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Time - Time Reversal

backward process forward processes

We perceive time as always proceeding forward into the future; there is no way we know of to travel into the past (except through memories). Time reversal refers the attempt to understand whether a process is moving forward or backward in time. For example, if you watch a movie of two isolated billiard balls colliding on a billiard table, can you tell if the movie is being shown forward or backward? If the two balls are exactly the same, and you do not see one of them being hit to start the process, probably not. If we were to film the process of an egg being dropped and hitting the ground, and then show the film, you could definitely determine in which direction the process continued. The ability to distinguish between forward and backward processes, between past and future on all scales, is crucial for scientific research.

To see if there really is an "arrow of time," or preferred direction in which time flows, researchers have been conducting many different types of experiments for more than 40 years. According to theory, we should be able to tell forward time processes from backward time processes. Experimental verifications of this theory delineate how difficult some experiments are to design and perform. The latest are designed to observe atomic particles as they undergo different processes. If the processes are then "run backward;" any differences that show up mean that we can tell a forward process from a backward running process.

Resources

Books

Ellis, G.F.R., and R. M. Williams. Flat and Curved Space-Times. Oxford: Clarendon Press, 2000.

Feynman, Leighton, and Sands. The Feynman Lectures on Physics. New York: Addison-Wesley, 1989.

Flaherty, Michael G. A Watched Pot: How We Experience Time New York: New York University Press, 2001.

Lineman, Rose, and Jan Popelka. Compendium of Astrology West Chester, PA: Schiffer, 1984.

Michelson, Neil F. (compiler). The American Ephemeris 1931 to 1980 & Book of Tables. San Diego: Astro Computing Services, 1982.

Periodicals

Cleere, Gail S. "Got a Second?" Natural History (June 1992).

Hunter, L.R. "Tests of Time-Reversal Invariance in Atoms, Molecules, and the Neutron." Science 252 (April 1991).

Itano, Wayne A., and Norman F. Ramsey. "Accurate Measurement of Time." Scientific American 269 (July 1993).

Moyer, Michael. "The Physics of Time Travel." Popular Science 260, no. 3 (2002): 52-54.

Tudge, Colin. "How Long is a Piece of Time?" New Scientist 129 (January 1991).

Westerhout, Gart, and Gernout M.R. Winkler. "Astrometry and Precise Time." Oceanus 33 (Winter 1990/1991).

Zee, A. "Time Reversal." Discover 13 (October 1992).


Peter K. Schoch

KEY TERMS

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Ephemeris time

—This time scale is based on the apparent repetitious motion of the Moon and planets due to the motion of the earth. It is corrected for, and based on, the mean solar motion.

Greenwich mean time

—The location of the Royal Observatory in Greenwich, England where the zero time scales corresponding to time zones have their origin. To synchronize clocks worldwide, all comparisons are made to this scale. It is the period between successive noontime transits of the Sun.

Time reversal

—The attempts to discover if we can tell if a process is moving forward or backward in time.

Universal time

—On January 1, 1972, this time scale replaced Greenwich Mean Time as the reference for scientific work. It is based upon an atomic clock, which keeps time from midnight to midnight.

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almost 8 years ago

I would like to time travel to the past and change some of my life. I would like to go back to when I was born, or in the womb, and have my spine straightened, have things I lost, taken away, stolen, given away, missed out on, have my ears pierced, take pictures, drive, go to Disneyland, go too a Beatles concert, have their records and other memorabilia, and enjoy the British Invasion, do better in school,go to college or a unniversity, have a career, my own house, be listened to when I was little, meet The Beatles, go to Liverpool and write a book on The Beatles, get a nose job, do my family tree, other things I forgot, etc., then travel back to the present.