>>One day the speed of light might be measured with a ruler and a clock. The next day, if the same measurement gave a different answer, no one could tell if the speed of light changed, the ruler length changed, or the clock ticking changed.
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>Not true.
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>The ruler and clock can change, but the speed of light ( or of every particle without mass) cannot.
>
>The Relativity theory is founded on this axiom:
>"the speed of light on the space" is a spacetime property.

Recent research has shown that the speed of light has varied slightly over time. (View the NOVA documentary on String Theory - first installment http://www.pbs.org/wgbh/nova/elegant/everything.html)

Forms of "Always" and "Never" are seldom true.

Once consumed by a black hole a star never emits light. However as the light emitting star material is absorbed there must be a moment where photons from that star that are moving away from the black hole are first slowed and then reversed by the graviational effect of the black hole as it absorbs the hot star matter. True or false?

http://www.sciencedaily.com/releases/2001/02/010212075309.htm