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This report on the BBC site suggests that optical photons have been prepared that travel slower than c in vacuum. How is this possible?

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    $\begingroup$ Read about group velocity and phase velocity. $\endgroup$ – Paul Jan 23 '15 at 13:11
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    $\begingroup$ Read the last part (peloton). it says.'' the main bunch of riders - may be moving at a constant speed. But within the bunch an individual rider may moving more slowly, dropping back for a rest or a drink." Meanwhile other riders in the bunch are moving faster to get to the front. The bunch is a beam of light, travelling at - yes - the speed of light. The riders are photons, travelling at their individual speeds.'' its all about group velocity ,phase velocity ,single velocity..read this mathpages.com/home/kmath210/kmath210.htm $\endgroup$ – Paul Jan 23 '15 at 13:45
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    $\begingroup$ Except the article is about single photons, not a light beam $\endgroup$ – user56903 Jan 23 '15 at 13:53
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    $\begingroup$ What you're missing, Dirk, is that there is no "single photon" while in the wavelike behaviour in free space. The mask is absolutely forcing the wave from a single emitted photon to become a multispectral packet, and the phase vs group velocity rules apply. This article looks suspiciously like a lot of hullaballoo over nothing. $\endgroup$ – Carl Witthoft Jan 23 '15 at 14:14
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    $\begingroup$ May I invite everyone here to submit a complaint to the BBC for their pure sensationalism. $\endgroup$ – lemon Jan 23 '15 at 16:45
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I believe that Paul's (http://www.mathpages.com/home/kmath210/kmath210.htm) source answers it brilliantly. "The main bunch of riders - may be moving at a constant speed. But within the bunch an individual rider may moving more slowly, dropping back for a rest or a drink." Coupled with Mr. Witthoft's response "What you're missing, Dirk, is that there is no "single photon" while in the wavelike behaviour in free space. The mask is absolutely forcing the wave from a single emitted photon to become a multispectral packet, and the phase vs group velocity rules apply. This article looks suspiciously like a lot of hullaballoo over nothing." you have your anwser.

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