Two different groups have tested a seemingly counter-intuitive property of the quantum world: That it’s possible to put a photon, a particle of light, in a superposition of states going forward and backward in time. This is not time travel and won’t lead to communicating with the past – but it is an intriguing demonstration of how time can be thought to work at a quantum level.
Unless you have a TARDIS or a DeLorean, time only flows in one direction (forward) for us. This annoying little fact that protects us from all sorts of paradoxes is called the arrow of time. It is believed to be related to the concept of entropy (which always increases in an isolated system like the universe) but it doesn’t seem to be as fundamental at the quantum level.
Instead, something that appears to be fundamental is the so-called CPT symmetry (charge, parity, and time reversal symmetry). This holds for all physical phenomena, and if a combination of two of them is violated (such as famously the CP violations) there ought to be a violation in time symmetry as well.
This has some fun theoretical applications, for example where an electron moving forward in time can be thought of as a mirrored positron (the antimatter equivalent) going back in time. Mathematically, it works – but it’s not like the little antimatter we see in the universe is doing a Back to the Future.
Recent research suggested that it was possible to have photons in a superposition of states backward and forward in time. Now, that idea has been demonstrated experimentally. Superposition is a quantum phenomenon where a particle is in multiple states at once. Only when it is measured does the particle settles into one of them.
In one of the experiments, a photon was placed on a superposition of processes as it moved through a crystal. Photons have no charge, so by changing parity (mirroring its motion in 3D) it is equivalent to the behavior of a photon moving back in time. The process called quantum time flip was demonstrated.
Again, the work doesn’t allow us to change the past. However, it might have some interesting applications in quantum computers, especially when it comes to logical operations, the crucial architecture of computer processes. The ability to have quantum time flips could allow for better performances and switching inputs and outputs.
The papers discussing these experiments are awaiting peer review and are available on the ArXiv here and here.
[h/t: New Scientist]