It seems like there is no such thing as uniform motion. Example: I just took my pup to the beach. She crashed into many waves. We're back and she's asleep (at rest). However, her position in spacetime is changing because she's attached to the earth, which is moving, and the earth is constantly changind its direction as it rotates. A train moving at a constant 80mph on a "straight track" is changing its direction (through spacetime) even if it doesn't realize it. Thanks for any help on this. .