In the movie "Back to the Future," Doc Brown builds a time machine into a Delorean. (Universal)
By proving that even a single photon must obey Einstein's theory that nothing can travel faster than the speed of light, Hong Kong physicists believe they have debunked the idea of time travel once and for all.
A Hong Kong University of Science and Technology research team led by Du Shengwang said they had proved that a single photon, or unit of light, "obeys the traffic law of the universe."
"Einstein claimed that the speed of light was the traffic law of the universe or in simple language, nothing can travel faster than light," the university said on its website.
"Professor Du's study demonstrates that a single photon, the fundamental quanta of light, also obeys the traffic law of the universe just like classical EM (electromagnetic) waves."
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The possibility of time travel was raised 10 years ago when scientists discovered superluminal -- or faster-than-light -- propagation of optical pulses in some specific medium, the team said.
If Shengwang and his team are correct, that possibility is now no more than ancient history.