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Is it true that positrons run down the time, and electrons run up the time? - Windows to the Universe

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# Is it true that positrons run down the time, and electrons run up the time?

#### The short answer is no they don't. The idea that positrons go one way in time and electrons go the other way comes from a comment made by Richard Feynman. He said that a positron going forward in time looks like an electron going backward in time.

Feynman diagrams are diagrams used by particle physicists to help them determine particle interactions.  The graphic on this page is an example of one.  In them, one can draw what looks like a particle going backward in time, but this does not mean that it actually does!  No one has ever observed particles travelling backward in time.

Part of the problem is that it is unclear what is meant by the direction of time.  Most agree that the forward direction of time is the direction of increasing "entropy". Entropy is a term used to describe the amount of disorder in a system.  Entropy applies only to systems of large numbers of particles.

Think of a china cup sitting on a table. The cup has a certain amount of entropy.  If the cup falls off the table and crashes on the floor and breaks into little bits, the amount of entropy has increased.  The direction of time is said to be the direction of increasing entropy because we would never see a broken cup jumping up off the floor and reassembling itself on the table.  But, at the individual particle level there is no entropy and therefore no specific direction to time.  (So maybe positrons really are electrons going backward in time???)

Submitted by Gougeon (Lyon, France)
(November 27, 1997)

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information.