Aristarchus - your misunderstanding is so severe that I can't resist one more post

Here's an excellent explanation of the relevant issue (emphasis is mine):

http://www.xs4all.nl/~johanw/PhysFAQ/Re ... light.htmlEinstein went on to discover a more general theory of relativity which explained gravity in terms of curved spacetime, and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: . . . according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [. . .] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Since Einstein talks of velocity (a vector quantity: speed with direction) rather than speed alone, it is not clear that he meant the speed will change, but the reference to special relativity suggests that he did mean so. This interpretation is perfectly valid and makes good physical sense, but a more modern interpretation is that the speed of light is constant in general relativity.

The problem here comes from the fact that speed is a coordinate-dependent quantity, and is therefore somewhat ambiguous. To determine speed (distance moved/time taken) you must first choose some standards of distance and time, and different choices can give different answers. This is already true in special relativity: if you measure the speed of light in an accelerating reference frame, the answer will, in general, differ from c.

In special relativity, the speed of light is constant when measured in any inertial frame. In general relativity, the appropriate generalisation is that the speed of light is constant in any freely falling reference frame (in a region small enough that tidal effects can be neglected). In this passage, Einstein is not talking about a freely falling frame, but rather about a frame at rest relative to a source of gravity. In such a frame, the speed of light can differ from c, basically because of the effect of gravity (spacetime curvature) on clocks and rulers.

If general relativity is correct, then the constancy of the speed of light in inertial frames is a tautology from the geometry of spacetime. The causal structure of the universe is determined by the geometry of "null vectors". Travelling at the speed c means following world-lines tangent to these null vectors. The use of c as a conversion between units of metres and seconds, as in the SI definition of the metre, is fully justified on theoretical grounds as well as practical terms, because c is not merely the speed of light, it is a fundamental feature of the geometry of spacetime.

Like special relativity, some of the predictions of general relativity have been confirmed in many different observations. The book listed below by Clifford Will is an excellent reference for further details.

Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies.

Aristarchus wrote:Physicist wrote:While I'm sure you can continue the quote mining ad infinitum

It's not called "quote mining." It's called citing sources/references.

Physicist wrote:I'm curious as to YOUR opinion. Do you think that the speed of light is constant in GR? How would YOU define it?

I'm offering authoritative cited sources that appear to defy your previous comment (viz),

"The gravitational field does NOT affect the speed of light - light always travels past any observer at the same speed c." My previous cited sources was from Warren F. Davis who obtained his PhD in Physics from M.I.T.

It's not for me to define, but for you to have an understanding of how this is actually understood in your field of physics. For example - here's more:

http://arxiv.org/PS_cache/math-ph/pdf/0 ... 0054v1.pdfThree Tests of General Relativity via Fermat’s

Principle and the Phase of Bessel FunctionsThe slowing down of clocks in a gravitational field will result in an apparent reduction in the speed of light. Light will therefore travel at the phase velocity u(r) = c/η(r), rather than c, as it would in vacuum.

What you stated above, i.e.,

"The gravitational field does NOT affect the speed of light - light always travels past any observer at the same speed c.", is patently untrue. This is according to the GR theory.

So the question becomes, how is it that in your internet research you were able to find so many references to light slowing down in a gravitational field?

The answer is that in applications like gravitational lensing, an approximation to GR is made. The approximation is that curved space is treated as flat space, but with an effective refractive index derived from the Newtonian gravitational potential. Hence that pesky word "apparent" in your quote above. The abstract of that paper makes it clear that the author is working in that approximation from the beginning.

If an observer is in the geometry in question, he or she will certainly see light going by at speed c. Assuming of course that you define the speed of light in an appropriate coordinate-independent fashion.

In conclusion - I think that you're certainly guilty of quote mining, although you probably weren't aware of it. There is no way you could have known the context of your quotes without putting in a great deal more work.

I'll skip a response to the rest of your post - this discussion is nasty enough already

Nereid wrote:For example, without a good grasp of the relevant background math, how could anyone judge the merits, or non-merits, of what S. Crothers has written?

Another example: without a good understanding of the details of a 'bending of light by the Sun' experiment (or observation), how could anyone judge the merits, or non-merits, of Mermet's paper?

I was wondering that too. But I think the main motivation of our friend Aristarchus, as well as of the aforementioned authors, is to create the impression that there is some kind of controversy wrt these issues in GR. I don't think that's the case.

Now I really am going to do some work

Later.