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OK, a look at Wikipedia says this experiment can't cause FTL communication, and certainly not backward-in-time communication.

It seems you somehow can't actually read the results from the signal detector until after the idler photon has reached its detector. Can't quite work out why, but maybe its because you need multiple photons to form an inteferance pattern.

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It seems you somehow can't actually read the results from the signal detector until after the idler photon has reached its detector. Can't quite work out why, but maybe its because you need multiple photons to form an inteferance pattern.



Well, I may have this wrong as it is very confusing, but the photon that reaches its detector first (and this is designed to always happen) will always show an interference pattern since it has not, at that point, any information as to which slit it went through.

After the idler photons have been detected we seem to have the situation where there is both information about which slit the original photons went through and also no information about this (according to which set of detectors is checked).

My understanding is (and I may be wrong about this) that the experimenter now has the choice of whether to keep the information about 'which-path' (the true path of the original photon, thus telling us which slit it went through) or erasing it. If the true-path info is kept then the signal photon will automatically change its orientation to a clump pattern, representing a particle like aspect. It will do that because now, information has been made available indicating which slit the original photon went through. On the other hand, destroying this information will simply mean the signal photon displays the interference pattern and behave like a wave since no information is available any longer.

Wiki is very conservative and tends not to express conclusions that seem unscientific, however, nobody has really explained this phenomenon to date, although many have tried.


Confused? You will be!!

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Originally Posted By: abacus9900

the signal photon will automatically change its orientation to a clump pattern, representing a particle like aspect. It will do that because now, information has been made available indicating which slit the original photon went


But it can't change because it's only one photon that only hits the detector once, then it's gone.

Regardless of Wikipedia, as long as this experiment has actually been performed, then it's not communicating FTL because that's still accepted as impossible.

Sure you might make some interpretation of what's happening and it somehow appears to be FTL. But at the end of the day, there's no way you can rig the machine to actually use as a telegraph to transmit data FTL.

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But it can't change because it's only one photon that only hits the detector once, then it's gone.



That is the paradox. How can a photon that has already been detected as being a wave change to a particle later on depending on the whim of a human being?


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Regardless of Wikipedia, as long as this experiment has actually been performed, then it's not communicating FTL because that's still accepted as impossible.



Well, the same principle operates on two entangled particles who could be at opposite ends of the universe. Although no information as such can pass between them one of the 'twins' is immediately aware of any measurement made on the other. The one that has not had some property measured on it directly will instantaneously take up a complimentary position of the property measured on the other one. It's a kind of symmetry. Somehow there is 'spooky action at a distance' taking place outside the normal relativistic constraints of space time, i.e. greater than luminal speed.


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Sure you might make some interpretation of what's happening and it somehow appears to be FTL. But at the end of the day, there's no way you can rig the machine to actually use as a telegraph to transmit data FTL.




No, but then these effects might be taking place outside of our normal concept of space and time, I really don't know. Certainly, this is the basic principle underpinning quantum computing, i.e. many parallel computations taking place over superimposed particles. David Deutsch sites this idea as supporting the idea of 'many worlds.'

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Originally Posted By: K
Retro-causailty!!! How can that not be hard to accept?


Of course it is hard to accept. At least it is for me, but the more P Sci. books I read, the more I wonder if that is just my poorly informed personal prejudice. I can usually find arguments to support my view, but that could be because I "don't have the math"


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Originally Posted By: abacus9900
No, but then these effects might be taking place outside of our normal concept of space and time,


Kallog will not like this, because it points towards a physical infinity, but if we are looking at photons which travel at "c", then they can be at every part of the experimental set-up at the same time, in their own F of R; so there is no question of FTL communication or retro-causality; only the appearance of such in the F of R of the observer.


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Originally Posted By: abacus9900

Although no information as such can pass between them one of the 'twins' is immediately aware of any measurement made on the other. The one that has not had some property measured on it directly will instantaneously take up


Yes, and I suppose it's amazing if you understand it well enough. But the overall effect is very non-amazing. If you didn't know how it worked, you'd just say "oh well when the entangled pair was together they agreed on what result to produce if ever either of them was observed."

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Originally Posted By: K
Yes, and I suppose it's amazing if you understand it well enough. But the overall effect is very non-amazing. If you didn't know how it worked, you'd just say "oh well when the entangled pair was together they agreed on what result to produce if ever either of them was observed."


If you regard QM as a "window" into the infinite; entanglement becomes understandable. It is no stranger than talking about needing to involve other dimensions in order to make sense of observations; especially if we then have to accept that these dimensions are rolled up "infinitely small".

If the quons, in their F of R, exist in infinity, there is never any question of physical separation between them, either in space or time. Instantaneous communication is always possible, because the separation is only in the F of R of the observer.


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