Don't make any long-term plans of going back to the Jurassic era to slay a T-Rex or two, or stepping into the future to save the world from Skynet
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Don't make any long-term plans of going back to the Jurassic era to slay a T-Rex or two, or stepping into the future to save the world from Skynet. 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 team at the Hong Kong University of Science and Technology, led by Professor Shengwang Du, has concluded that single photons cannot travel faster than the speed of light in a vacuum.
The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum...
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, Professor Du said in a statement. The experiment is apposite because for the past 10 years it has been thought optical pulses may be able to travel faster than light under distinct circumstances. This is called superluminal. However, it was later proved wrong, but left scientists with questions about how fast a photon could travel as it would be the one particle that could potentially break the speed of light barrier.
As a result, Du's team measured the maximum speed of a single photon, which showed that it obeys the universe's speed limit and "confirms Einstein's causality; that is, an effect cannot occur before its cause," researchers said.
In their tests, physicists managed to separate the optical precursor, a wave-like structure at the front of an optical pulse, from the rest of the photon wave packet. To do this, they generated 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 (EIT). For the first time, they successfully observed optical precursors of a single photon.
"The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum. The main wave packet of the single photon travels no faster than the speed of light in vacuum in any dispersive medium, and can be delayed up to 500 nanoseconds in a slow light medium. Even in a superluminal medium where the group velocity (of an optical pulse peak) is faster than the speed of light in vacuum, the main part of the single photon has no possibility to travel faster than its precursor," researchers said.
So, time travellers out there, you're still just fiction.
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