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entirelyuseless comments on The Philosophical Implications of Quantum Information Theory - Less Wrong Discussion

5 Post author: lisper 26 February 2016 02:00AM

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Comment author: lisper 02 March 2016 06:02:07PM -1 points [-]

So I read the paper, and it is kind of a cool experiment, but it does not show that "future choices can affect a past measurement's outcome." Explaining why would require a separate article (maybe time to re-open main!) But the TL;DR version is this: if you want to argue that A affects B then you have to show a causal relationship that runs from A to B. If you can do that, then you can always come up with some encoding that will allow you to transmit information from A to B. That's what "causal relationship" means. But that is (unsurprisingly) not what Aharonov et al. have done. They have merely shown correlations between A and B, and then argue on purely intuitive grounds that there must have been some causal relationship between A and B because "Bell's theorem forbids spin values to exist prior to the choice of the orientation measured." While this is true, it's misleading because it implies that spin values do exist after a strong measurement. But that is not true. There is no fundamental difference between a strong and a weak measurement. There is a smooth continuum between weak and strong measurements, and at no point during the transition from weak to strong does the spin value begin to "actually exist" (a.k.a. wavefunction collapse).

Comment author: entirelyuseless 02 March 2016 10:03:10PM 0 points [-]

I disagree: if you interpret EPR experiments as wavefunction collapse rather than many worlds, then you can conclude that either one measurement affects the other, or both affect each other. But you cannot come up with any encoding that will allow you to transmit information.

Comment author: lisper 02 March 2016 10:36:33PM 0 points [-]

Yes, of course that's true. But collapse is only an approximation to the truth. It is a very good approximation in many common cases. But the Aharonov experiment is interesting precisely because it is a case where collapse is no longer a good approximation to the truth, and so of course if you view it through the lens of collapse things are going to look weird. To see why collapse is not always a good approximation to the truth, see the references in the OP.