```X-Message-Number: 19151
From: "Lee Corbin" < var s1 = "lcorbin"; var s2 = "tsoft.com"; var s3 = s1 + "@" + s2; document.write("<a href='mailto:" + s3 + "'>" + s3 + "</a>"); >
Subject: Re: Velocity of Light
Date: Sat, 25 May 2002 21:53:11 -0700

Thomas Donaldson writes

> I ASSUME that the reference you give for relativity includes general
> relativity, in which the speed of light DOES change due to gravity.

It will be retorted that what is meant is that the speed of light
will always be measured to be the same by any observer, and this
is correct.  But this is only correct if we take a limited meaning
to "measured".  In other words, yes, the speed of light is always
the same *locally* as measured by any observer; indeed, the speed
of light as inferred from any of its physical effects is the same.

However, in the coordinate system of a distant observer the speed
of light, as Thomas says, is not constant.  This coordinate system
is, in my opinion, just as valid as is the local one.  Moreover,
the distant observer using Schwarzchild coordinates---his best and
most logical choice---must assume that the speed of light varies.

An example will explain what I'm saying.  Suppose that in deep
space a signal is sent far into a gravitating source to be
reflected back.  Due to general relativistic time dilation,
the light goes slower and slower, the deeper into the gravity
well it travels.  Next, suppose that it strikes a mirror and is
reflected back to the deep space observer.  The distant observer
has no choice but to say that the frequency of the light as it
goes past each point of its journey remains the same (else the
light would get backed up).  But the distance between crests
becomes shorter, i.e., a shorter wavelength, which in this case
does not mean a higher frequency, but instead means slower speed.
Thus in the distant observer's coordinate system the speed of
light does slow.

How are the above events seen by an apparatus on the surface of
a planet in the gravity well?  That apparatus measures the light
as going at speed c.  The real reason that it does so is that
it suffers from time dilation.  What it "thinks" to be one
second is, in the non-accelerated frame of the distant observer
outside the gravity well, more than one second.

So one has to specify which observer one means.  As for me, I'm
certainly not going to argue with the distant observer who is
*not* in an accelerated frame of reference, and for whom the
speed of light definitely changes.

Lee Corbin

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