On May 7, 10:40�am, PD wrote:
On May 7, 11:25�am, rbwinn wrote:


Well, no, if lightining strikes the front and rear of a moving train,
leaving marks on the train and the track, the marks on the track will
be the length of the train apart, which relativity of simultaneity
cannot explain.

Sure, it explains it.

Here's how:
The track observer goes back after the train has passed and notes that
the marks on the track are 400 m apart. Remember that, for the track
observer, the strikes hit simultaneously. Since the marks are also on
the train, the track observer correctly notes that the train is 400 m
long. (This is what a length measurement entails anyway: marking the
locations of the ends of an object at the same time.)

Now the train observer goes back after the strikes have hit and notes
that the marks on the train are 500 m apart. But it's also true that
this observer saw the front strike happen before the rear strike ---
the strikes are not simultaneous in this frame. So it doesn't bother
this observer at all that the track observer sees the marks 400 m
apart. That's exactly what you'd expect if you marked the location of
the front of a moving object before you marked the location of the
rear of the moving object -- and that's exactly what the train
observer is sure happened.

The length of the train is frame-dependent. Whether the marks at the
end of the train were made at the same time, is also frame-dependent.

Does this help you understand?

The train has an actual length. You are going to have to decide
whether you want it to be 400 m or 500m. If you run this experiment
as I have described it, the marks on the railroad track are going to
be the length of the train apart.
Robert B. Winn