Last updated at 1:20 PM on 25th July 2011
For those that while away their days dreaming about travelling into the distant past or future, it is disappointing news.
But scientists claim to have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light - meaning time travel is impossible.
Their findings close a decade-long debate about the speed of a single photon, the fundamental unit of light.
No heading Back To The Future: Scientists have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light and that time travel is therefore impossible
Lead researcher Professor Shengwang Du, from the Hong Kong University of Science and Technology (HKUST), said: '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.
'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.'
Professor Du and his team found that a single photon obeys the traffic law of the universe.
Einstein claimed that the speed of light was the traffic law of the universe - or, simply, that nothing can travel faster than light.
The HKUST team is the first to show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon wave packet even in a so-called 'superluminal' - or faster than light - medium.
Mankind's long-held dream of time travel was given a shot in the arm ten years ago with the discovery of superluminal propagation of optical pulses in some specific medium.
But scientists later realised that it is only a visual effect - where the superluminal 'group' velocity of many photons could not be used for transmitting any real information.
Hard at work: Lead researcher Shengwang Du and his team measured the ultimate speed of a single photon with controllable waveforms
Researchers then set their hope on single photons because of the possibility that a single photon may be able to travel faster.
Due to a lack of evidence of single photon velocity, this has also been an open debate among physicists.
To confront this impasse, Professor Du's team measured the ultimate speed of a single photon with controllable waveforms.
Their study confirmed Einstein's theory that an effect cannot occur before its cause.
The researchers not only produced single protons but separated the optical precursor - the wave-like propagation at the front of an optical pulse - from the rest of the photon wave packet.
To do so, 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.
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 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.
The study is published in the journal Physical Review Letters.
25 Jul, 2011--
Source: http://www.dailymail.co.uk/sciencetech/article-2018498/Time-travel-sci-fi-fantasy-Scientists-prove-travel-faster-speed-light.html?ITO=1490
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