No dark matter?

[little_monkey]

The speed of light does not vary because some observer happens to be ignorant of the laws of physics. His corrections can amply be supplied by one who does know relativity.

I didn't say that. I meant that if he doesn't know to correct his coordinate time interval such that light is travelling on a ds=0 path, then the coordinate speed he measures can differ from c. Does this mean that the "true" speed of light is not c? Well, that depends entirely on what you mean by the speed of light in a non-inertial frame. Perhaps the best way to deal with the problem is just to construct a locally inertial frame, in which it is obvious and the coordinate speed and the proper speed have the same value.

The way to do physics properly is to start with Lorentz invariant equations and then figure out what it means in a specific coordinate system. From there you can figure out what is velocity or force or any other quantity. Needless to say that these equations will be written in tensorial form. To make matter easy, you define the proper time (τ) of the worldline of a particle as the parameter that parametrize the worldline of that particle. You then define the velocity u = dx/dτ, which has components ( dt/dτ, dx/dτ, dy/dτ, dz/dτ), etc. In that development of relativity, the speed of light is ALWAYS constant. If you throw that out, then you might as well throw out relativity altogether.