From wikipedia: Retrocausality (also called retro-causation, backward causation and similar terms) is any of several hypothetical phenomena or processes that reverse causality, allowing an effect to occur before its cause.
According to Paul March from a Talk Polywell comment: Dr. Cramer’s retrocausal experiment should be completed by the end of this year. And if verified it would buttress Dr. Woodward’s M-E (Mach Effect) arguments and provide a path to finally merging GRT (General relativity) with QM (Quantum Mechanics).
Entanglement: The separated but “entangled” parts of the same quantum system can only be described by referencing the state of other part.
The possible outcomes of measurement M2 depend of the results of measurement M1, and vice versa. This is usually a consequence of conservation laws.
Nonlocality: This “connectedness” between the separated system parts is called quantum nonlocality. It should act even of the system parts are separated by light years. Einstein called this “spooky actions at a distance.”
* A series of EPR experiments, beginning with the 1972 Freedman-Clauser experiment, have demonstrated convincingly that measurements performed on one of a pair of polarization-entangled photons affect the outcome of measurements performed on the other entangled photon.
In quantum mechanics, the EPR paradox (or Einstein–Podolsky–Rosen paradox) is a thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities and the values that can be accounted for by a physical theory. “EPR” stands for Einstein, Podolsky, and Rosen, who introduced the thought experiment in a 1935 paper to argue that quantum mechanics is not a complete physical theory
The EPR paradox draws on a phenomenon predicted by quantum mechanics, known as quantum entanglement, to show that measurements performed on spatially separated parts of a quantum system can apparently have an instantaneous influence on one another.
This effect is now known as “nonlocal behavior” (or colloquially as “quantum weirdness” or “spooky action at a distance”).
Before delving into the complicated logic that leads to the ‘paradox’, it is perhaps worth mentioning the simple version of the argument, as described by Greene and others, which Einstein used to show that ‘hidden variables’ must exist.
A positron and an electron are emitted from a source, by pion decay, so that their spins are opposite; one particle’s spin about any axis is the negative of the other’s. Also, due to uncertainty, making a measurement of a particle’s spin about one axis disturbs the particle so you now can’t measure its spin about any other axis.
Now say you measure the electron’s spin about the x-axis. This automatically tells you the positron’s spin about the x-axis. Since you’ve done the measurement without disturbing the positron in any way, it can’t be that the positron “only came to have that state when you measured it”, because you didn’t measure it! It must have had that spin all along. Also you can now measure the positron’s spin about the y-axis. So it follows that the positron has had a definite spin about two axes – much more information than the positron is capable of holding, and a “hidden variable” according to EPR.
* It is now well established that quantum nonlocality really does “connect” the separated parts of the same quantum mechanical system (c.f. Freedman-Clauser, Aspect, etc.)
* There are several “No-Signal Theorems” in the literature (c.f. P. Eberhard, A. Shimony, …) showing that quantum nonlocal signaling is impossible, e.g., a change on one measurement has no observable effect on the other, in the absence of coincidence links.
* However, Peacock and Hepburn have argued that these “proofs” are tautological and that certain key assumptions (e.g., measurements are local) are inconsistent with the quantum formalism (e.g., Bose-Einstein symmetrization).
Therefore, the question of nonlocal signaling remains “open” (at least a crack) and should be tested.
Status of the UW Test of Nonlocal Quantum Communication with Momentum-Entangled Photon Pairs
The Cramer Symposium was just held on Sept 10-11, 2009 where there were several interesting presentations listed and linked from this program. This included an update on the Cramer retrocausal experiments.