Image copyright Other Image caption The speed of light in extremely cold gases of atoms can be varied widely

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 travelling 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.

All relative

Now, Shengwang Du and colleagues at the Hong Kong University of Science and Technology have measured what is known as an optical precursor.

Like the wind that moves ahead of a speeding train, optical precursors are the waves that precede photons in a material; before now, such optical precursors have never been directly observed for single photons.

By passing pairs of photons through a vapour of atoms held at just 100 millionths of a degree above absolute zero - the Universe's ultimate low-temperature limit - the team showed that the optical precursor and the photon that caused it are indeed limited to the vacuum speed of light.

"By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon," said Professor Du.

Thus, photons cannot time travel, and moving information around at faster-than-light speeds is impossible.

But the work has more prosaic implications.

"Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information," said Professor Du.

Time travel by other means, however, is not entirely ruled out.

Einstein's theory of general relativity, in which space and time are two intertwined aspects of the same medium, would permit the bending of the medium to join two different times - a situation popularised as creating a "wormhole".