You did and the author is correct: modern GR treats that speed of light as constant rather than the equivalent formulism of GR where it varies. This is because the experimental results support that the speed of light is constant.
I reiterate, experimental results show that the speed of light varies. We define our time using the motion of electromagnetic waves, essentially light in the wider sense:
“Under the International System of Units, the second is currently defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom...”
Check out
the NIST caesium fountain clock. It employs the hyperfine transition, which is an electromagnetic "spin flip" change to one of the atom's electrons. As a result a microwave photon is emitted with a given "frequency". But imagine you're the detector. You sit there counting the arriving microwave peaks, and when you get to 9,192,631,770 you tick off a second. Since frequency is measured in Hertz which is cycles per second, you haven't actually measured the frequency, the frequency is 9,192,631,770
by definition. So if electromagnetic phenomena proceed at a reduced rate, the second is bigger. In other words, if the light moves slower, the second is bigger, and you use it to measure the speed of light. The metre isn't affected because provided you avoid the radial length contraction, the slower light and the bigger second cancel each other out.
The fact is that clocks clock up
motion, not time. When atomic clocks or light clocks "run slow" in a region of low gravitational potential, they do this because the speed of light at that location is less than it is up in space. That’s why we have the GPS clock adjustment, see
http://en.wikipedia.org/wiki/Gps#Relativity. A GPS clock is an atomic clock that employs microwaves. The clock runs slower because the light moves slower. This is why
the Shapiro delay is called what it is. It’s a
delay, the light moves slower when it passes the limb of the sun. And this doesn’t just affect light. It affects everything. It affects electromagnetic things like electrons, because of what
pair production is showing us. The evidence is there. Electrons are literally made from light.
Whether charge is "fundamental" has nothing to do with it. Electromagnetic waves are electromagnetic waves. They do not contain charges. They contain electromagnetic waves.
It's got everything to do with it. Many people who consider themselves to be knowledgeable in physics are stumped when pressed for an explanation of things they consider to be intrinsic or fundamental, and give nonsense non-answers like
electromagnetic waves contain electromagnetic waves. Show them the clear evidence that leads to deeper understanding, and they reject it.
We have been talking about gravity - that is what GR is about.
Agreed. I did start on a new thread on this as promised, but it seems there was a problem:
http://www.internationalskeptics.com/forums/showthread.php?t=171080. If you want to talk about gravity further, we may have to take it offline or elsewhere.