Time
Time Reversal
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.
See also Relativity, special.
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
Additional topics
Science EncyclopediaScience & Philosophy: Thallophyta to ToxicologyTime - Time Measurement Systems, Time Measuring Devices, Time Reversal