Spooky Action: EPR vs. breach of Bell’s inequality

[tranx]

I think I understand that Aspect, and later The Gisin Team in Geneva, showed that Bell's inequality was violated and that therefore there were no EPR ‘hidden variables’. However, and according to Relativity, it seems that ‘messages’ could have been sent between the entangled particles, without invoking an especially ‘spooky’ interaction

 

Supposing the detectors were separated by a light year when one of the two randomly positioned detectors detected a particle:- As follows it might seem that the other detector could simultaneously take an equivalent reading when a light-speed message travelled between them, while no time would pass for the message on the trip

 

From the point of view of an observer near the central emitter, it would take 6 months before he could ‘see’ the particle being detected by both detectors, and from the point of view of an observer near a detector it would take 1 year before he could ‘see’ the other detector taking its reading. It would appear to him to have taken just a year for the light to arrive from other detector i.e. a light-speed message could arrive at a detector, from the other, at the same moment it made its own measurement

 

So, just relying on Relativity, perhaps it could be possible for some sort of light-speed message to influence an entangled particle at any distance without the need to regard it as being especially ‘spooky’

 

How is this wrong please?

[BenTheMan]

It would appear to him to have taken just a year for the light to arrive from other detector i.e. a light-speed message could arrive at a detector, from the other, at the same moment it made its own measurement

 

I don't quite understand this.

 

Let me try to start from the beginning. Two detectors one light year apart are both set up to measure an EPR pair. Both detectors measure the spins of the particle (presumably at the same time). Detector A sends a message to Detector B and vice versa.

 

So how can A possibly receive B's message at the same time it makes the measurement? What am I missing?

[tranx]

So how can A possibly receive B's message at the same time it makes the measurement? What am I missing?

You are not missing anything but apologies and thanks again, I see that my last post did not make any sense at all

 

And I see that a message travelling at the speed of light could 'think' it was everywhere in its path at once, since no time would seem to pass for it between B and A if it travelled at c., but to an observer at A it would still seem to take a year - a bit late to make any difference indeed!

[BenTheMan]

no worries.

[Norman Albers]

Do they need to measure at the same time in their common frame? What if one is further?

[tranx]

Do they need to measure at the same time in their common frame? What if one is further?

I think this sort of thing is mentioned in in: http://www.mtnmath.com/whatth/node62.html : a later paragraph discusses whether one or other detection can be considered to 'cause' the collapse of the state of uncertainty in both

 

I think the point is that, for a particular pair, it would not matter when, or at what distance from the source, one or other was detected because they are supposed to be entangled anyway. 'if one is further' it would be detected 'later', but supposedly its state would have already collapsed at the moment the earlier detection occurred. Vice versa would seem untestable but I think it has been demonstrated in double slit experiments

[Norman Albers]

Thanks for the good source, tranx. It is more clear than some others I've seen so I will chew on it. Clearly precise distance cannot matter if "precise" means "same number of wavelengths".

[webplodder]

Thanks for the good source, tranx. It is more clear than some others I've seen so I will chew on it. Clearly precise distance cannot matter if "precise" means "same number of wavelengths".

 

 

Norman, In 1935 Einstein and two other physicists in the United States, Boris Podolsky and Nathan Rosen, analyzed a thought experiment to measure the position and momentum in a pair of interacting systems or 'entangled' particles, if you like.

 

These entangled particles can be sent off in opposite directions to anywhere in the universe.

 

The proton, like the electron, has 1/2 a 'spin' and no matter what direction is chosen for measuring the spin, or angular momentum, the values are always + in one direction and - in the opposite direction. This means, therefore, the overall aggegate spin is 0 (+ x -x = 0) right? It turns out that when you measure the spin of, say, of a photon in one direction it's entangled 'twin', which in theory could be seperated at any distance anywhere in the universe, 'knows' it's first twin has been measured in a particular direction and automatically 'becomes' measured in the opposite direction instantaneously without worrying about the speed of light.

 

Einstein and his two collaborators thought that this conclusion was so obviously false that the quantum mechanical theory on which it was based must be incomplete. They concluded that the correct theory would contain some hidden variable feature that would restore the determinism of classical physics.

 

However, in 1964, John Bell showed, by using a statistical argumant, that no hidden variables could possibly be involved and that non-local effects as described above were in fact part of reality despite appearing to violate the normal laws of nature.

 

So far, there are no satisfactory answers to these questions, although there are several schools of thought from the role of the observer to the collapse of a wave function, to the 'many worlds' interpretation of quantum mechanics.