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Simultaneity Video

 The very notion of "now" is relative. For more information, see the World Year of Physics 2005 feature. A RealPlayer streaming version of the video also is available. To see the clip with the captions on, you will need to select Tools > Preferences > Content > Use Supplemental text captioning when available, or deselect it if you do not wish to see the captions. Then re-start the video. Credit: Trent Schindler, National Science Foundation

 Video Transcript: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. General Restrictions: Images and other media in the National Science Foundation Multimedia Gallery are available for use in print and electronic material by NSF employees, members of the media, university staff, teachers and the general public. All media in the gallery are intended for personal, educational and nonprofit/non-commercial use only. Videos credited to the National Science Foundation, an agency of the U.S. Government, may be distributed freely. However, some materials within the videos may be copyrighted. If you would like to use portions of NSF-produced programs in another product, please contact the Video Team in the Office of Legislative and Public Affairs at the National Science Foundation. Additional information about general usage can be found in Conditions.

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