Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Does this recent Zeilinger group delayed choice entanglement experiment imply backward-in-time influences? http://arxiv.org/abs/1203.4834
From the abstract: "This can also be viewed as “quantum steering into the past ”."

share|improve this question
add comment

3 Answers

No, there is no retrocausal causation in the delayed choice entanglement swapping experiment (or any other experiment or process in the Universe, for that matter), see

http://motls.blogspot.com/2012/03/has-anton-zeilinger-created-time.html

for a detailed explanation. Correlations between Alice, Bob, and Victor's outcomes may obviously be verified only after all of them complete their measurements.

But one may easily prove – by a simple application of the completeness relation i.e. independence of some inner product on the choice of bases – that if Alice and Bob measure their particles before Victor, their outcomes (the probabilistic distributions governing them) will be totally unaffected by Victor's later decisions. For this reason, the cause behind correlations in any group – for example the group of four photons in this experiment – is always hiding in the fact that these components have been in contact in the past. Correlation isn't causation, at least not a randomly picked causation, and there's never any causation between spacelike-separated events and there's never any causation that would go backwards in time, either. Quantum mechanics fully respects this statement.

Also, Heisenberg's equations make it totally obvious that the degree of entanglement between two isolated subsystems isn't changing with time. In fact, this experiment is really designed so that the polarizations of any of the photons don't change with time at all after the photons are produced in the special four-photon initial state. This makes it obvious that the results of the measurements don't depend on the timing of the measurements and it makes it silly to be surprised by this independence on the timing.

In all such bizarre nonlocal or retrocausal interpretations of quantum measurements, the error hides in the people's attempts to create a "classical model" which already possesses objective properties before they are measured. But there's no valid classical model that would coincide with the Universe, a system that respects inequivalent, quantum rules. One must carefully wait for the moment of the measurements but whenever we learn the results of the measurements, the most accurate predictions of probabilities of new, future experiments has to take all the previously measured outcomes into account; we must switch to the appropriate conditional probabilities which some people materialistically interpret as "collapsing wave function".

So if Victor does the experiment after Alice and Bob and he wants the most accurate predictions, he must "collapse the wave function" for his photons according to the results obtained by Alice and Bob. If he doesn't know these results, he won't be able to collapse them and he will only predict things according to the initial distribution. Whether he knows something or not, quantum mechanics unambiguously predicts that there would be correlations between the outcomes obtained by Alice, Victor, and Bob and no retrocausal influences are needed in quantum mechanics to produce these – experimentally verified – predictions.

share|improve this answer
1  
Interesting and informative answer. The very last sentence in the paper is this: "Some registered phenomena do not have a meaning unless they are put in relationship with other registered phenomena." I like that because it sums up the situation pretty nicely, and not that differently from the situation with space-separated effects where you can't see the need for spooky interactions until you correlate the final events. –  Terry Bollinger Mar 24 '12 at 8:26
    
"backwards in time" is this another name for reversing the order of events compared to "forward in time"? –  Physiks lover Sep 11 '12 at 11:05
    
Yes, "backwards in time" means that the cause (an event) takes place after its consequence (another event). –  Luboš Motl Apr 6 '13 at 6:41
add comment

"...and there's never any causation between spacelike-separated events" The integral form of the Schrödinger equation is over all space. If there is no action at a distance, then AFAICS it is wrong. My understanding is that this was the basis of Einstein's issue with QM and he (and others) designed the EPR experiment to prove QM was incorrect but when it was performed it only confirmed QM.

share|improve this answer
2  
Why on earth should the fact that one can integrate over all space imply that there is "action at a distance?" Care to share this particular "integral form" of the Schrodinger equation that necessitates such a conclusion? It does not matter whether one is expressing physical laws as derivatives or integrals. What matters is whether the Lagrangian/Hamiltonian is Lorentz covariant, and the Standard Model is. One can cook up effective theories that appear to violate this (i.e. take a nonrelativistic limit) but it is just an illusion of low speed where we cannot see the time delay in propagation. –  Doug Packard Sep 11 '12 at 9:37
add comment

A pretty good non-technical summary of the experiment and its possible interpretations is given by Chad Orzel here:

Entangled In the Past: “Experimental delayed-choice entanglement swapping”

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.