Why does quantum entanglement not allow ftl communication


by macd
Tags: communication, entanglement, quantum
Ken G
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#55
May12-08, 01:36 PM
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Quote Quote by ThomasT View Post
Thus, the appropriate path to take in considering all the stuff related to EPR, Bell, quantum entanglement, etc. is to assume that nature is local -- at least until something a bit more compellingly suggestive of ftl or instantaneous actions or connections is discovered or invented.
I agree, and that's why I object whenever I hear someone claim that Bell-type experiments exhibit a nonlocal influence when a measurement is made. I see it as entirely local influences, being used to intentionally "unpack" nonlocal information. You only run into trouble when you ask "where is the information stored", and combine local thinking with realism. But these are problems for philosophy, not physics, and really just say that we need to tailor successful philosophies more carefully for them to be informed by physics. Philosophies should not, on the other hand, be used to inform physics-- the history of trying that is pretty clear on that point. (Even the principle of relativity, which is often pointed to as a kind of philosophy-informing-physics, is actually just philosophy-informing-form, i.e., informing pedagogy, not physics itself. In my view, anyway-- there's a relativity thread on this which draws much fire for that position and I'd have to say it's still unresolved).
Degeneration
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#56
Jul9-08, 09:40 PM
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I dont understand. Isnt it that there is no SIGNIFICANT information sent? Suppose I wish to receive a signal to turn on a lamp and I have one of two entangled particles. When the particle has an up spin, I am to turn on the lamp. My partner, a couple lightyears away decides to do something to his particle to change its spin to down. My particle instantly reacts with an up spin meaning that I am to turn on my lamp. Isnt information sent here, as primitive of a method it might be?
Ken G
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Jul10-08, 02:55 AM
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Quote Quote by Degeneration View Post
When the particle has an up spin, I am to turn on the lamp. My partner, a couple lightyears away decides to do something to his particle to change its spin to down. My particle instantly reacts with an up spin meaning that I am to turn on my lamp. Isnt information sent here, as primitive of a method it might be?
No, because your partner cannot "decide" to make his particle be down, and expect that will make your particle be up. If the partner makes a decision and gets a certain spin by design, that would break the entanglement. The entanglement is only unbroken if the partner makes no such decision and simply measures the spin-- but then he has no way to influence whether or not you turn on the lamp. Nothing is transmitted, nonlocal information is simply being "unpacked" by the experiment.

It only seems like a nonlocal "influence" if you imagine that the information being unpacked is somehow stored in the two particles, such that changing that information represents a physical change in both particles, but I would argue that such is a purely philosophical picture that is clearly problematic and retains no value in quantum mechanics, any more than imagining that any wave function is "stored" in the same region of space as it takes on its values. I would say that the place a wave function "resides" is in the mind of the physicist using it, not in the region of space where it takes on its values, and many people may not even realize they are implicitly assuming the latter instead of the former when they agonize over entanglement and delayed choice.
Dar Kthulu
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#58
Apr14-10, 07:56 AM
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hello everyone, I am very new to this discussion and i have just a few questions regarding this topic.

1.) how are these particles affected by speed. do they gain mass? can that be measured?
2.) Is the communication of these paricles affected by gravity such as the gravity well around massive objects.
3.) what do you suppose of this experiment on an entangled pair? One is left here on earth and the other is placed aboard the International Space Station. both are observed.

