24 INTRODUCTORYstatistical origins of the light-quantum hypothesis. Einstein's paper of March
1905 contains not one but two postulates. First, the light-quantum was conceived
of as a parcel of energy as far as the properties of pure radiation (no coupling to
matter) are concerned. Second, Einstein made the assumption—he called it the
heuristic principle—that also in its coupling to matter (that is, in emission and
absorption), light is created or annihilated in similar discrete parcels of energy
(19c). That, I believe, was Einstein's one revolutionary contribution to physics
(2). It upset all existing ideas about the interaction between light and matter. I
shall describe in detail the various causes for the widespread disbelief in the heu-
ristic principle (19f), a resistance which did not weaken after other contributions
of Einstein were recognized as outstanding or even after the predictions for the
photoelectric effect, made on the grounds of the heuristic principle, turned out to
be highly successful (19e).
The light-quantum, a parcel of energy, slowly evolved into the photon, a parcel
of energy and momentum (21), a fundamental particle with zero mass and unit
spin. Never was a proposal for a new fundamental particle resisted more strongly
than this one for the photon (18b). No one resisted the photon longer than Bohr
(22). All resistance came to an end when experiments on the scattering of light by
electrons (the Compton effect) proved that Einstein was right (21f, 22).
Quantum mechanics was born within a few months of the settling of the photon
issue. In (25) I describe in detail Einstein's response to this new development. His
initial belief that quantum mechanics contained logical inconsistencies (25a) did
not last long. Thereafter, he became convinced that quantum mechanics is an
incomplete description of nature (25c). Nevertheless, he acknowledged that the
nonrelativistic version of quantum mechanics did constitute a major advance. His
proposal of a Nobel prize for Schroedinger and Heisenberg is but one expression
of that opinion (31).
However, Einstein never had a good word for the relativity version of quantum
mechanics known as quantum field theory. Its successes did not impress him.
Once, in 1912, he said of the quantum theory that the more successful it is, the
sillier it looks (20). When speaking of successful physical theories, he would, in
his later years, quote the example of the old gravitation theory (26). Had Newton
not been successful for more than two centuries? And had his theory not turned
out to be incomplete?
Einstein himself never gave up the search for a theory that would incorporate
quantum phenomena but would nevertheless satisfy his craving for causality. His
vision of a future interplay of relativity and quantum theory in a unified field
theory is the subject of the last scientific chapter of this book (26), in which I
return to the picture drawn in the preface.
Finally, I may be permitted to summarize my own views. Newtonian causality
is gone for good. The synthesis of relativity and the quantum theory is incomplete
(2). In the absence of this synthesis, any assessment of Einstein's vision must be
part of open history.