Clarification on light speed experiment

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SUMMARY

The forum discussion clarifies the light speed experiment referenced in the Physics World article, emphasizing that the phenomenon observed is a distinction between phase velocity and group velocity. The consensus among physicists is that the speed of information is defined by group velocity, which is not superluminal. The discussion also highlights the intent of the research team to further analyze scenarios involving pulses with minimal photon counts, reinforcing the established understanding of signal velocity.

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  • Knowledge of signal velocity definitions in physics
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http://physicsworld.com/cws/article/news/2810

Can anyone provide some clarification on this experiment? Is this just a case of 'phase velocity' surpassing group velocity? If so, why does the last paragraph in the article seem to point to FTL?

"There is no widespread agreement among physicists about the speed at which information is carried by pulses in such experiments. One definition is that it is the speed at which the point of half the maximum intensity on the leading edge of the pulse travels, but this velocity is superluminal in the Princeton experiment. The team intend to analyse this further, including cases in which the pulse contains only a few photons."

Thanks
 
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Yes. It is just a phase vs. group velocity issue.

And the last paragraph is simply false. There is complete agreement (it is easily provable) on what the 'speed of information' is, in this case its simply the group velocity.

See: http://en.wikipedia.org/wiki/Signal_velocity
 

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