Any light travelling through a medium other than a vacuum travels below c as a result of the time lag between the polarization response of the medium and the incident light. However, certain materials have an exceptionally high group index and a correspondingly low group velocity. In 1999, a team of scientists led by Lene Hau were able to slow the speed of a light pulse to about 17 metres per second,[2] and in 2001, they were able to momentarily stop a beam.[3]



I understand that light is slowed as it passes through dense materials, but what principal lets it be slowed to ~17m/s. Is c not based so much on a medium's density, but more on some other property? For instance, the number of energy levels in an element? I thought I read somewhere that this was done by passing light through a bose-einstein condensate, altho currently i cannot relocate that article.

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I'm like one of those idiot savants...well, except for the savant part.

light is ElectoMagentic in nature. different medias play with that characterestic to change the velocity as it is absorbed and reemited by the individual atoms. Light itself still moves at c, but the processes it goes through take time. We only see the final result.

I see you quoted Wikipedia, so I won't link you there.

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"I will do my best to understand and explain the universe from big to small without invoking miracles, unrepeatable events, or divine intervention. In place of those things I will use observations, mathematics, and science."


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