a/A double-slit interference pattern made with neutrons. (A. Zeilinger,
R. Gahler, C.G. Shull, W. Treimer, and W. Mampe, ̈ Reviews of Modern
Physics, Vol. 60, 1988.)- Electrons have mass, and photons don’t.
- Photons always move at the speed of light, but electrons can
move at any speed less thanc. - Photons don’t have electric charge, but electrons do, so electric
forces can act on them. The most important example is the
atom, in which the electrons are held by the electric force of
the nucleus. - Electrons cannot be absorbed or emitted as photons are. De-
stroying an electron or creating one out of nothing would vio-
late conservation of charge.
(In section 13.4 we will learn of one more fundamental way in which
electrons differ from photons, for a total of five.)
Because electrons are different from photons, it is not immedi-
ately obvious which of the photon equations from chapter 11 can be
applied to electrons as well. A particle property, the energy of one
photon, is related to its wave properties viaE=hfor, equivalently,
E=hc/λ. The momentum of a photon was given byp=hf/cor
p=h/λ. Ultimately it was a matter of experiment to determine
which of these equations, if any, would work for electrons, but we
can make a quick and dirty guess simply by noting that some of
the equations involvec, the speed of light, and some do not. Since
cis irrelevant in the case of an electron, we might guess that the
Section 13.3 Matter as a wave 891