MadSci Network: Physics

Re: Can a shadow go faster than light?

Date: Thu Feb 17 16:19:39 2000
Posted By: Ken Wharton, Post-doc, Laser/Plasma Physics
Area of science: Physics
ID: 946851598.Ph

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If you think of a shadow as a solid object, then the object appears to be moving faster than light. Of course, it's NOT a solid object; it's not any sort of object at all, so there's nothing that's really moving faster than light.

You don't even need a shadow to get this effect; just sweep a laser pointer across the sky. No single photon travels faster than light, but (at a far enough distance) the beam seems to sweep from point A to point B faster than light could ever travel. Does this violate relativity? No. It's the entire "beam" that appears to be moving faster than light, not any one particle. And the "beam" is just a way of thinking about the collection of individual photons, it's not a real object.

More importantly, the above example wouldn't transmit any information from A to B -- all of the information is coming from your laser pointer -- so no information is travelling faster then the speed of light, and relativity is safe.

[Moderator note: On the more technical side, you may want to look up the terms phase velocity and group velocity. Also,in astronomy there are many observations that appear to be superluminal, but are actually not. Here is one example:
Radio Map of Quasars]

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