For The Encyclopedia of the Social Sciences. Limit 500 words.
Nothing seems so ubiquitous—so absolute and universal—as time. Yet, in his 1905 special theory of relativity Albert Einstein showed that the times measured on clocks are different for clocks that are moving with respect to one another—an effect called time dilation. This called into question some of our deepest intuitions of time. No moment in time can be labeled a universal "present." There is no past or future that applies to every point in space. Two events separated in space can never be judged to be objectively simultaneous. The whole notion of cause and effect has to be carefully rethought.
Unless one is making highly precise measurements with atomic clocks, time dilation is important only when the relative speeds of clocks are near the speed of light. So, we do not notice these effects in everyday life. However Einstein's theory has been confirmed by a century of experiments involving high-energy particles that move near the speed of light, as well as low speed measurements with atomic clocks. While we do not need to take into account the relativity of time in the social sphere, it is important not to draw universal, philosophical, or metaphysical conclusions based on notions related to time that are inferred from normal human experience.
Philosophers and theologians have introduced alternate "metaphysical times" more along the lines of common experience, but these have no connection with scientific observations. Scientific models uniformly assume that time is, by definition, what is measured on a clock and that time is relative.
Until recently, the passage of time was registered by familiar regularities such as day and night and the phases of the moon, or more accurately by the apparent motions of certain stars. The second was defined by the ancient Babylonians to be 1/84,600 of a day. Our calendars are still based on astronomical time using the Gregorian calendar, introduced in 1582, in which the year is defined as 365.2425 days. With the rise of science, the second has undergone several redefinitions to make it more useful in the laboratory.
The most recent change occurred in 1967 when the second was redefined by international agreement as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine energy levels of the ground state of the Cesium133 atom at rest at absolute zero. The minute remains sixty seconds, the hour remains sixty minutes, and the day remains twenty-four hours, following ancient traditions. The day is still taken to be 84,600 seconds, as in ancient Babylonia. Our calendars need to be corrected occasionally to keep them in harmony with the seasons because of the lack of complete synchronization between atomic time and the motions of astronomical bodies.
Davies, Paul 1995. About Time: Einstein's Unfinished Revolution. New Yok: Simon and Schster.
Price, Huw 1996. Time's Arrow and Archimedes' Point, New York, Oxford : Oxford University Press.
Stenger, Victor J. 2000. Timeless Reality: Symmetry, Simplicity, and Multiple Universes. Amherst, NY: Prometheus Books.
Stenger, Victor J. 2006. The Comprehensible Cosmos: Where Do the Laws of Physics Come From? Amherst, NY: Prometheus Books.
Victor J. Stenger
Emeritus Professor of Physics and Astronomy
University of Hawaii at Manoa
Adjunct Professor of Philosophy
University of Colorado at Boulder