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Unit 7: Astrophysics Page 42

Think about two high-speed planes moving in opposite directions where
you’re standing on the ground watching them pass overhead:

In this reference frame (above), both planes are viewed as moving, and

their ends coincide at the same time. The tails of both planes meet at the
same time and are simultaneous.

However, if you imagine you’re in the upper plane watching the lower one,
you’ll see the lower plane contract in size along its length and also the right
ends of the two planes coincide before the left ends do. So now these events
(of the tails lining up at exactly the same time) are not simultaneous.

And in the frame of reference where you’re watching the upper plane from
the lower plane, the upper appears foreshortened and the left ends coincide
first. The time order has reversed!

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