Retrocausality, Space-Time, and Photon Entanglement

Cause and effect is pretty easy to understand. I pick up a remote control, press the power button and my TV turns on. Me pressing the power button is the cause, the TV turning on is the effect. We all learned about it in grade school.

But what if that were reversed? What if my TV turned on, and that caused me in the past to press the power button on the remote control? It goes against everything we think we know about the way the universe works, but that is the essence of the theory of retrocausality. The effect creates the cause. It sounds like something straight out of the pages of science fiction, but many scientists today believe that retrocausality could be a real phenomenon.

Space-Time

In 1895 HG Wells published his classic science fiction novel The Time Machine. Although Wells provided little details in the actual functionality of his fictional machine, his main character, known to the reader only as the Time Traveller, provided a basic theoretical concept which made his time travel possible. He posited that time existed as a fourth dimension no different than height, width and depth, the three dimensions we are already familiar with. Just as I can walk forward or backward, move from side to side or climb up and down a ladder in these three dimensions, so can I move forward and backward through the dimension of time.

Albert Einstein, in his famous writings on relativity, somewhat validated this view of time. He referred to Space-Time, where time is included as a fourth dimension and is inseparable from the three dimensions of space. We cannot separate one from the other.

Does that mean Albert Einstein thought time travel was possible? Well, not exactly. However his revolutionizing of the way we view the universe, and perhaps even more the later development of quantum theory and quantum mechanics (much of which Einstein rejected), have made retrocausality seem theoretically possible, at least to some.

Photon Entanglement

One of the hotly debated topics of quantum mechanics over the past several decades has been the idea of photon entanglement. Photons are the basic building blocks of light, a very unusual aspect of our reality because they can be viewed both as a particle (matter) or as a wave (energy). The theory of photon entanglement only makes photons all the more strange.

The idea is that two photons (or more) can become entangled with each other, meaning that what happens to one photon effects what happens to the other photon. If one photon goes up, the other goes down to balance it out instantaneously. This isn't such a hard thing to buy on the surface, but the problem arises when you realize that it doesn't matter how far apart entangled photons are for this effect to occur: they could be on opposite sides of the universe and both would still effect the other instantaneously.

Einstein, in fact, rejected this idea wholesale, because it meant that information could be communicated faster than the speed of light. As we all know, Einstein's theories on relativity showed that the speed of light was constant and that it was impossible for anything to move faster than that speed. However, numerous experiments have been done that show that this photon entanglement is in fact a real phenomenon. How, then, can we explain the relationship?

Experiments in Retrocausality and Photon Entanglement

One of the ideas that have been proposed in solving the seeming paradox of how photon entanglement operates is that of retrocausality. The idea is that there is a physical signal being sent between two photons that are entangled which allows them to operate as they do, but it's not going faster than the speed of light: it's going backwards in time.

John W. Cramer, a physicist at the University of Washington in Seattle, came up with this theory back in the 1980's and is still its primary proponent. Now he is planning to put his theories to the test.

Cramer's first test is fairly fundamental, and is based on work that has already been done by other scientists. Basically an ultraviolet laser is shot into a crystal, and out pop two entangled photons. These photons are then sent through a split screen detector to detect the photons as either waves are particles. This sort of experiment has been done before, and the effect is that if one photon is detected as either a wave or a particle, the other photon must be detected as a wave or a particle as well. Cramer's primary purpose in this is not to prove the effect of the entanglement, which has already been done, but to try to find evidence of a signal being sent between the two photons: something that is as of yet unproven.

It is the next phase of his experiments which hold the most interest for retrocausality. Once again he will shoot an ultraviolet laser into a crystal to produce two entangled photons. One of those photons will be sent immediately through a split screen, as in the first experiment, but the other photon will be sent through two 10 km lengths of fiber-optic cable before going through its own split screen.

By sending the second photon through 20 km worth of fiber optic cable, a 50 microsecond (one millionth of a second) delay between the two photons. When that second photon is detected as either a wave or a particle, it will force the other photon to be detected as either a wave or a particle. However, the other photon had already been detected 50 microseconds earlier, meaning that the effect had been witnessed before the cause: retrocausality.

Will the experiment work? Even Cramer has his doubts. If it succeeds it will be the first ever experimental evidence that the present can effect the past and the future can effect the present. If retrocausality is proven it will fundamentally change the way that we understand time.