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From: tan@ihlpg.UUCP (Bill Tanenbaum - AT&T Bell Labs - Naperville IL)
Newsgroups: net.physics
Subject: Re:  Light
Message-ID: <857@ihlpg.UUCP>
Date: Mon, 15-Jul-85 13:43:11 EDT
Article-I.D.: ihlpg.857
Posted: Mon Jul 15 13:43:11 1985
Date-Received: Wed, 17-Jul-85 06:19:35 EDT
References: <393@sri-arpa.ARPA>
Organization: AT&T Bell Laboratories
Lines: 21

> From:  Doug Gwyn (VLD/VMB) 
> 
> Gravity doesn't "bend light waves" according to general relativity;
> it warps the structure of spacetime.  Light follows a null
> geodesic (think of it as the "shortest path") in all cases,
> so that it appears to take a bent path to an observer who does
> not take the space-time curvature into account.
> 
> It is really meaningless to question whether the "speed of light
> is constant" without describing how units of space and time
> measurement are to be determined, since speed will be measured in
> such units.  The concept of "null geodesic" is invariant with
> respect to any choice of units.

The statement "speed of light is constant" is meaningful and correct if the
speed is always measured locally.  The curvature of space-time can be ignored
if the domain is small enough, i.e. space-time is locally flat.  It does
not matter what units we use.  The magnitude of a physical quantity does
not depend on the units of measurement.
-- 
Bill Tanenbaum - AT&T Bell Labs - Naperville IL  ihnp4!ihlpg!tan