London: Scientists claim to have confirmed that nothing can travel faster than the speed of
light, thus putting an end to a decade-long debate about the speed of a single photon, the fundamental unit of light.
   
An international team, led by the Hong Kong University of Science and Technology, says their findings reveal a single photon obeys Albert Einstein's traffic law of the universe - meaning time travel is impossible.
   
Einstein claimed that the speed of light was the traffic law of the universe - or, simply, that nothing can travel faster than 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.”
   
"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," lead scientist Prof Shengwang Du said.
   
In fact, the team claims to be the first to show that optical precursors exist at the single-photon level, and that they're the fastest part of the single-photon wave packet even in a "superluminal" - or faster than light - medium.

Mankind's long-held dream of time travel was given a shot in the arm 10 years ago with discovery of superluminal propagation of optical pulses in some specific medium. But, it was later realised that it is only a visual effect - where the superluminal "group" velocity of many photons couldn't be used for transmitting any real information.
   
Researchers then set their hope on single photons because of the possibility that a single photon may be able to travel faster. Due to a lack of evidence of single photon velocity, this has also been an open debate among physicists.

To confront this impasse, Prof Du's team measured the ultimate speed of a single photon with controllable waveforms. Their study confirmed Einstein's theory that an effect cannot occur before its cause.
   
The team not only produced single protons but separated the optical precursor - the wave-like propagation at the front of an optical pulse - from the rest of the photon wave packet.
   
To do so, they generated a pair of photons, and then passed one of them through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency.
   
For the first time, they successfully observed optical precursors of a single photon. The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum.
   
Even in a superluminal medium where the group velocity is faster than the speed of light in vacuum, the main part of the single photon has no possibility to travel faster than its precursor, say the scientists.
   
The findings have been published in the 'Physical Review Letters' journal.

(Agencies)