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THE PHOTON 407

v (and h). It is a compatibility condition. Its physical content is this: in order that
Eqs. 21.7 and 21.8 may lead to Planck's law, it is necessary that the transitions
m ^5 n are accompanied by a single monochromatic radiation quantum. By this
remarkable reasoning, Einstein therefore established a bridge between blackbody
radiation and Bohr's theory of spectra.
About the assumptions he made in the above derivation, Einstein wrote, 'The
simplicity of the hypotheses makes it seem probable to me that these will become
the basis of the future theoretical description.' That turned out to be true.
Two of the three papers under discussion [E10, Ell] contained another result,
one which Einstein himself considered far more important than his derivation of
the radiation law: light-quanta carry a momentum hv/c. This will be our next
topic.

21c. The Completion of the Particle Picture

1. Light-Quantum and Photon. A photon is a state of the electromagnetic field
   with the following properties.


2. It has a definite frequency v and a definite wave vector k.


3. Its energy E,
   (21.13)

(21.14)

(21.15)

and its momentum p,

satisfy the dispersion law

characteristic of a particle of zero rest mass.*

3. It has spin one and (like all massless particles with nonzero spin) two states of
   polarization. The single particle states are uniquely specified by these three
   properties [Wl].
   The number of photons is in general not conserved in particle reactions and
   decays. I shall return to the nonconservation of photon number in Chapter 23, but
   would like to note here an ironic twist of history. The term photon first appeared
   in the title of a paper written in 1926: 'The Conservation of Photons.' The author:
   the distinguished physical chemist Gilbert Lewis from Berkeley. The subject: a
   speculation that light consists of 'a new kind of atom ... uncreatable and inde-
   structible [for which] I ... propose the name photon' [L2]. This idea was soon
   forgotten, but the new name almost immediately became part of the language. In

*There have been occasional speculations that the photon might have a tiny nonzero mass. Direct

experimental information on the photon mass is therefore a matter of interest. The best determina-

tions of this mass come from astronomical observations. The present upper bound is 8 X 10~^49 g

[Dl]. In what follows, the photon mass is taken to be strictly zero.