c is usually set to 1 if you’re doing relativity. It’s more a scale factor, and more a statement about the rest of physics than about relativity. The same is t

c is usually set to 1 if you’re doing relativity. It’s more a scale factor, and more a statement about the rest of physics than about relativity. The same is true for G. It’s an important constant of course, but it’s an… adapter. I’m not sure if I’d say relativity has any free parameters at all, but really “relativity” is just “perspective, when one of the dimensions involved is time.” …special relativity anyway. For a universe with massive particles, massless particles and four dimensions, there are only about three ways you can build its geometry and have it make sense. Of the two others, 4-Euclidean spacetime had trouble with causality, and Galilean relativity (infinite speed of light) has trouble with everything else. So, in a sense, relativity is super-determined. There certainly are other possible universes that work out to be consistent, but they’re further away and fewer in number than you’d think. Einstein was able to do a lot of work in his head, with no experiments, because he figured out that every other option was inconsistent. We’d very much like a final theory of everything to be similar.