I was mostly just teasing you. Yes, c can vary in the absolute value but it is always the speed of a photon since it's massless."That's the measured two-way speed of light, sometimes called the "canonical" speed of light, c."
"The one-way speed of light or Conventionality of Simultaneity Thesis - which we shall call CST, originally conceived by Hans Reichenbach[14] and verified by others. [5, 6, 8-9, 11, 12] The proposition that nature does not prefer one convention of simultaneity over another.:
We will use a spherically symmetric system with spherical coordinates. In the CST the speed of light as a function of direction relative to the observer is θ where θ is given by, cθ = c/(1-cosθ) where c is the canonical value of the speed of light. When θ = 0 this indicates the direction directly toward the observer. I.e., When θ = 0 this indicates the speed of light is infinite in the direction of the observer. The reason we’re using spherical coordinates is that when using the CST all events everywhere in the universe are observed in real time..."
From, "Relativity, Light Travel, and SpaceMajor Topics in Physics Handout
By Me
References: Jammer, Max, Concepts of Simultaneity: from Antiquity to Einstein, and Beyond, The Johns Hopkins University Press, 2006; Salmon, W.C., The Philosophical significance of the one-way speed of light, Nous, pp. 253-292, 1977; Sarkar, S., and Statchel, J., Did Malament prove the non-conventionality of simultaneity in the Special Theory of Realtivity?, Philosophy of Science, 1999; Winnie, J.A., Special Relativity without one-way velocity assumptions Part I, Philosophy of Science, Vol. 37, 1970a; Winnie, J.A., Special Relativity without one-way velocity assumptions Part II, Philosophy of Science, Vol. 37, 1970b; Einstein, A., Lorentz, H.A., Minkowski, H., and Weyl, H, The Principle of Relativity: A Collection of Original Papers on the Special and General Theory of Relativity, Dover: New York, 1924; Edwards, W. F., Special Relativity in Anisotropic Space, American Journal of Physics; 31 (7): pp. 482–489, 1963; Giannoni, C., Relativistic Mechanics and Electrodynamics without One-Way Velocity Assumptions, Philosophy of Science, Vol. 45, 1978; Eddington, A., Space, Time, and Gravitation, Cambridge University Press, 1920. (This enlightening paper can also be retrieved from:
http://strangebeautiful.com/other-texts/eddington-space-time-grav.pdf ) ;
Reichenbach, H., The Philosophy of Space and Time, Dover: New York, 1958.
PM me if you would like more information on this very interesting topic. Hans Reichenbach was a student of Einstein.
c is also a multi-vector as well as being a constant. So in many ways we all travel at c but it depends on which vector. At rest we are speeding through time at c.