Postprint version. Published in Physics Today, Volume 41, Issue 2, February 1, 1988, pages 86-88.
Copyright © 1988 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Physics Today.
The definitive version is available at http://dx.doi.org/10.1063/1.2811326.