Which phenomenon describes how time changes for an object moving near the speed of light?

Which phenomenon describes how time changes for an object moving near the speed of light?

• A. Time dilation
• B. Length contraction
• C. Lorentz factor
• D. On the Electrodynamics of Moving Bodies

Answer: (A)

Time dilation is the key idea here. In special relativity, moving clocks run slower compared to clocks at rest when viewed from a stationary frame. The amount of slowing is described by the Lorentz factor, gamma = 1 / sqrt(1 - v^2/c^2). As an object's speed approaches the speed of light, gamma increases without bound, so time for the moving object appears to pass more and more slowly to an outside observer. Keep in mind that from the object's own point of view, its internal clock ticks normally; the dilation is what outside observers notice. Length contraction is a related but separate effect where lengths in the direction of motion shorten, not a change in time. The Lorentz factor is the numerical term that ties these relativistic effects together. The remaining option is the title of Einstein’s 1905 paper introducing these ideas, not a phenomenon describing how time changes.

Time dilation is the key idea here. In special relativity, moving clocks run slower compared to clocks at rest when viewed from a stationary frame. The amount of slowing is described by the Lorentz factor, gamma = 1 / sqrt(1 - v^2/c^2). As an object's speed approaches the speed of light, gamma increases without bound, so time for the moving object appears to pass more and more slowly to an outside observer. Keep in mind that from the object's own point of view, its internal clock ticks normally; the dilation is what outside observers notice.

Length contraction is a related but separate effect where lengths in the direction of motion shorten, not a change in time. The Lorentz factor is the numerical term that ties these relativistic effects together. The remaining option is the title of Einstein’s 1905 paper introducing these ideas, not a phenomenon describing how time changes.