I found that the author gave a great setup for an explanation, and then balked at giving the actual answer.
> For right now, if you just believe that four-velocities can never stretch or shrink because that's just the way it is
In other words, nothing can go faster than the speed of light because that's the way it is? The author needs to explain why the magnitude of this four-velocity vector is the speed of light! I was hooked after the first few paragraphs, but then felt like it dead-ended in a circular argument.
> For right now, if you just believe that four-velocities can never stretch or shrink because that's just the way it is
In other words, nothing can go faster than the speed of light because that's the way it is? The author needs to explain why the magnitude of this four-velocity vector is the speed of light! I was hooked after the first few paragraphs, but then felt like it dead-ended in a circular argument.