sci.physics.particle

Pmb wrote:
> "Pentcho Valev" wrote in message
> news:4136f86e-5fc3-4849-92e6-d42985e1f21c@c65g2000hsa.googlegroups.com...
> On Apr 1, 12:52 am, Tom Roberts wrote in
> sci.physics:
>> Anon wrote:
>>> Because as you get closer to a massive body light is slowed down, this
>>> bends
>>> the rays of light towards the massive object. The same is true of other
>>> massive objects. That is what Einstein said.
>> As I said before, Einstein said that in 1911, early on the
>> then-unfinished journey to General Relativity. GR itself does not really
>> have this property.
>
> Note: Einstein's first work on general relativity (e.g. the equivalence
> principle, gravitational time dilation. deflection of light, etc.) was in a
> paper published in 1907 called "On the Relativity Principle and the
> Conclusions Drawn from it," A. Einstein, Jahrbuch der Radioaktivitat und
> Elektronik 4 (1907): 411-462. And contrary to Roberts claim, GR *does( have
> this property.

Had you read my full statement, you would know that I said GR has this
property IN THE APPROPRIATE APPROXIMATION, SPEAKING LOOSELY: i.e. for a
suitable meaning of "speed" -- not a LOCAL speed but a NON-LOCAL speed
or a COORDINATE speed. And as I said, 'without approximation, or to
speak more accurately, one must discuss geodesic deviation, not "light
slowed down".'

The primary fault is Valev's, as he is the one who omitted my
explanatory text. But my original post is in this thread.


> It is a very simple thing to derive. E.g.
> http://www.geocities.com/physics_world/gr/c_in_gfield.htm

That link says explicitly that the computation is the COORDINATE speed
of light. That is, the computed value is a ratio of coordinate
intervals, NOT what one would measure over a local distance using
standard clocks and rulers (one always measures c when one does that).

As I keep saying, and you both (Valev and PMB) keep ignoring: ONE MUST
BE CAREFUL TO AVOID PUNS. There are multiple meanings of the word
"speed" here and you are both oblivious to that fact. Doing physics
requires precision in thought and word, and such unacknowledged puns
destroy the discussion or argument.

The normal meaning of "speed" is what one measures in the obvious way
using standard clocks and rulers; with gravitation this must be over a
local path (short enough so variations in gravitational potential can be
ignored), so the two clocks can be synchronized. The coordinate speed of
light can be essentially anything, and the speed of light measured over
a non-local path can differ from c (e.g. Shapiro time delay). But if you
call those simply "speed" you are sure to confuse your reader (and most
likely yourself) -- that has CLEARLY happened in this thread.


Tom Roberts