Two different groups tested a seemingly illogical property of the quantum world, namely the theory that it is possible to place a photon (particle of light) in an overlapping of states traveling back and forth in time (superposition). This theory does not suggest time travel and will not allow communication with the past. However, it can be considered as an interesting demonstration of how time can be thought of at the quantum level.
Unless you have a TARDIS or a DeLorean (at least the version in the movie) for us, time only flows in one direction (forward). This little truth that protects us from any paradox is called the arrow of time. It’s thought to be related to the concept of entropy (always increasing in an isolated system like the universe), but it doesn’t seem like such a fundamental concept at the quantum level.
Instead, a concept called CPT symmetry (charge, parity, and time reversal symmetry) seems to be fundamental. This applies to all physical phenomena, and if a combination of the two is violated (like the famous CP violations), there must be a violation of time symmetry as well.
This has some fun theoretical applications. For example, it is possible to think of an electron traveling forward in time as a reversed positron (antimatter equivalent) traveling back in time. While this concept works mathematically, it’s not like the little antimatter we see in the universe is going Back to the Future.
Recent research has suggested that it is possible for photons to exist in superpositions of forward and backward states in time. And now, this idea has been proven experimentally. Superposition is a quantum phenomenon in which a particle is in more than one state at the same time. The particle only settles in one of these positions when measured.
In one of the experiments, a photon was placed on a superposition of processes while moving through a crystal. Because photons have no charge, by changing the parity (reflecting its motion in 3D) an equivalent behavior can be created for a photon traveling back in time. The process, called quantum time transformation, was demonstrated during testing.
Papers discussing these experiments are awaiting peer review and can be found on ArXiv.
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