Sorry, I could not even finish reading the whole post before I demonstrate my psychic abilities and predict the answer, "variable"?

(This was meant as humor for those who might mistake my intent.)

Humor appreciated, but that's not what I meant. I wasn't asking the

**value** (though that comes next), I was asking the

**name**. See, everywhere you see "c" defined, including in citations Farsight posts, it is defined as "the speed of light". If you take away that definition of "c", you need to replace it with something else, because just repurposing the name doesn't eliminate the term from the equation.

Farsight said:

The answer Russ was looking for is that the locally measured speed of light is constant....

Fair enough. So we can re-label "c" in every textbook and internet link discussing Relativity to be "the locally measured speed of light". Next, then, is the value:

...by tautological definition. George Ellis admitted it. You use the local speed of light to define your second and your metre, and then you use them measure the local speed of light.

I don't know who George Ellis is and that link contains no quotes from him, but yes,

**today**, the constancy of the speed of light is tautological. Why? Because the local speed of light truly is constant. You, of course, know this since you just acknowledged it above by providing the alternate definition of "c" that preserves it. You bring this up just as an additional diversion, which you then follow with a lie about history. I'll just jump ahead and debunk it right now:

The equation you keep posting:

$$t_0 = t_f \sqrt{1 - \frac{2GM}{rc^2}} = t_f \sqrt{1 - \frac{r_0}{r}}$$

was derived in 1916, long before the definition was made tautological. The locally measured speed of light is constant because it

**is** constant. The definition just codified that already known fact. And it is then also a historical fact that the definition was made tautological because it reflects the truth that the local speed of light is constant. Arguing against that is just another one of your common lies. And I really don't get it because it doesn't help you any. Even if you were right about the history and even if you were right about the definition being recursive and therefore useless, it

**still** doesn't eliminate "c" from the equation.

And by the way, that God frame of yours, where the "real" value of "c" resides: what is the value of "c" in that frame? I'll give you a hint: you already posted the answer in this thread.