Is the speed of light in free space always c?

General interest seminar

Is the speed of light in free space always c?

  • Event time: All day
  • Event date: 22nd October 2015
  • Speaker: Miles Padgett (Kelvin Professor of Natural Philosophy, University of Glasgow)
  • Location: Lecture Theatre A,

Event details


Light propagates more slowly when passing through materials like water or glass but goes back to its higher velocity as soon as it returns to free space again. Or at least it did until now. Two and a half years ago, we set out to see if we could slow down light just a little - and then keep it moving more slowly, even in free space.

We race our photons in pairs. One photon is in its normal state, the other photon is sent through a special mask which gives the transmitted photon a transverse structure, ie shape. If both photons have been travelling at the speed of light it would be a dead heat, but we find that the re-shaped photon comes in second. So it seems that even in free-space the speed of light can be reduced.

About General interest seminars

Our General Interest Seminars are an opportunity for distinguished speakers to present new research in physics and related areas. The material presented is suitable for undergraduate level upwards and all members of the School are welcome to attend..

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