Now, Shengwang Du and colleagues at the Hong Kong University of Science and Technology have measured what is known as an optical precursor.
Like the wind that moves ahead of a speeding train, optical precursors are the waves that precede photons in a material; before now, such optical precursors have never been directly observed for single photons.
By passing pairs of photons through a vapour of atoms held at just 100 millionths of a degree above absolute zero – the Universe’s ultimate low-temperature limit – the team showed that the optical precursor and the photon that caused it are indeed limited to the vacuum speed of 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,” said Professor Du.
Thus, photons cannot time travel, and moving information around at faster-than-light speeds is impossible.
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