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From: tan@ihlpg.UUCP (Bill Tanenbaum - AT&T Bell Labs - Naperville IL)
Newsgroups: net.physics
Subject: Re:  Light
Message-ID: <773@ihlpg.UUCP>
Date: Mon, 8-Jul-85 19:20:58 EDT
Article-I.D.: ihlpg.773
Posted: Mon Jul  8 19:20:58 1985
Date-Received: Tue, 9-Jul-85 07:06:54 EDT
References: <345@sri-arpa.ARPA> <778@inuxd.UUCP>
Organization: AT&T Bell Laboratories
Lines: 29

> The speed of light is not always constant.  Einstein assumed that
> the speed of light was constant through any round trip.  The
> speed of light through one leg of that trip can be greater than
> the speed through another leg.
> 
> Take for example a light pulse being sent from the earth to the moon
> and back.  Light will travel faster on the way back than on the way
> there because of gravity effects.  Does general relativity take this
> into account somehow?  It is proven that gravity bends light waves
> (through sun eclipse experiments) so why shouldn't gravity also increase
> the speed of the light wave?  Has there been an experiment that has
> measured the speed of light during a one way trip through some
> gravitational potential?  Most measurements I've heard of involve
> the reflection back and forth of a light wave here on earth.
> 
> Can anyone explain this to me.
> 
> Dave Claus

You are incorrect in several respects.  The speed of light in vacuum IS
constant, i. e. the same to all observers.  Light traveling from the earth
to the moon is red-shifted due to the difference in gravitational potential,
and light travelling from the earth to the moon is blue-shifted for the
same reason.  The travel times, however, are frequency independent (assuming
transmission in vacuum) and the same in both directions.  General relativity
does account for the frequency shifting.
The answer to your question about the experiment is YES.
-- 
Bill Tanenbaum - AT&T Bell Labs - Naperville IL  ihnp4!ihlpg!tan