Imagine two observers, one seated in the center of a speeding train car,
and another standing on the platform as the train races by. As the
center of the car passes the observer on the platform, he sees two bolts
of lightning strike the car - one on the front, and one on the rear.
The flashes of light from each strike reach him at the same time, so he
concludes that the bolts were simultaneous, since he knows that the
light from both strikes traveled the same distance at the same speed,
the speed of light. He also predicts that his friend on the train will
notice the front strike before the rear strike, because from her
perspective on the platform the train is moving to meet the flash from
the front, and moving away from the flash from the rear.
But what
does the passenger see? As her friend on the platform predicted, the
passenger does notice the flash from the front before the flash from the
rear. But her conclusion is very different. As Einstein showed, the
speed of the flashes as measured in the reference frame of the train
must also be the speed of light. So, because each light pulse travels
the same distance from each end of the train to the passenger, and
because both pulses must move at the same speed, he can only conclude
one thing: if he sees the front strike first, it actually happened
first.
Whose interpretation is correct - the observer on the
platform, who claims that the strikes happened simultaneously, or the
observer on the train, who claims that the front strike happened before
the rear strike? Einstein tells us that both are correct, within their
own frame of reference. This is a fundamental result of special
relativity: From different reference frames, there can never be
agreement on the simultaneity of events
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