Time Dilation | Definition, Equation, Examples, & Facts | Britannica

Ask the Chatbot Games & Quizzes History & Society Science & Tech Biographies Animals & Nature Geography & Travel Arts & Culture ProCon Money Videos time dilation Introduction References & Edit History Related Topics Images & Videos Quizzes Physics and Natural Law Contents Science Physics Matter & Energy CITE verifiedCite While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. Select Citation Style MLA APA Chicago Manual of Style Copy Citation Share Share Share to social media Facebook X URL https://www.britannica.com/science/time-dilation Feedback External Websites Feedback Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Select a type (Required) Factual Correction Spelling/Grammar Correction Link Correction Additional Information Other Your Feedback Submit Feedback Thank you for your feedback

Our editors will review what you’ve submitted and determine whether to revise the article.

External Websites

• The University of Mississippi High Energy Physics Group - Time Dilation
• CORE - Derivation of Gravitational time dilation from principle of equivalence and special relativity
• Nature - Quantum clocks observe classical and quantum time dilation
• University of Iowa Pressbooks - Simultaneity And Time Dilation
• Physics LibreTexts - Time Dilation
• Khan Academy - Time dilation
• International Journal of Advances in Engineering and Management - A Journey through Time Dilation in Space and on Earth (PDF)
• Live Science - What is time dilation?
• OpenStax - University Physics Volume 3 - Time Dilation
• University of New South Wales - School of Physics - Einsteinlight - Time dilation and length contraction in Special Relativity
• International Journal of Scientific and Research Publications - Time Dilation
• IOPscience - Time dilation in quantum systems and decoherence
• IOPscience - Physics Education - The invisibility of time dilation (PDF)

time dilation physics Ask Anything Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Nov. 13, 2025 •History Britannica AI Ask Anything Table of Contents Table of Contents Ask Anything

time dilation, in the theory of special relativity, the “slowing down” of a clock as determined by an observer who is in relative motion with respect to that clock. In special relativity, an observer in inertial (i.e., nonaccelerating) motion has a well-defined means of determining which events occur simultaneously with a given event. A second inertial observer, who is in relative motion with respect to the first, however, will disagree with the first observer regarding which events are simultaneous with that given event. (Neither observer is wrong in this determination; rather, their disagreement merely reflects the fact that simultaneity is an observer-dependent notion in special relativity.) A notion of simultaneity is required in order to make a comparison of the rates of clocks carried by the two observers. If the first observer’s notion of simultaneity is used, it is found that the second observer’s clock runs slower than the first observer’s by a factor of Square root of√(1 − v2/c2), where v is the relative velocity of the observers and c equals 299,792 km (186,282 miles) per second—i.e., the speed of light. Similarly, using the second observer’s notion of simultaneity, it is found that the first observer’s clock runs slower by the same factor. Thus, each inertial observer determines that all clocks in motion relative to that observer run slower than that observer’s own clock.

A closely related phenomenon predicted by special relativity is the so-called twin paradox. Suppose one of two twins carrying a clock departs on a rocket ship from the other twin, an inertial observer, at a certain time, and they rejoin at a later time. In accordance with the time-dilation effect, the elapsed time on the clock of the twin on the rocket ship will be smaller than that of the inertial observer twin—i.e., the non-inertial twin will have aged less than the inertial observer twin when they rejoin.

Related Topics: time relativity (Show more) See all related content

The time-dilation effect predicted by special relativity has been accurately confirmed by observations of the increased lifetime of unstable elementary particles traveling at nearly the speed of light. The clock paradox effect also has been substantiated by experiments comparing the elapsed time of an atomic clock on Earth with that of an atomic clock flown in an airplane. The latter experiments, furthermore, have confirmed a gravitational contribution to time dilation, as predicted by the theory of general relativity.

Britannica Quiz Physics and Natural Law The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Adam Augustyn.