Doc's super fast car won't do it. Neither will Bill and Ted's magic telephone booth. Physicists at the Hong Kong University of Technology and Science have just proved that no machine will ever allow a person to travel through time because time travel is flat-out impossible. Not just unlikely, or we don't have the technology yet, but, beyond the limits of the physical laws of the universe.

You might think time travel has 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.

Bill (Alex Winter) and Ted (Keanu Reeves) are joined by Socrates (Tony Steedman) in "Bill & Ted's Excellent Adventure". Photo: Fox Broadcasting

But in a study published in the peer-reviewed scientific journal Physical Review Letters, Shengwang Du and his team measured the ultimate speed of a single photon and showed that it cannot move faster than the speed of light.

"The results add to our understanding of how a single photon moves. They also confirm the upper bound on how fast information travels with light," Du said in a statement put out by the Hong Kong University of Technology and Science.

"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."

For more on the science behind time travel, check out "How to build a time machine", written last year by the one and only Stephen Hawking.

LA Times