The quantum world is full of phenomena scientists are still largely grappling with on a theoretical level. One such quantum theory is quantum entanglement. Although there are a number of tests that demonstrate what Einstein called “spooky action at a distance,” — many merely assume that it happens, without being able to explain how. At least not yet. But two physicists have proposed an alternative that might just be able to explain this quantum effect.
Essentially, quantum entanglement assumes that measurements of quantum properties within the state of one entangled particle occurs simultaneously with its entangled pair, regardless of how far apart they are. There isn’t any known mechanism that would explain that kind of influence, though, which is why physicists Matthew S. Leifer at Chapman University and Matthew F. Pusey at the Perimeter Institute for Theoretical Physics have offered an alternative: the team has asserted the idea of “retrocausality” as a possible explanation for this “spooky action.” Their findings were published in the journal Proceedings of The Royal Society A in June.
“There is a small group of physicists and philosophers that think this idea is worth pursuing, including Huw Price and Ken Wharton [a physics professor at San José State University],” Leifer told Phys.org. “There is not, to my knowledge, a generally agreed upon interpretation of quantum theory that recovers the whole theory and exploits this idea. It is more of an idea for an interpretation at the moment, so I think that other physicists are rightly skeptical, and the onus is on us to flesh out the idea.”
Simply stated, retrocausality assumes that influences can travel backwards in time. When an experimenter decides how to measure a particle, that choice can influence the properties of that particle — or, an entangled particle in the past. This, therefore, makes the “action at a distance” part of Einstein’s definition unnecessary. Instead, the entanglement effect becomes retrocausal influence. That being said, it’s not the same thing as sending signals back in time.
Retrocausal theory, then, could offer a better quantum theory. “The only options seem to be to abandon realism or to break out of the standard realist framework,” Leifer explained. “Abandoning realism is quite popular, but I think that this robs science of much of its explanatory power and so it is better to find realist accounts where possible.” Retrocausality entails a number of assumptions, though: including one that reformulates the idea of time symmetry.
At any rate, Leifer and Pusey think that retrocausality can offer a generalized standard quantum theory. “This might be needed to construct the correct theory of quantum gravity, or even to resolve some issues in high-energy physics given that the unification of the other three forces is still up in the air in the light of LHC results,” Leifer added. Perhaps it could even help improve quantum computing technology.
Needless to say, as is the case with most everything in the world of quantum physics, the work is largely theoretical. “As far as direct experimental tests of retrocausality go, the status is not much different from other things in the foundations of quantum mechanics,” Leifer said. “We never test one assumption in isolation, but always in conjunction with many others, and then we have to decide which one to reject on other grounds.”