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Time Travel More Complicated Than Previously Thought

July 26, 2011

In a blow to science fiction writers and daydreamers all over, it appears that the hopes of building a time machine may be dashed once and for all, reports the Los Angeles Times (LA Times) and other media outlets.

Physicists at the Hong Kong University of Technology and Science have just proved that no machine will ever allow a person to travel through time because time travel is flat-out impossible and beyond the limits of the physical laws of the universe.

Until now, it was thought that even though light is the ultimate speed limit of the universe, experiments in recent years suggested that single photons might be able to pass that limit. If they could, theory allows for the prospect of time travel.

However, a paper in Physical Review Letters shows that individual photons too are limited to the vacuum speed limit. That means that photons maintain the principle of causality laid out in Einstein’s theory of special relativity – that is, an event’s effect cannot precede its cause by traveling faster than light.

It is a violation of this causality that would, in principle, permit time travel. Shengwang Du and his team measured the ultimate speed of a single photon and showed that it cannot travel faster than the speed of light.

While the limit in vacuum is a fixed number at around 300,000 km/second, the speed of light can vary widely in different materials. These differences explain everything from why a straw looks bent in a glass of water to experiments in cold gases of atoms in which light’s speed is actively manipulated.

“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,” Du said in a statement put out by the Hong Kong University of Technology and Science.

“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.”

Du and colleagues 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. But by passing pairs of photons through a vapor of atoms held at just 100 millionths of a degree above absolute zero, the 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 Du. Thus, photons cannot time travel, and moving information around at faster-than-light speeds is impossible.

But the work has more prosaic implications. “Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information,” said Professor Du.

Time travel by other means, however, is not entirely ruled out. Einstein’s theory of general relativity, in which space and time are two intertwined aspects of the same medium, would permit the bending of the medium to join two different times, a situation popularized as creating a “wormhole”.

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