Train A with proper length L moves eastward at speed v, while train B with proper length 2L moves west at speed v. The point where their fronts coincide is an event E1, while the point when their backs coincide is an event E2. How much time passes between these events in a) The ground frame? b) A's frame? c) B's frame? d) Verify that the invariant interval between the events, s2 = |E1 E22, is the same in all three frames.