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From: AI.Mayank@MCC.ARPA
Newsgroups: net.physics
Subject: Re: Light
Message-ID: <376@sri-arpa.ARPA>
Date: Wed, 10-Jul-85 14:13:04 EDT
Article-I.D.: sri-arpa.376
Posted: Wed Jul 10 14:13:04 1985
Date-Received: Sat, 13-Jul-85 11:32:39 EDT
Lines: 45

From:  Mayank Prakash 

>From: ihnp4!inuxc!inuxd!claus@UCB-Vax.ARPA (David Claus)
>Subject: Re:  Light
>Article-I.D.: <778@inuxd.UUCP>
>In-Reply-To: Article(s) <345@sri-arpa.ARPA>
>
>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
>AT&T/Indy

According to general relativity, the presence of mass curves space-time around
it. That is, it causes the notions of length and time to alter from point to
point in such a way that the speed of light is the same at each point, when
measured in terms of the length and time apropriate for that point. The average
speed of light, say between moon and earth may be different from c, but
measuring that difference would be beyond the current technology, I would
think. Remember that the gravitational fields of the earth and moon are
extremely weak, as is for that matter, that of the sun, in terms of the
curvature produced by them. That is the reason why Newton's Law of gravitation
is almost all you need to describe the solar system. (To give you an idea of
its weakness, a  ray of light just grazing the sun's surface is bent by a
measly 1.75 seconds of an arc. The only measurable deviation from the Newton's
law is the perihelion advance of Mercury's orbit at the rate of a few seconds
of an arc per century.  The same effect for the other (more distant) planets is
far too small to be measured.)

- mayank.
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