Time, as far as we know, moves only in one direction. But last year, researchers found events in some gamma-ray burst pulses that seemed to repeat themselves as though they were going backwards in time.

Now, new research suggests a potential answer for what might be causing this time reversibility effect. If waves within the relativistic jets that produce gamma-ray bursts travel faster than light — at ‘superluminal’ speeds — one of the effects could be time reversibility.

Such speeding waves could actually be possible. We know that when light is travelling through a medium (such as gas or plasma), its phase velocity is slightly slower than c — the speed of light in a vacuum, and, as far as we know, the ultimate speed limit of the Universe.