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236 RELATIVITY, THE GENERAL THEORY

The portrait of Einstein the scientist in 1913 is altogether remarkable. He has

no compelling results to show for his efforts. He sees the limitations of what he

has done so far. He is supremely confident of his vision. And he stands all alone.

It seems to me that Einstein's intellectual strength, courage, and tenacity to con-

tinue under such circumstances and then to be supremely vindicated a few years

later do much to explain how during his later years he would fearlessly occupy

once again a similar position, in his solitary quest for an interpretation of quantum

mechanics which was totally at variance with commonly held views.

13b. The Einstein-Fokker Paper

Adriaan Daniel Fokker received his PhD degree late in 1913 under Lorentz. His

thesis dealt with Brownian motions of electrons in a radiation field [Fl] and con-

tains an equation which later became known as the Fokker-Planck equation.

After this work was completed, Lorentz sent Fokker to Zurich to work with Ein-

stein. The resulting collaboration lasted one semester only. It led to one brief paper

which is of considerable interest for the history of general relativity because it

contains Einstein's first treatment of a gravitation theory in which general covar-

iance is strictly obeyed [El8].

The authors first rewrite Eq. 13.13:

from which they conclude that the Nordstrom theory is a special case of the Ein-

stein-Grossmann theory, characterized by the additional requirement that the

velocity of light be constant. Yet the theory is, of course, more general than special

relativity. In particular, it follows from Eq. 13.21 that neither the real rate dt of

a transportable clock nor the real length dl of a transportable rod have the special

relativistic values dt 0 and dl 0 , respectively. Rather (as Nordstrom already knew)

dt 0 = dt/4/, dl 0 = dl/\{/, compatible with the i/'-independence of the light velocity.

This paper is particularly notable for its new derivation of the field equation

(Eq. 13.17). 'From the investigation by mathematicians of differential tensors,'

this field equation must be of the form (they state)

where

is the curvature scalar derived from the Ricci tensor R^ (Eq. 12.20) in which the
g^ are, of course, given (in the present instance) by Eq. 13.21. Einstein and Fok-
ker go on to prove that Eq. 13.22 (with the constant adjusted) is equivalent to
Eq. 13.17!
The paper concludes with the following remark: 'It is plausible that the role
which the Riemann-Christoffel tensor plays in the present investigation would