There are those who believe the key to time travel is being able to exceed the speed of light. Unfortunately for them, Professor Shengwang Du from the Hong Kong University of Science and Technology has proven such a feat is impossible.

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

His experiments generated photon pairs and then blasted them through cooled rubidium atoms to achieve the electromagnetically induced transparency (pdf) effect. This would separate the photons from their optical precursor and then allow measurements to be made against slowed, normal and superluminal light where they found no variations in the precursor. The idea was to test causality. and as a result, they found that photons obey Einstein's laws of relativity.