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Unified Field Theory


17a. Particles and Fields around 1920
Einstein died early on a Monday morning. The day before, he had asked for his
most recent pages of calculations on unified field theory. The awareness of unfin-
ished work was with him, and not just in those final hours when he knew that
death was near. It had been so throughout his life. Nearly forty years earlier, he
had written to Felix Klein:
However we select from nature a complex [of phenomena] using the criterion
of simplicity, in no case will its theoretical treatment turn out to be forever
appropriate (sufficient). Newton's theory, for example, represents the gravita-
tional field in a seemingly complete way by means of the potential . This
description proves to be wanting; the functions g^, take its place. But I do not
doubt that the day will come when that description, too, will have to yield to
another one, for reasons which at present we do not yet surmise. I believe that
this process of deepening the theory has no limits. [El]
That was written in 1917, shortly before he began his search for the unification
of gravitation and electromagnetism. Those were still the days in which he knew
with unerring instinct how to select complexes from nature to guide his scientific
steps. Even then he already had a keen taste for mathematical elegance as well,
but did not yet believe that formal arguments alone could be relied upon as mark-
ers for the next progress in physics. Thus, later in 1917, when Felix Klein wrote
to him about the conformal invariance of the Maxwell equations [Kl], he replied:
It does seem to me that you highly overrate the value of formal points of view.
These may be valuable when an already found [his italics] truth needs to be
formulated in a final form, but fail almost always as heuristic aids. [E2]
Nothing is more striking about the later Einstein than his change of position in
regard to this advice, given when he was in his late thirties. I do not believe that
his excessive reliance in later years on formal simplicity did him much good,
although I do not accept the view of some that this change was tragic. Nothing in
Einstein's scientific career was tragic, even though some of his work will be
remembered forever and some of it will be forgotten. In any event, when Einstein
embarked on his program for a unified field theory, his motivation was thoroughly


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