Proof that time travel is, as Einstein said, fiction
Physicists at the Hong Kong University of Science and Technology have proved Albert Einstein's theory that nothing travels faster than light is correct - showing that time travel is only possible in science fiction.
Research team leader Du Shengwang, 36, assistant professor of physics, said his study should 'close a decade-long debate about the speed of a single photon (a unit of light)'.
Einstein famously stated in his special theory of relativity that nothing, under any circumstances, can travel faster than light, since according to his theory of relativity, faster-than-light would literally mean going back in time, and anything that attempts this type of travel would have negative mass.
Yet the travelling speed of photons has been a subject of debate for scientists and science-fiction writers for years.
Du's team of six, including postgraduate students Shanchao Zhang, Jiefei Chen and Chang Liu, and fellow professors Michael Loy and George Wong, showed that a single photon cannot travel faster than the speed of light in a vacuum.
Their study, which took three years, was published last month in Physical Review Letters, one of the most prestigious journals in physics, with the title, 'Single photons obey the speed limits'.
Du said he hoped to change how people think. 'The results add to our understanding of how a single photon moves. They also confirm the limit on how fast information travels with light,' he said.
'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.'
The prospect of possibly being able to travel back in time excited the world when 10 years ago scientists found superluminal - or faster-than-light - movements of optical pulses in some specific medium.
It was later found to be a visual effect, but the physicists began a debate about the single photon's speed limit.
Believing Einstein was right, Du decided to measure the ultimate speed of a single photon with controllable waveforms.
In the experiment, the team 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.
They did it by generating 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.
Du said the findings could help scientists get a clearer picture of how to transmit quantum information, for instance in cryptography, 'so they can only use the fastest method'.
Loy said: 'We have made it very difficult for others to doubt if there's anything which travels faster than single photons. The results were shown very cleanly and clearly.'
He added: 'Some people may still have doubts, but the majority of them will see this as an answer.'
University of Hong Kong associate professor of physics Chau Hoi-fung applauded the researchers' efforts in developing the experimental technique.
'The highlight of the experiment, I think, is how they went about doing it instead of the result, because it was very difficult to do,' he said.
He said the question of whether anything could travel faster than light was not in itself important to scientists. They knew the answer was that nothing could. The difficulty was in proving it.
Chau said scientists 'must question what people think of as fundamentals' from time to time. For instance, scientists might ask whether the result will hold true under changing environments, he said.
'There are still people who question if 1+1=2, and we need those people,' said Chau. 'We never know what results they will get in the end. But of course, the question this study asked is not as fundamental as that.'
Although Einstein's theory of special relativity was produced in 1905, Chau said scientists would be proving or challenging his ideas for some time yet. 'As we know more about the world and do experiments better, we should question what we already know. It's a long process,' he said.