Retrocausality Experiment

University of Washington physicist John Cramer has devised a an experiment to see if an entangled photon can signal its state backwards in time. If it works, a signal could be recieved before it is sent.

In that final phase, one of the entangled photons will be sent through a slit screen to a detector that will register it as either a particle or a wave -- because, again, the photon can be either. The other photon will be sent toward two 10-kilometer (6.2-mile) spools of fiber optic cables before emerging to hit a movable detector, he said.

Adjusting the position of the detector that captures the second photon (the one sent through the cables) determines whether it is detected as a particle or a wave.

The trip through the optical cables also will delay the second photon relative to the first one by 50 microseconds, Cramer said.

Here's where it gets weird.

Because these two photons are entangled, the act of detecting the second as either a wave or a particle should simultaneously force the other photon to also change into either a wave or a particle. But that would have to happen to the first photon before it hits its detector -- which it will hit 50 microseconds before the second photon is detected.