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HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light

Sep 1, 2010
5,901 (2.22/day)
Someone might say "pah" but ...


Discovery of superluminal propagation of optical pulses in some specific medium 10 years ago has evoked the world's dream of time travel, but later scientists realized that it is only a visual effect where the superluminal 'group' velocity of many photons could not be used for transmitting any real information. Then people set their hope on single photons because in the strange quantum world nothing seems impossible -- a single photon may be possible to travel faster than the speed limit in the classical world. Because of lack of experimental evidence of single photon velocity, this is also an open debate among physicists.
Damn, I demand superluminal velocity NAO! :mad:

A group of physicists at the Hong Kong University of Science and Technology led by Prof Shengwang Du reported the direct observation of optical precursor of a single photon and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon.
Yeah, boffins say effect cannot occur before its cause. Shame.

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.
Meh.... Anyway it's interesting.


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But a shadow can travel faster than light! No, this is not BS - I did read about it ages on a science website somewhere. Just think about it carefully.
Sep 1, 2010
5,901 (2.22/day)
whose to say things smaller then protons can't and or don't.
Riiiight. And anyway if something travels faster than speed of light it'll pass undetected .. I suppose. Maybe in other space-time or something like that.
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But a shadow can travel faster than light! No, this is not BS - I did read about it ages on a science website somewhere. Just think about it carefully.
A shadow isn't really there it is just the absence of light.


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A shadow isn't really there it is just the absence of light.
absence of reflection of light - human eyes see reflected light, so a shadow is nothing more than a surface reflecting less light than its surroundings (due to that source being blocked in that area)