Just some random thoughts and questions from a non student. Thanks for your time and information. :)
Ken G
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#59
Apr14-10, 09:15 AM
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Quote Quote by Dar Kthulu View Post

1.) how are these particles affected by speed. do they gain mass? can that be measured?
They don't change rest mass, and if you consider the change in what is known as "relativistic mass", that is just a frame-of-reference issue, not a physical difference that should affect entanglement.
2.) Is the communication of these paricles affected by gravity such as the gravity well around massive objects.
The GR effects should just affect the background spacetime through which the system moves, but I don't see a direct impact on entanglement except perhaps in strong gravity environments where we would need a combined theory of quantum mechanics and gravity.
3.) what do you suppose of this experiment on an entangled pair? One is left here on earth and the other is placed aboard the International Space Station. both are observed.
I think the normal quantum mechanical expectations, referenced to the system proper times, should work fine there.
DrChinese
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Apr14-10, 09:33 AM
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Quote Quote by Ken G View Post
The GR effects should just affect the background spacetime through which the system moves, but I don't see a direct impact on entanglement except perhaps in strong gravity environments where we would need a combined theory of quantum mechanics and gravity.
This is one of those areas where there are some interesting opportunities to consider QM and GR as a pair. If GR is correct, and there is no graviton, then you would certainly expect that entanglement is not affected by gravitational field. That might not be true, on the other hand, if the graviton exists. There have been a few papers that have speculated on this point. Of course without a specific QG candidate to work with, it is hard to say too much. But there might be some limits which could be derived to steer a potential candidate theory.

http://arxiv.org/abs/0910.2322

"We propose a thought experiment to detect low-energy Quantum Gravity phenomena using Quantum Optical Information Technologies. Gravitational field perturbations, such as gravitational waves and quantum gravity fluctuations, decohere the entangled photon pairs, revealing the presence of gravitational field fluctuations including those more speculative sources such as compact extra dimensions and the sub-millimetric hypothetical low-energy quantum gravity phenomena and then set a limit for the decoherence of photon bunches and entangled pairs in space detectable with the current astronomical space technology. "
Ken G
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#61
Apr14-10, 12:51 PM
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That's interesting, it would be somewhat ironic if gravity waves are first detected via their interaction with sublimely constructed entangled quantum states, rather than the more brutely classical application of watching them make masses jiggle!
foxy1968
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#62
Jun22-10, 02:01 AM
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By using a minimum of 2 sets of qubits in isolation and by freezing the spin of the entangled particles and specifying that set 1 is used to indicate the start of a message and set 2 is used to send the message. By influencing the spin of the particles at one site and monitoring the spin changes at the other site why is this not possible. In this manner would it not be possible to send data over an infinate distance with no delay and therefore ftl.
K^2
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#63
Jun22-10, 02:55 AM
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Because you can't check whether the spin was "influenced" by the sender or by your own attempt to check whether it was influenced. Both give you exactly the same result, and so no information is carried.
Demystifier
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#64
Jun22-10, 04:26 AM
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Sec. 3 of
http://xxx.lanl.gov/abs/1006.0338
gives a simple explanation why entanglement cannot be used for ftl signalization.
It also proposes how this inability (to use it for ftl signalization) could, in principle, be overcame.
Brokinarrow
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#65
Aug8-11, 09:53 PM
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So basically the reason FTL communication is not possible using quantum entanglement: Currently we cannot control the state of the entangled particles, we can only observe the changes that nature is making to the state of the particles. If we could figure out a way to control the state of these particles, FTL communications would be possible.
Albert V
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#66
Aug10-11, 05:42 AM
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I've read some papers that were aiming to use linearly and circularly polarized light as a protocol for communication - however, it seems difficult/impossible to distinguish these two when you have to rely on incident photons (eg. Physics Letters A
Volume 251, Issue 5, 1 February 1999, Pages 294-296). Anyone with an idea?

I recently saw another ideá from Arxiv.org. I am not able to discover the flaw in his argument, but I suspect that there will be no interference?

http://arxiv.org/abs/1106.2257
Albert V
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#67
Aug30-11, 07:33 AM
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This is a discussion of the Cornwall paper on superluminal communication from

http://arxiv.org/abs/1106.2257

quantum theory says that whatever you do on one side does not change what you observe on the other:

Total state |Phi> = (|H>|V> + |V>|H>)/sqrt2.

