EINSTEIN'S VISION 465
Overdetermination was and remained Einstein's hope for an answer to the
quantum problem. In addressing Planck, six years later, he made his point quite
emphatically: the understanding of quantum phenomena does not demand a
weakening of classical causality, as is done in quantum mechanics. On the con-
trary, classical causality should be strengthened.
Natural phenomena seem to be determined to such an extent that not only the
temporal sequence but also the initial state is fixed to a large extent by [phys-
ical] law. It seemed to me that I should express this idea by searching for over-
determined systems of differential equations. ... I strongly believe that we will
not end up with a Subkausalitat [subcausality] but that, in the indicated sense,
we will arrive at an Uberkausalitat [supercausality]. [E23].
At long last, I can now explain Einstein's vision. He was looking for a unified
field theory, but to him that concept meant something different from what it meant
and means to everyone else. He demanded that the theory shall be strictly causal,
that it shall unify gravitation and electromagnetism, that the particles of physics
shall emerge as special solutions of the general field equations, and that the quan-
tum postulates shall be a consequence of the general field equations. Einstein had
all these criteria in mind when he wrote, in 1949, 'Our problem is that of finding
the field equations of the total field' [E3]. Einstein's scientific evolution can there-
fore be schematized by the picture given in the preface:
Special relativity Statistical physics
I I
General relativity Quantum theory^ Unified "^
field theoryIn Chapter 171 discussed that portion of Einstein's work on unified field theory
that dealt with the synthesis of gravitation and electromagnetism. Here I add a
few remarks on the quantum aspects.
Einstein's correspondence shows that the unified field theory and the quantum
problems were very often simultaneously on his mind. Here are but a few exam-
ples. In 1925, while he was at work on a theory with a nonsymmetric metric, he
wrote to a friend, 'Now the question is whether this field theory is compatible
with the existence of atoms and quanta.' [E24]. He discussed the same generalized
theory in a letter written in 1942. 'What I am doing now may seem a bit crazy
to you. One must note, however, that the wave-particle duality demands some-
thing unheard of,' [E25]. In 1949 he wrote, 'I am convinced that the... statistical
[quantum] theory ... is superficial and that one must be backed by the principle
of general relativity' [E26]. And in 1954, 'I must seem like an ostrich who forever
buries its head in the relativistic sand in order not to face the evil quanta' [E27].