Email Print Share

Time Dilation Video

Is time different to observers moving at different speeds? 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:
Two spaceships are traveling together through the galaxy at close to the speed of light. Mounted on one ship is a laser that can fire pulses of light, and on the other, a mirror. The pilot of the first ship fires a pulse at the mirror, and watches as it is reflected back. A clock on board measures how long the round trip takes.

But now suppose that he does this as the ships are passing an observer on a nearby asteroid. According to relativity theory, this observer sees the pulse moving through space at exactly the same speed that the pilot does -- namely, the speed of light. But he also sees the pulse traveling a longer distance, because from his perspective, he must add the forward motion of the ships to the motion of the pulse between them. So he measures a longer time interval for the round trip than the pilot does, because he is watching the pulse go farther without going any faster. This effect is called time dilation: if one observer is moving with respect to another, each perceives that the other's time is flowing more slowly.

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.