IMPOSSIBLE?! Looks like it's back to the drawing board for Back to the Future's 'Doc'.

Scientists have crushed the dreams of anyone who has ever tried to turn their car, Jacuzzi or a telephone booth into a time machine.

Physicists at the Hong Kong University of Technology and Science have ruled out the possibility of time travel by proving that nothing can travel faster than the speed of light.

You might think time travel always belonged in the world of fantasy, but 10 years ago some scientists began to believe time travel might actually be possible when superluminal - or faster than light - propagation of some specific medium were discovered. It was later found to be a visual effect, but the idea that a single photon could exceed the speed of light lingered, and with it, the possibility of time travel.

Until now, that is.

Einstein claimed that the speed of light was the traffic law of the universe - or, simply, that nothing can travel faster than light.

Shengwang Du and his team's research, which was published in the journal Physical Review Letters, sought to end the debate, and kill mankind's long-held dream of time travel, by measuring the ultimate speed of a single photon, which had not been done before.

Their study shows that even single photons, the smallest particle of light, travel at the same maximum speed.

"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," Du said.

In other words, time travel looks to remain solely within the realm of Doctor Who.