204 RELATIVITY, THE GENERAL THEORYstein? Why would he ever write about a static gravitational field coupled to a
nonstatic Maxwell field and hope to make any sense? I would certainly have asked
him this question, were it not for the fact that I never laid eyes on these papers
until many years after the time I knew him. I can offer nothing better than the
reply I imagine he might have given me.
The time is about 1950. Einstein speaks: 'Ja, wissen Sie, that time in Prague,
that was the most confusing period in my life as far as physics was concerned.
Before I wrote down my equation Ac = kcp, I had, of course, thought of using
the Dalembertian instead of the Laplacian. That would look more elegant. I
decided against that, however, because I already knew that gravitation would have
to lead me beyond the Lorentz transformations. Thus I saw no virtue in writing
down DC = kcp, since Lorentz in variance was no longer an obvious criterion to
me, especially in the case of the dynamics of gravitation. For that reason, I never
believed what Abraham and others were doing at that time. Poor Abraham. I did
not realize, I must admit, that one can derive an equation for a time-dependent
scalar gravitational field that does satisfy the weak equivalence principle. No, that
has nothing to do with the wrong value for the perihelion obtained from a scalar
theory. That came some years later. I thought again about a scalar theory when
I was at first a bit overawed by the complexity of the equations which Grossmann
and I wrote down a little later. Yes, there was confusion at that time, too. But it
was not like the Prague days. In Zurich I was sure that I had found the right
starting point. Also, in Zurich I believed that I had an argument which showed
that the scalar theory, you know, the Nordstrom theory, was in conflict with the
equivalence principle. But I soon realized that I was wrong. In 1914 I came to
believe in fact that the Nordstrom theory was a good possibility.
'But to come back to Prague. The only thing I believed firmly then was that
one had to incorporate the equivalence principle in the fundamental equations.
Did you know that I had not even heard of the Eo'tvos experiments at that time?
Ah, you knew that. Well, there I was. There was no paradox of any kind. It was
not like the quantum theory in those days. Those Berlin experiments on blackbody
radiation had made it clear that something was badly amiss with classical physics.
On the other hand, there was nothing wrong with the equivalence principle and
Newton's theory. One was perfectly compatible with the other. Yet I was certain
that the Newtonian theory was successful but incomplete. I had not lost my faith
in the special theory of relativity either, but I believed that that theory was likewise
incomplete. So what I did in Prague was something like this. I knew I had to start
all over again, as it were, in constructing a theory of gravitation. Of course, New-
tonian theory as well as the special theory had to reappear in some approximate
sense. But I did not know how to proceed. I was in no-man's land. So I decided
to analyze static situations first and then push along until inevitably I would reach
some contradictions. Then I hoped that these contradictions would in turn teach
me what the next step might be. Sehen Sie, the way I thought then about New-
tonian theory is not so different from the way I think now about quantum