Sorry, time travelers, you’re still just fiction

If a photon can’t outrun light, none of us can


Time to start crying in your beer, people: according to researchers at the Hong Kong University of Science and Technology, not only is it impossible to break the speed of light, it will always be that way.

Forget time travel, forget trips to distant galaxies, and ditch the idea that there’s a more advanced civilization than ours that’s made the visit to Earth. A group led by Professor Shengwang Du (the name translates as “killjoy”, The Register hopes) claims to have proven that a single photon can’t break the speed of light in a vacuum.

By confirming photons’ conformance to Einstein’s theory that nothing breaks the speed of light, the researchers hope to put that argument to rest.

The paper is published in Physical Review Letters, unfortunately available only to subscribers, which means this author has to try to disentangle what’s actually been demonstrated at second-hand.

In essence, Professor Du’s team has accomplished two observations. The first was an observation of what’s referred to as the “optical precursor” of the photon, separately from the photon itself; the second was the speed at which both the precursor and the photon can travel.

A good discussion of the notion of the “optical precursor” can be found here, but it amounts to this: if a modulated pulse of light enters a dispersive medium, it causes excitations in the medium that seem to travel ahead of the wave-front (yes, I know about wave-particle duality; here, I’m summarizing a pretty complex experiment first conducted in 1907).

The observation of these “optical precursors” had led physicists to speculate as to whether, given the right medium, the precursor could break light-speed – could it arrive “ahead” of the photon, even if the photon is traveling at c? If the answer is “yes”, then Einsteinian causality would fail – the effect (the precursor) would be observed before the cause (the arrival of the photon).

Professor Du’s experiment says “no”. In their experiment, according to Physorg, the researchers generated a pair of photons, and passed one of the pair through a group of laser-cooled rubidium atoms, taking advantage of an effect called electromagnetically induced transparency.

The combination of the single photon and the material is apparently what allowed the researchers to be the first to observe the precursors associated with a single photon – and they also found that the precursor wavefront is limited by the speed of light. Meanwhile, the “main” wave packet of that photon could be slowed down (by up to 500 nanoseconds), but also could not be sped up beyond the speed of light in a vacuum.

“By showing that single photons cannot travel faster than the speed of light, our results bring to a closure the debate on the true speed of information carried by a single photon,” Physorg quotes Professor Du as saying. “Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information.”

Professor Du’s killjoy associates collaborators in the experiment were postgrad students Shanchao Zhang, Jiefei Chen and Chang Liu, along with Chair Professors Michael Loy and George Wong. ®

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