Not using the polarizing filter (no modulation)

rho = |Phi><Phi|
= (1/2) ( |H>|V><H|<V| + |H>|V><V|<H| + |V>|H><H|<V| + |V>|H><V|<H| ).

In order to see what we observe on the left side we have to "trace out" the right side

rho_right = Tr_left(rho) = (1/2) (|H><H|+|V><V|),

which is eihter a photon in the mode H or a photon in the mode V, which will give no interference.

Am I mistaken?
N468989
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#68
Mar12-12, 11:58 AM
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If I understand correctly, then quantum entanglement is explained by the simple fact that two particles behave the same way after being separated.

Take Machine A and B, each compute numbers from 1 to 10 and are synchonized. Separate the machines and get the output at a given moment in time. We know what the other machine reads, is this correct?

Another thing is to assume that something is propagating through space... (http://www.youtube.com/watch?v=QErwOK3S5IE)
DrChinese
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#69
Mar12-12, 01:37 PM
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Quote Quote by N468989 View Post
If I understand correctly, then quantum entanglement is explained by the simple fact that two particles behave the same way after being separated.

Take Machine A and B, each compute numbers from 1 to 10 and are synchonized. Separate the machines and get the output at a given moment in time. We know what the other machine reads, is this correct?
This is true in a sense. And the description you give works fine for identical measurements on the individual particles. But it does not yield a suitable explanation for Bell tests. I.e. it predicts the wrong results. This fact was not noticed for many years after the EPR paper appeared, until Bell discovered it around 1964.

Best way to think of it is to imagine polarization of a pair of Type II entangled photons Alice and Bob at angles 0, 120 and 240 degrees. I.e. 1/3 of the way around a circle. After a while, you will realize that using your example, there is an average of at least a 1/3 chance that 2 adjoining measurements (one on Alice, the other on Bob) yielding the same value. However, experiments yield a value of 25% which is in agreement with the quantum expectation value.
unified
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Mar12-12, 08:11 PM
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Quote Quote by DrChinese View Post
This is true in a sense. And the description you give works fine for identical measurements on the individual particles. But it does not yield a suitable explanation for Bell tests. I.e. it predicts the wrong results. This fact was not noticed for many years after the EPR paper appeared, until Bell discovered it around 1964.

Best way to think of it is to imagine polarization of a pair of Type II entangled photons Alice and Bob at angles 0, 120 and 240 degrees. I.e. 1/3 of the way around a circle. After a while, you will realize that using your example, there is an average of at least a 1/3 chance that 2 adjoining measurements (one on Alice, the other on Bob) yielding the same value. However, experiments yield a value of 25% which is in agreement with the quantum expectation value.

Agreed. But this leaves the question of how the entangled particles "know" what to do. If the correlation can't be explained in terms of a past interaction, I don't see how you can ever escape from "what I do over hear influences what happens over there". I think that's the whole point of Bell's theorem. It's not that hidden variables must be non-local, but any theory explaining this must be non-local.
DrChinese
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Mar13-12, 08:50 AM
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Quote Quote by unified View Post
Agreed. But this leaves the question of how the entangled particles "know" what to do. If the correlation can't be explained in terms of a past interaction, I don't see how you can ever escape from "what I do over hear influences what happens over there". I think that's the whole point of Bell's theorem. It's not that hidden variables must be non-local, but any theory explaining this must be non-local.
Welcome to PhysicsForums, unified!

You've probably seen some of the different interpretations that are currently in play. Of course the Bohmian view should be right up your alley. There are several others, including the time symmetric group. In those, the mantra is: "what I do now affects the past" and locality is preserved in the sense that influences do not propagate faster than c.
questionpost
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#72
Mar13-12, 06:36 PM
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Quote Quote by macd View Post
quite simply the above question.
Why does quantum entanglement not allow for faster than light communication?
Thanks
They are the same particle, there's nothing to send information between. It would be like bouncing a ball and asking "why aren't other balls magically bouncing now?"


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