Physicists have confirmed the ultimate speed limit for the packets of light called photons - making time travel even less likely than thought.
The speed of light in vacuum is the Universe's ultimate speed limit, but experiments in recent years suggested that single photons might beat it.
If they could, theory allows for the prospect of time travel.
Now, a paper in Physical Review Letters shows that individual photons too are limited to the vacuum speed limit.
That means that photons maintain the principle of causality laid out in Einstein's theory of special relativity - that is, an event's effect cannot precede its cause by traveling faster than light. It is violation of this causality that would, in principle, permit time travel.
While the limit in vacuum is a fixed number - some 300,000km per second - the speed of light can vary widely in different materials.
These differences explain everything from why a straw looks bent in a glass of water to experiments in cold gases of atoms in which light's speed is actively manipulated.
Some of those experiments showed "superluminal" behaviour, in which photons travelled faster than the speed of light in a given medium.
It remained, however, to determine whether or not individual photons could exceed the vacuum limit