Talk:No-communication theorem

message: I know, I Know, this is not a forum. But I honestly think this theorem is very, very broken. He shoves dozens of mathematical obscure formulas to try to furiously deny that it can happen something that is logically simple to analyze: We have two points, each with a photon "tied" to another by quantum entanglement. We know that if the spin of the photon "A" change, this change is reflected immediately in the photon "B".
Knowing this, the observer "B" does _not_ need dozens of mathematical expressions to know that if the photon "B" spin change, was because the observer "A" changed the spin of the photon "A". How can this be so difficult? More valid questions would be:
a) The change in spin is really instantaneous? This can be tested, although it is difficult;
b) The change in spin can be caused by external interference? If yes, so which ones?
c) The act of trying to measure the spin breaks, or can break, the entanglement?
Less "Oh No!! It will break my preferred theory! heretic!", more "Let's see what happens" science please. 200.189.118.162 (talk) 17:11, 18 March 2013 (UTC)
thread_title:  Sorry, but... 

message: Added conflict of interest label, clear dispute by the author(s)on the subject, weasel words and general disagreement. Certainly not an expert in the subject; suggest weaving opposing viewpoints into single article that presents both ideas before removing label.  —Preceding unsigned comment added by 130.215.232.181 (talk) 03:24, 5 December 2009 (UTC)
replies: text: You have to realize that the distinction between "instantly" and "faster than the speed of light" disappears in relativity. "Simultaneity" loses its meaning. That is the significance of the boost transformations of the Lorentz group acting on Minkowski space. 178.38.115.176 (talk) 09:18, 5 May 2015 (UTC)

text: According to Special Relativity, for any two events which are connected by a super-luminal signal (which therefore are the endpoints of some space-like vector) there is a frame in which an observer would see the events as exactly simultaneous. In other words, for any space-like vector, there is a Lorentz transformation which makes the extremes to have the same time coordinate t (and vice-versa). Therefore, proving the theorem (in a Special Relativistic setup) for all simultaneous events means having proved it for any super-luminal speed (unless I'm missing something which goes beyond Special Relativity) 89.202.228.211 (talk) 14:06, 26 July 2010 (UTC)

text: The coincidence counters are what prevents FTL communication. Those counters are needed because a quantum eraser will actually produce two sets of interference patterns phase shifted from each other. So if you superimpose those interference patterns, you get essentially the same result as you would without an interference pattern. 184.100.111.110 (talk) 17:59, 15 November 2010 (UTC)
thread_title:  Untitled 

message: This so called theorem has many problems on this wiki because there is more than one version of the quantum eraser experiment, at least three.   Each of them cannot be analyzed for FTL communication (or lack thereof) in the exact same way.  
I suggest we analyze each QE experiment first before we give reasons as to why there is or there is not a no communication theorem.  I suggest we just state the no comm theorem in this page and give no reasoning behind it.  We could even demote it to the no comm hypothesis.
Let's discuss the viability of the no comm hypothesis as we analyze all three versions of the quantum eraser experiments.  Then once a consesus is reached we can come back and give reasoning for why the hypothesis is or is not valid.
--97.104.194.126 (talk) 03:29, 5 July 2011 (UTC)
thread_title:  New Attack Plan 

