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Equivalence Principle



Apropos the continuing appeal to the equivalence principle to try to
justify the inclusion of fictitious forces in physics, the clear and
cogent analysis of the principle by Edward A. Desloge (Nonequivalence of a
uniformly accelerating reference frame and a frame at rest in a uniform
gravitational field, Am. J. Phys., 57, 12, December, 1989, Pp. 1121 -
1125) should definitely be read by anyone interested in the issue. The
author shows that it is always possible in principle to distinguish
whether one is in an elevator in a uniform gravitational field or in an
elevator uniformly accelerating in a zero gravitational field, and this no
matter how small the region occupied by the elevator is. It is only when
one restricts the accuracy of the measurements permitted in the elevator
that the two cases can seem approximately equivalent; the author points
out (with references supplied) that, of course, Einstein never claimed
more for the principle, and clearly recognized its approximate nature.

The new emphasis in the paper is on the fact that *uniform* gravitational
fields both exist (contrary to claims made sometimes) and can be
distinguished from acceleration in a field free space. How such an
approximate equivalence can be used to justify inclusion of fictitious
forces in physics has always been beyond my understanding, so I won't try
to discuss that.

A. R. Marlow E-MAIL: marlow@loyno.edu
Department of Physics, Box 124 PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453