John: if by relative speed, you mean the relative speed as described by us in our reference frame, yes, it's 2c.
If you mean "How fast does one beam go from the point of view of
the other?" we can't answer it, because we know the rules for switching
points of view only for things travelling at LESS THAN c relative to
us.
If you were to ask about two protons travelling at 0.99c opposite
directions, we can give a real answer. Their relative speed in our
frame is 1.98c. But the speed of one as measured by the other is only
about 0.99995 c. Strange but true- space and time don't behave the way
we feel that they should.
Mike W
(published on 10/22/2007)
