 So now let's consider time dilation. So Alice is on a train traveling at speed V, and she's carrying a stopwatch. Now at time ta equals zero, she starts the stopwatch and then turns it off at some later time. So we're going to be very specific here. We're going to pick two events. The first being when Alice turns on her watch in her frame, and the second when she turns off the watch in her frame. So the time interval between these two events. So Alice measures delta ta, and in Bob's frame he measures delta tb. So here are the Lorentz transformations. What is delta xa? The space Alice measures between these two events. And then using this and the Lorentz transformation, how do delta ta and delta tb relate?