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This Quantum Theory Says Time Can Flow Backwards

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Quantum physics throws all the rules of classical physics out the window. In the quantum world, particles can pass through solid walls, be in two places at once, and communicate over an infinite distance. And, if a handful of physicists are right, they can affect the past just as easily as they affect the future. That's a theory known as quantum retrocausality, and researchers have good reasons to believe it's true.

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Don't Need No Entropy To Ride This Train

One of the fundamental oddities about quantum physics is the principle of superposition, which says that a particle exists in multiple states at once until it's measured. That means that while classical physics deals in solid numbers, quantum mechanics deals in fuzzy probabilities. There's an ongoing debate, however, about why that is: is it that we just don't have the right precision to understand the true states of individual particles so they just look fuzzy, or is the reality of the quantum world that everything is fuzzy until it's measured?

In 2012, physicist Huw Price argued that if the latter is true — if the quantum world really is a soup of superposition — then it opens the door to some intriguing possibilities. Both classical and quantum don't say time can only go forward.

In theory, all physical processes can run forward and backward; it's just that in classical physics, the second law of thermodynamics puts a stop to things like time going backward and toast un-burning. But that red light doesn't apply to quantum mechanics. So if the quantum world really does involve multiple states and time can flow backward, Price says, measuring the state of a particle today could technically affect its state in the past.

Not So Spooky Anymore

If Price is right (ha!), it could be the solution to something that has puzzled physicists all the way back to Albert Einstein: quantum entanglement, which Einstein ridiculed as "spooky action at a distance." The idea is that you can link the states of two particles so that even when they're separated over an infinite distance, the state of one affects the state of the other. The problem with this is that it seems to require information to be sent faster than the speed of light, which is impossible. That's what's so "spooky" about it.

In 2017, Matthew Leifer and Matthew Pusey applied Price's idea to the puzzle of quantum entanglement. If the current state of a particle can affect its state in the past, then maybe quantum entanglement isn't action at a distance but action back in time. That is, if time can run backwards, a particle can take the action of its measurement — the event that makes it "choose" a state — back in time to when it was linked with its partner. Trading faster-than-light travel for time travel doesn't sound like much of an improvement, but when it comes to the rules of quantum physics, it solves a lot of problems.

Of course, there are huge caveats. First, this doesn't mean that a physicist could send a message to her younger self, or that you could go back in time to kill Hitler. We still live in the macroscopic world of classical physics, where entropy is still a thing. Second, this is a very fringe idea in the physics world. "There is not, to my knowledge, a generally agreed upon interpretation of quantum theory that recovers the whole theory and exploits this idea," Leifer told Phys.org. "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."

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