The interaction of one photon with another can beunderstood
by assuming that each photon can temporarily become a“virtualâ€
electron-positron pair in free space, and the respective
pairs can then interact electromagnetically. (a) How longdoes
the uncertainty principle allow a virtual electron-positronpair
to exist if h 2mc2, where m is the electron mass? (b) If
h   2mc2, can you use the notion of virtualelectron-positron
pairs to explain the role of a nucleus in the production ofan
actual pair, apart from its function in ensuring theconservation
of both energy and momentum?
