Now that NASA's space shuttle era has come to an end, some of us among humanity may have the wish to see a new travel vehicle - the time machine.

While we may have put our hope on the advancement of science to realize it, our dream to join Michael J. Fox in "Back to the Future" was ruthlessly shattered by a recent discovery by scientists. What an irony. "Warp speed" in Star Trek too, will remain within the Sci-Fi world.

A group of physicists at the Hong Kong University of Science and Technology found out that a single photon cannot travel faster than the speed of light in vacuum. The study reaffirms Einstein's theory that nothing travels faster than light, and most sadly, proves the impossibility of achieving humanity's dream of time travel as well as faster-than-light warp drive.

"The study, which showed that single photons also obey the speed limit, confirms Einstein's causality; that is, an effect cannot occur before its cause," the university said in a press release.

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

Professor Du's study demonstrated that a single photon, which is the fundamental quanta of light, also obeys the traffic law of the universe.

This revelation, the scientists say, closes the book on time travel once and for all. One theory from 10 years ago said time travel may be possible through superluminal propagation of optical pulses in some specific medium. However, Du and his crew said this phenomenon is only a visual effect and could not be used for transmitting any real information.

There was also hope that a single photon may be able to travel faster than the speed of light. However, there was no experimental evidence. Du and his colleagues decided to test this and measured the ultimate speed of a single photon with controllable waveforms. The results confirmed Einstein's causality.

The research team in their demonstration not only produced single photons, but also separated the optical precursor, which is the wave-like propagation at the front of an optical pulse, from the rest of the photon wave packet. To do so, a pair of photons were generated, and one of them was passed through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency (EIT). It was the first time for them to successfully observed optical precursors of a single photon.

As a result, it was found out that the precursor wave front always travels at the speed of light in vacuum, and that a single photon has no possibility to travel faster than its precursor.

"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."

NASA