Is it possible for a patch of light (or a shadow to travel faster than the speed of light? i.e. if a searchlight were swung around very fast, would the patch of light on the clouds be able to move at ANY speed, depending how far away the clouds are?
Asked by: Mark Griffiths


Yes, a PATCH of light can appear to travel at any speed. The Special Theory of Relativity sets c (the speed of light in a vacuum) as the speed limit for any physical object or packet of energy.

Your examples do not violate Relativity principles because any single photon does not exceed c. Imagine throwing tennis balls along a wall, with each ball hitting some distance to the right of the previous one. The apparent path of each hit against the wall moves at a rate that depends only on the distance between them and how frequently the balls are thrown. The speed of the balls is not important. The 'patch of light' example just replaces tennis balls with photons.

Note from the editor: However, even with your clever scheme, Mark, you can NOT transfer useful information faster than speed of light. Just think about it. Try to devise any scheme to transfer information by swinging a search light - it is impossible!
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
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