Retrocausation?

It says something about me that I take breaks from revising for my Biochemistry finals by reading about quantum mechanics. Exactly what it says I'm not sure. :|

John Cramer has published an update on his progress to try to demonstrate retrocausal signalling. I'll summarise in a few sentences for those who don't have time to read the article or prefer a less technical phrasing:

Quantum mechanics allows non-local signalling between entangled particles. This essentially means that an interaction with one particle that changes its state can change the property of its entangled partner at arbitrary distance, instantly (not just at light speed). The 'reality' of this phenomenon is disputed but there is a great deal of experimental evidence that it exists.

If we take two entangled photons and pass one into a long optical fibre, we can delay the arrival of the second photon at its destination by a few microseconds. We detect both photons, but the delayed photon is detected in a specific fashion that changes its state - the non-delayed photon is detected without forcing it to take any particular state.

If the quantum prediction holds, the two photons will always be in the state induced by the measurement of the delayed photon, even though several microseconds passed between the first 'free' detection and the delayed detection.

So what?

Cramer's paper is a progress report and doesn't speculate about applications of the phenomenon if it's demonstrated to be possible, but any number of science fiction authors have considered possible 'future-scope' devices, some more credible than others. I have a feeling Greg Egan wrote one such story, but I don't recall in which collection.

The most obvious application is a device for 'signalling back in time'; by delaying one entangled particle for longer than a few microseconds (this is hard, of course, without breaking the entanglement - the same problem is encountered in quantum computing - but not impossible), i.e. for minutes or hours, we can then 'immediately' receive information on what the state of the future will be. This throws up all sorts of interesting potential paradoxes (although it may simply deal a final decisive blow to the concept of free will).

Spooky Sensing at a Distance

Slightly less obvious is the idea of using the phenomenon as a sensor - fire one photon at a distant object (like an extrasolar planet), and observe how the entangled partner changes. This should in principle reveal information about the 'target', again 'instantly' - even though the sensor photon takes subjective time to reach the target. Only certain kinds of information could be retrieved, but astronomers are very good at making sense of sparse data.

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More obscurely - and perhaps most interestingly, although I have a niggling feeling it may prove to be impossible - one could in theory perform long computations 'without actually performing them, by 'sending the answer back' to the beginning of the computation. If that's really true, then a suitable computer can perform any finite-length computation 'instantly'.