Retrocausality

[Cross-posted and edited lightly from the Kentucky Satellite blog]

Alan Boyle at Cosmic Log offers more detail on experiments in backward causality that have recently been funded. Related to Albert Einstein's notion of quantum entanglement, the basic idea is that an effect might be observed before its cause. Cosmic Log:

Boiled down to its basics, the experiment involves splitting laser light into two beams, so that characteristics of one beam are reflected in the other beam as well. That's an example of what physicists call quantum entanglement. Specifically, [University of Washington physicist John] Cramer has been planning to fiddle with one of the entangled laser beams such that it takes on the property of waves or particles. If one beam behaves like particles, the entangled photons of light in the other beam should behave like particles, too.

Should that experiment yield the expected results, Cramer plans another that would send the split laser along two separate paths, one of which is much shorter. Again, Cosmic Log:

... [Cramer] would send one of the entangled beams (call it Signal A) through a circuitous detour - say, a few miles of fiber-optic cable - then fiddle with it when it came out of the cable. If the principles behind nonlocal communication held true, the evidence of that fiddling should be detected at a corresponding place in the other entangled beam (call it Signal B).

Now brace yourself for the backward-causality part: Because Signal B followed a shorter route to its detector, the fiddling in Signal A could theoretically show up in Signal B before Cramer actually fiddles with Signal A. It would be as if Cramer's actions had an effect that worked backward in time.

Story tip: Centauri Dreams. To get the Full Monty on these concepts, see the two links below. The second link begins with an important distinction between time travel and backward causality.

Wayne

Wikipedia: quantum entanglement, Standford Encyclopedia of Philosophy: Backward Causation