message: We should go back to what the intended purpose of Wikipedia is. It is not a forum to discuss the validity of some proposed theory or to introduce new ideas that have not been widely introduced before, specially through published articles or books. 
There is a note of this on the top of this talk page referring to this page itself, but it also applies (even more) to the main Article page.
If there is such a No-Communication Theorem, who has stated or proven it?  What does no-communication mean? If someone has published such a theorem, why is it that there is no mention of the physicist's name?  
Here "no-communication" is made to mean no-faster-than-light communication. If this where the prevalent meaning attached to "no-communication", we would have to accept this, even if it were not reasonable. But it would seem more reasonable to define no-communication in quantum mechanics as the impossibility of communication between two spacially separated (not necessarily space-like) points through the use of quantum entangled systems by measurement or modification of each of the systems but excluding any other classical communication channel. But of course, we could only define it this way if had been presented so before in published articles.
If there is a published article (or several) that present a "No-communication theorem", then we would have to use the definition/s used by the author/s and mention the author/s. 
With respect to this topic, before reading it in Wikipedia I came across an article by Phillippe H. Eberhard and Ronald R. Ross titled "Quantum Field Theory Cannot Provide Faster-Than-Light Communication"
The kind of proof that Eberhard and Ross present fits the meaning of the theorem described in the main article here, even though they dont call it just "no-communication". 
With respect to the various versions of the quantum eraser experiment, I think we would have to see if the authors, or anybody else who has analyzed these experiments has presented a "no-communication" theorem in connection with them. 
If what we are talking here does not refer to a particular theorem but to a family of theorems which is in the literature referred to as "no-communication" then we state it that way. 
For the moment, I would suggest that we start contributing in this talk page with some information about the different mentions of this in the literature. I also suggest that you take a look at Eberhard's  article (which I think could be considered a theorem). Talking about theorems, There is a note on the top of this page that says that this article fits within Wiki Mathematics. I doubt it. I would classify it as only belonging to Physics.   
Alexepascual (talk) 17:36, 30 August 2012 (UTC)
thread_title:  Need for improvement 

message: I removed the following from the article:
Re operators S,T acting on hlibert spaces H_A, H_B:
:which however need not be states on the subsystems (that is non-negative of trace 1)
because it's garbled.  I changed it to the following text:
:It is not required to assume that Ti and  Si  are state projection operators: i.e. they need not necessarily be non-negative, nor have a trace of one.
However, this is a non-standard defintion of a density operator, so perhaps sigma should not be called that?? User:Linas (talk) 19:51, 17 November 2013 (UTC)
thread_title:  Misc edits 

message: The "Opposing Viewpoint" section of this article has multiple issues:
# The section sounds highly confrontational. The theorem, as stated in the article, is mathematically correct; therefore, the statement that "the proof is wrong" is not factually accurate.
# The viewpoint being expressed seems to be that certain assumptions of the proof, or their relation to terminology expressed in the informal statement of the proof, may not hold in certain experimental contexts. In this case, the author should elaborate on which specific assumptions are in question, the particular cases in which they do not hold, and the terminology they relate to.
# The example given has major stylistic issues and should be rewritten.
For the time being, I am tagging the section as misleading, for lack of a better label.
Ubershaman (talk) 16:03, 30 January 2014 (UTC)
thread_title:  Issues in "Opposing Viewpoint" 

message: Quantum states that cannot be written as a mixture of other states are called pure quantum states, all other states are called mixed quantum states. 
What is this? --Дядько Ігор (talk) 17:50, 26 August 2017 (UTC)
thread_title:  Pure state 

message: This article made me rethink the no-communication theory. Even if not used on a couple of entangled particles, it can be used on a series of particles.
Worth reading, in my opinion: https://arxiv.org/pdf/1704.02587.pdf
Do you think it is worth adding inside the article?  — Preceding unsigned comment added by Le petit poussin (talk • contribs) 23:28, 31 August 2018 (UTC)
thread_title:  The entanglement might actually be exploited for communication 

message: There is one edit that could dramatically improve the clarity of this article, IMO, and that was indicated above:
Changing the spin of system A does change the spin of system B, of course. The point of the theorem is that a scientist at lab A cannot control whether the system decoheres into spin up or spin down (in the entangled photon example- whatever analogue is applicable for other systems) along the axis of measurement. The result is that, regardless of anything the scientist in lab A does, system B has (not knowing the result of the measurement of system A) a 50/50 chance of observing spin up or spin down. A scientist at lab A can, after measuring her system, determine what system B will be measured as, but can in no way affect that measurement.
Can we have a vote as to whether to include this text, or something like it, please? It resolved my doubt about "spooky action at a distance", and may help others, too. I had imagined that a particle could be forced into a particular spin state, instead of just measured to see what its spin state was. David Spector (talk) 13:10, 6 December 2019 (UTC)
thread_title:  Please edit the article to improve clarity