Re: speed of light

George Murphy (""
Sat, 13 Nov 1999 16:07:26 -0500

Moorad Alexanian wrote:
> Dear George,
> I am not sure I follow your argument. The meter is defined as the distance
> light travels in vacuum in 1/299,792,458 seconds. If clocks can be taken
> back into time and there is no time dilation or contraction, wouldn't a
> meter have different lengths and wouldn't it be shorter in the past if the
> speed of light in vacuum is reduced?

Correct with this (relatively new) definition of the meter, which makes use of c
in my second sense (i.e., the speed at which light actually does travel. Of course the
definition is predicated on the assumption that c _doesn't_ change - i.e., that it's
identical with c in my first sense. Logically we ought to designate these by different
symbols, c1 & c2. What we know now is consistent with c1 = c2. It's easier to
understand how c2 could change with time than c1.

George L. Murphy