During a discussion of the results obtained at a joint meeting of
the Royal Society and the Royal Astronomical Society held especially
for that purpose recently in London, it was the general opinion that
Einstein's prediction might be regarded as justified, and warm tributes
to his genius were made on all sides. Nevertheless, I cannot refrain,
while I am mentioning it, from expressing my surprise that, according
to the report in The Times there should be so much complaint about
the difficulty of understanding the new theory. It is evident that
Einstein's little book "About the Special and the General Theory of
Relativity in Plain Terms," did not find its way into England during
wartime. Any one reading it will, in my opinion, come to the conclusion
that the basic ideas of the theory are really clear and simple; it is
only to be regretted that it was impossible to avoid clothing them in
pretty involved mathematical terms, but we must not worry about that.
I allow myself to add that, as we follow Einstein, we may retain
much of what has been formerly gained. The Newtonian theory remains
in its full value as the first great step, without which one cannot
imagine the development of astronomy and without which the second
step, that has now been made, would hardly have been possible. It
remains, moreover, as the first, and in most cases, sufficient,
approximation. It is true that, according to Einstein's theory,
because it leaves us entirely free as to the way in which we wish to
represent the phenomena, we can imagine an idea of the solar system
in which the planets follow paths of peculiar form and the rays of
light shine along sharply bent lines--think of a twisted and distorted
planetarium--but in every case where we apply it to concrete questions
we shall so arrange it that the planets describe almost exact ellipses
and the rays of light almost straight lines.
It is not necessary to give up entirely even the ether. Many natural
philosophers find satisfaction in the idea of a material intermediate
substance in which the vibrations of light take place, and they
will very probably be all the more inclined to imagine such a medium
when they learn that, according to the Einstein theory, gravitation
itself does not spread instantaneously, but with a velocity that at
the first estimate may be compared with that of light. Especially in
former years were such interpretations current and repeated attempts
were made by speculations about the nature of the ether and about
the mutations and movements that might take place in it to arrive
at a clear presentation of electro-magnetic phenomena, and also of
the functioning of gravitation. In my opinion it is not impossible
that in the future this road, indeed abandoned at present, will once
more be followed with good results, if only because it can lead to the
thinking out of new experimental tests. Einstein's theory need not keep
us from so doing; only the ideas about the ether must accord with it.
Nevertheless, even without the color and clearness that the ether
theories and the other models may be able to give, and even,
we can feel it this way, just because of the soberness induced
by their absence, Einstein's work, we may now positively expect,
will remain a monument of science; his theory entirely fulfills
the first and principal demand that we may make, that of deducing
the course of phenomena from certain principles exactly and to the
smallest details. It was certainly fortunate that he himself put the
ether in the background; if he had not done so, he probably would
never have come upon the idea that has been the foundation of all
his examinations.
Thanks to his indefatigable exertions and perseverance, for he had
great difficulties to overcome in his attempts, Einstein has attained
the results, which I have tried to sketch, while still young; he is
now 45 years old. He completed his first investigations in Switzerland,
where he first was engaged in the Patent Bureau at Berne and later as a
professor at the Polytechnic in Zurich. After having been a professor
for a short time at the University of Prague, he settled in Berlin,
where the Kaiser Wilhelm Institute afforded him the opportunity to
devote himself exclusively to his scientific work. He repeatedly
visited our country and made his Netherland colleagues, among whom he
counts many good friends, partners in his studies and his results. He
attended the last meeting of the department of natural philosophy of
the Royal Academy of Sciences, and the members then had the privilege
of hearing him explain, in his own fascinating, clear and simple way,
his interpretations of the fundamental questions to which his theory
gives rise.