Re:When Time Flows Backwards
by tjradcliffe on Sunday February 15, 2015 @02:51AM
(#9052041)
It is worth emphasizing that weak measurements only (and can only) apply to ensemble averages. They work by making weak measurements on many copies of the same system (or equivalently, many identically-prepared systems) and then drawing inferences about "the system" from this. But it is not, in fact "the system" but the average over many copies of a system.
Bohr would likely have made a big deal over this, because there is no actual measurement on any single system that corresponds to the retrodicted inference, and that matters a great deal in Bohr's quantum ontology.
Measurement in QM is not an all-or-nothing process (and hence there is no such thing as "wave function collapse"). You can measure "just a little bit" by carefully controlling the entanglements of the system you are "measuring". These are called "weak measurements". When you make weak measurements, the probabilities of various outcomes can be non-intuitive because of how the quantum math shakes out, similar to the Bell inequality, except that the entanglements in this case are across time instead of across space. It's cool experimental work, but there's nothing fundamentally new here. It was already well known that the quantum wave function is timeless. This experiment just confirms it.