Question: emily said that there is a particle that can go back in time can you tell me more?

Andy Buckley answered on 8 Mar 2019:

Hmm, I don’t know exactly what was said! Usually talk of particles travelling backward in time is a reference to the maths of antiparticles. Without getting into a pile of detail, how particles “evolve” with time is related to energy x time. Whether this number is positive or negative sort-of governs whether the hands on a little clock inside the particle are going clockwise or anticlockwise: one way is for particles and the other for antiparticles. It doesn’t matter whether the minus sign for anticlockwise comes from the energy or from the time. We usually think of it the first way, but you can also think of them as particles travelling backwards in time.

It turns out that you can also get something like this by travelling faster than the speed of light, but apart from a few people who love to be wacky we don’t think that particles that do (called tachyons) that can exist. In 2011 there was a sudden burst of excitement around some experimental data suggested that neutrinos were travelling from CERN in Switzerland to the middle of Italy faster than light, but in the end it (fortunately!) turned out to be a problem with some measurement equipment.

I say fortunately because faster than light stuff would imply that our understanding of the basic structure of space and time is wrong. Which would be quite exciting, but I guess would throw our whole field into chaos for years! It also comes with the teeny problem that the equation making all those little clock hands turn would also transform into one where the whole universe decays in the blink of an eye… which doesn’t seem to be happening, so if tachyons do exist, they don’t work in the most obvious way.

Phew, I hope that was slightly understandable!!