PRELIMINARIES 361controversy over the meaning of the negative energy solutions of the Dirac equa-
tion. During that period, one participant expressed fear for 'a new crisis in quan-
tum physics'[Wl]. The crisis was short-lived, however. The experimental discov-
ery of the positron in 1932 was a triumph for theoretical physics. The positron
theory belongs to the most important advances of the 1930s.
And then there was the photon, the first particle to be predicted theoretically.
Never, either in the first half-century or in the years thereafter, has the idea of
a new particle met for so long with such resistance as the photon. The light-quan-
tum hypothesis was considered somewhat of an aberration even by leading phys-
icists who otherwise held Einstein in the highest esteem. Its assimilation came
after a struggle more intense and prolonged than for any other particle ever pos-
tulated. Because never, to this day, has the proposal of any particle but the photon
led to the creation of a new inner frontier. The hypothesis seemed paradoxical:
light was known to consist of waves, hence it could not consist of particles. Yet
this paradox alone does not fully account for the resistance to Einstein's hypoth-
esis. We shall look more closely at the situation in Section 19f.
18c. The Quantum Theory: Lines of InfluenceThe skeleton diagram below is an attempt to reduce the history of the quantum
theory to its barest outlines. At the same time, this figure will serve as a guide to
the rest of this paper; in it X -* Y means 'the work of X was instrumental to an
advance by Y.' Arrows marked M and R indicate that the influence went via the
theory of matter and radiation, respectively.
If Planck, Einstein, and Bohr are the fathers of the quantum theory, then Gus-
tav Robert Kirchhoff is its grandfather. Since he was the founder of optical spectra
analysis (in 1860, together with Robert Bunsen [Kl]), an arrow leads from him
and Bunsen to Johann Jakob Balmer, the inventor of the Balmer formula [Bl].
From Balmer we move to Bohr, the founder of atomic quantum dynamics.
Returning to Kirchhoff as the discoverer of the universal character of blackbody
radiation [K2], we note that his influence goes via Wien to Planck (see further,
Section 19a).
The arrow from Wien to Planck refers to the latter's formulation of his black-
body radiation law and the triangle Wien-Planck-Einstein to the mutual influ-
ences which led to the light-quantum hypothesis (Sections 19b-d).
The arrow from Bose to Einstein refers to Bose's work on electromagnetic
radiation and its impact on Einstein's contributions to the quantum statistics of a
material gas (Chapter 23, wherein Einstein's influence on Dirac is also briefly
mentioned).
The triangle Einstein-de Broglie-Schroedinger has to do with the role of Ein-
stein as the transitional figure in the birth of wave mechanics, discussed in Chapter
24.
The h marking the arrow from Planck to Bohr serves as a reminder that not