Subatomic Physics

196 Part III. Symmetries and Conservation Laws
Other symmetries are “broken,” and the corresponding conservation law holds only
approximately. There are two kinds of symmetry breaking; one is a symmetry broken
by small effects.Invariance under mirroring (parity) provides one example of
such a broken symmetry. A second kind of symmetry breaking is called “spontaneous”.
Here the forces have the symmetry, but the ground state does not. We
shall encounter both types of symmetry breaking, the first kind in Chapter 7 and
the second kind in Chapter 12. At the present time it is not understood why some
symmetries are broken and others are not. It is not even clear whether the question
should be phrased “Why are symmetries broken?” or “Why are some symmetries
perfect?” We must continue to explore symmetries and their consequences and hope
that a more complete understanding will be reached at some point. (1)
1 The meaning of symmetries in physics, and more generally, in human endeavor are beautifully
described in the following references: R. P. Feynman, R. B. Leighton, and M. L. Sands, The
Feynman Lectures on Physics, Vol. I, Addison-Wesley, Reading, Mass., 1963, Chapter 52; H.
Weyl, Symmetry, Princeton University Press, Princeton, N.J., 1952; E. P. Wigner, Symmetries
and Reflections, Indiana University Press, Bloomington, 1967; C. N. Yang, Elementary Particles,
Princeton University Press, Princeton, N.J., 1962; R. P. Feynman, The Character of Physical Law,
MIT Press, Cambridge, MA, 1965; A. V. Shubnikov and V. A. Kopstik, Symmetry in Science and
Art, Plenum, New York, 1974; J. P. Elliott and P. G. Dawber, Symmetry in Physics, Oxford
University Press, New York, 1979; F. Close, Lucifer’s Legacy, the Meaning of Asymmetry, Oxford
University Press, New York, 2000.