FTL information transfer, but that it all comes down to just that. An illusion.

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  #43  
Old 09-22-2019, 04:07 AM
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I understand. I have understood. The point of the analogy was not to give another example of things being "entangled," but the other half of this thread, which is how a variety of scenarios can be concocted that give the illusion of FTL information transfer, but that it all comes down to just that. An illusion.
I think most people, myself included, reacted to statements such as these:

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Originally Posted by ASL v2.0 View Post
Sorry, not buying it. There is (or can be) as much a concept of "now" between two individuals separated by arm's width as two individuals separated by a mile, the diameter of the Earth, the distance from Earth to the moon, a light-year, and beyond. Being local has nothing to do with it.

[...]

Two people can synchronize watches, move apart, and take whatever means necessary to be reasonably confident (as confident as they need to be, to within whatever margin of error they deem acceptable for the experiment) that their watches remain synchronized, and then be "reading" the clocks at the same time, coming up with the same time (each having access to the same information) but without being able to communicate.
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I donít think so. Take points A, B, and, C. Put B midway between A and C. Flip a light on at B. A and C both receive news that the light has turned on at B at the same timeóor merely "about the same time" if you prefer.
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Originally Posted by ASL v2.0 View Post
A and C both act off a signal from B in a way that suggests communication between A and C, but does not actually involve transfer of information from A to C, only the illusion of it due to the pre-arranged plan (which did require communication between A and C at some point, but was not done faster than light).
What they suggest, on a straightforward reading, is that you claim that there would be some absolute sense in which one could put an order to events arriving at A and C---i. e. that it looks like A sends a message 'FTL' to which C immediately reacts, sends a message back, thus getting a conversation between the two that happens at apparent light speed, by merely following a fixed script. But that's just not the case---in some reference frames, C will seem to reply before A sends the first message, and hence, it will not 'look like' there is communication between the two. And neither reference frame is objectively the right one.

But in the original problem, one could claim that a sort of information exchange has taken place, although it would not be clear in which direction---i. e. which general measured first, thus causing the entangled state to collapse.
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Old 09-22-2019, 10:33 AM
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I understand. I have understood. The point of the analogy was not to give another example of things being "entangled," but the other half of this thread, which is how a variety of scenarios can be concocted that give the illusion of FTL information transfer, but that it all comes down to just that. An illusion.
In quantum mechanics though there is non-locality, so it is not an illusion in the sensse that there is, or can be, a explanation that is 'classical-like' (i.e. realist) and doesn't involve FTL information transfer (i.e. is local) for what happens. A local realist explanation could explain the correlation between two entangled particles, but Bell's inequalties shows it would fail in other situations.

What I basically mean is that any explanation of how quantum non-locality doesn't 'break' causality will defy a simple classical analogy.
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Old 09-22-2019, 11:19 AM
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In quantum mechanics though there is non-locality
Well, although it's often glossed like that, I think one should point out that it's far from a settled issue, even though some would claim otherwise. On most accounts, the Many Worlds interpretation is thoroughly local---even local realistic, getting around Bell's theorem by violating the (implicit) assumption that only one outcome of a range of possibilities ever obtains. Even on other interpretations, Bell's theorem doesn't force you to give up locality; all that a Bell inequality really is, is just a necessary condition for a certain set of variables (the experimental outcomes) to have a joint probability distribution, so their violation just means that no such distribution exists. And after all, what would be the meaning of the probability for observing, say, some simultaneous spin values for orthogonal directions, when there is no possible way to actually make such an observation?
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