PURPOSE AND PLAN 15although we now know that they will have to be replaced by others farther
removed from the sphere of immediate experience, if we aim at a profounder
understanding of relationships. [Ell]
However, in one respect Einstein forever continued to side with Newton and
to quote his authority. That was in the matter of causality. On the occasion of the
bicentenary of Newton's death, Einstein wrote to the secretary of the Royal Soci-
ety, 'All who share humbly in pondering over the secrets of physical events are
with you in spirit, and join in the admiration and love that bind us to Newton',
then went on to comment on the evolution of physics since Newton's day and
concluded as follows:
It is only in the quantum theory that Newton's differential method becomes
inadequate, and indeed strict causality fails us. But the last word has not yet
been said. May the spirit of Newton's method give us the power to restore
unison between physical reality and the profoundest characteristic of Newton's
teaching—strict causality. [E12]
What is strict Newtonian causality? As an example, if I give you the precise
position and velocity of a particle at a given instant, and if you know all the forces
acting on it, then you can predict from Newton's laws the precise position and
velocity of that particle at a later time. Quantum theory implies, however, that I
am unable to give you that information about position and velocity with ideal
precision, even if I have the most perfect instrumentation at my disposal. That is
the problem I discussed with Einstein in our conversation about the existence of
the moon, a body so heavy that the limitations on the precision of information on
position and velocity I can give you are so insignificant that, to all astronomical
intents and purposes, you can neglect the indeterminacy in the information you
obtained from me and continue to talk of the lunar orbit.
It is quite otherwise for things like atoms. In the hydrogen atom, the electron
does not move in an orbit in the same sense as the moon moves around the earth,
for, if it did, the hydrogen atom would be as flat as a little pancake whereas
actually it is a little sphere. As a matter of principle, there is no way back to
Newtonian causality. Of course, this recognition never diminished Newton's stat-
ure. Einstein's hope for a return to that old causality is an impossible dream. Of
course, this opinion, held by modern physicists, has not prevented them from rec-
ognizing Einstein as by far the most important scientific figure of this century.
His special relativity includes the completion of the work of Maxwell and Lorentz.
His general relativity includes the completion of Newton's theory of gravitation
and incorporates Mach's vision of the relativity of all motion. In all these respects,
Einstein's oeuvre represents the crowning of the work of his precursors, adding to
and revising the foundations of their theories. In this sense he is a transitional
figure, perfecting the past and changing the stream of future events. At the same
time he is a pioneer, as first Planck, then he, then Bohr founded a new physics
without precursors—the quantum theory.