In November 1922, when Einstein and Elsa were visiting
Japan as part of an extended tour of the Far East, they received
the news that Einstein had been awarded the 1921 Nobel Prize in
Physics. Although Einstein was most famous for his theory of relativity,
the prize was officially awarded for his work on quantum theory.
Throughout the first quarter of the century, Einstein made many important
contributions to this field, the first of which was his 1905 paper
on the photoelectric effect. From 1905 to 1923, he was one of
the only scientists to take seriously the existence of light quanta, or
photons. However, he was strongly opposed to the new version of
quantum mechanics developed by Werner Heisenberg and Erwin Schroedinger
in 1925-26, and from 1926 onwards, Einstein led the opposition to
quantum mechanics. He was thus both a major contributor to and
a major critic of quantum theory.

Einstein's early contributions to quantum theory include
his heuristic suggestion that light behaves as if it is composed
of photons, and his exploration of the quantum structure of the
mechanical energies of particles embedded in matter. In 1909,
he introduced what was later called the wave-particle duality, the
idea that the wave theory of light had to be supplemented by an
equally valid yet contradictory quantum theory of light as discrete
particles. Many of Einstein's quantum ideas were incorporated
into a new model of the atom developed by the Danish physicist
Niels Bohr in the first decades of the century. Bohr explained
that electrons occupy only certain well-defined orbits around a
dense nucleus of protons and neutrons. He showed that by absorbing
a discrete quantum of energy, an electron can jump from one orbit
to another. In 1916, Einstein found that he could explain Max
Planck's blackbody spectrum in terms of the interaction of photons
with the new Bohr atoms. Although his arguments for light quanta
were well founded, the physics community did not take them seriously
until 1923. In this year, the American physicist Arthur Compton
measured the transfer of momentum from photons to electrons as
they collide and scatter, an observation that made sense only in
terms of the particle nature of light.

In spite of his contributions to the Bohr model of the
atom, Einstein remained deeply troubled by the notion that atoms
seemed to emit photons at random when their electrons change orbits.
He considered this element of chance to be a major weakness of
the model, but he hoped that it would soon be resolved when the
quantum theory was fully developed. However, by 1926 the problem
of chance remained, and Einstein became increasingly alienated
from the developments in quantum theory; he insisted that "God
does not play dice," and thus there is no room for fundamental
randomness in physical theory.

The year 1926, was a critical turning point in quantum
theory, because it witnessed the emergence of two new forms of
quantum mechanics. The first, wave mechanics, was a mathematically
accessible theory based on Louis de Broglie's idea that matter
can behave as waves just as electromagnetic waves can behave as
particles. This idea received its strongest support from Einstein,
Planck, de Broglie, and the Austrian physicist Erwin Schroedinger.
The opposing camp, led by the German physicists Bohr, Max Born,
and Werner Heisenberg, as well as the American Paul Dirac, formulated the
theory of matrix mechanics. Matrix mechanics was far more mathematically
abstract and involved those elements of chance and uncertainty
that Einstein found so philosophically troubling.

In 1928, Heisenberg, Bohr, and Born developed the "Copenhagen
interpretation," which joined the matrix and wave mechanical formulations
into one theory. The Copenhagen interpretation relies on Bohr's
complementarity principle, the idea that nature encompasses fundamental
dualities and observers must choose one side over another in making
observations. The interpretation is also based on Heisenberg's
uncertainty relations, which state that certain basic properties
of an object, such as the position and momentum of a subatomic
particle, cannot be measured simultaneously with total accuracy.
Thus the Copenhagen interpretation explained that while quantum
mechanics provides rules for calculating probabilities, it cannot
provide us with exact measurements.

Following the formulation of this new interpretation,
Born and Heisenberg proclaimed that the "quantum revolution" had
come to an end: quanta were a mere means of calculating probablilities,
but did not account for phenomena as they actually occur. However, Einstein
could not accept a probabilistic theory as the final word. As
he saw it, the very goal of physics was at stake: he yearned to
produce a complete, causal, deterministic description of nature.
In an ongoing debate with Bohr that started at the Solvay conferences
in 1927 and 1930 and lasted until the end of his life, Einstein
raised a series of objections to quantum mechanics. He tried to
develop thought experiments whereby Heisenberg's uncertainty principle might
be violated, but each time, Bohr found loopholes in Einstein's reasoning.
In 1930, Einstein argued that quantum mechanics as a whole was
inadequate as a final theory of the cosmos. Whereas he was once
regarded as too radical in his quantum theories, he now appeared
to be too conservative in his defense of classical Newtonian ideas.