Say we have two objects: A and B. In A's IRF, B moves with velocity +v. Then, B undergoes time dilation, and from B's perspective, the passage of time for A has sped up.
However, in B's IRF, A moves with velocity -v. Then, A undergoes time dilation, and from B's perspective, the passage of time for B has sped up.
This is a contradiction, the passage of time in A can't have sped up relative to B. What am I missing?
