Welcome to Gaia! ::

If his experiment with splitting photons actually works, says University of Washington physicist John Cramer, the next step will be to test for quantum "retrocausality."

That's science talk for saying he hopes to find evidence of a photon going backward in time.

...

"It doesn't seem like it should work, but on the other hand, I can't see what would prevent it from working," Cramer said. "If it does work, you could receive the signal 50 microseconds before you send it."

Uh, huh ... what? Wait a minute. What is that supposed to mean?

Roughly put, Cramer is talking about the subatomic equivalent of arriving at the train station before you've left home, of winning the lottery before you've bought the ticket, of graduating from high school before you've been born -- or something like that.

....

Basically, the idea is that interacting, or entangled, subatomic particles such as two photons -- the fundamental units of light -- can affect each other no matter how far apart in time or space....This is where going backward in time comes in. If the entanglement happens (and the experimental evidence, at this point, says it does), Cramer contends it implies retrocausality. Instead of cause and effect, the effect comes before the cause. The simplest, least paradoxical explanation for that, he says, is that some kind of signal or communication occurs between the two photons in reverse time.

...

For the first phase of the experiment, to be started early next year , they will look for evidence of signaling between the entangled photons. Finding that would, by itself, represent a stunning achievement. Ultimately, the UW scientists hope to test for retrocausality -- evidence of a signal sent between photons backward in time.

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.

That is what quantum mechanics predicts should happen. And if it does, signaling would have gone backward in time relative to the first photon.

Link

That said, does anyone know of a theoretical reason to have causality? From what I know, we insert it by hand — e.g. in QFT you use a Dirac delta function to set the effect of one point in a field to be zero if it is outside the lightcone [there is no casual way for you region to know anything about that point, so it cannot have an effect]

About your shoes: Wouldn't the analogy be something like you put the shoes in the boxes, and sent them to the two places, and before the shoes arrived, the people that were receiving them already knew what they were getting, without being told?

If i'm completely wrong on this, just ignore me. I thought I knew a decent amount, and I came in here and I feel like a 6th grader.