200 RELATIVITY, THE GENERAL THEORYA as 7 X 10'° cm, M « 2 X 1033 g, and a = 0?87 (Einstein found (K'83). This
is the answer to which four years later he would supply a further factor of 2.
The paper ends with a plea to the astronomers: 'It is urgently desirable that
astronomers concern themselves with the question brought up here, even if the
foregoing considerations might seem insufficiently founded or even adventurous.'
From this time on, Einstein writes to his friends of his hopes and fears about
gravitation, just as we saw him do earlier about the quantum theory. Shortly after
he completed the paper discussed above, he wrote to Laub:The relativistic treatment of gravitation creates serious difficulties. I consider it
probable that the principle of the constancy of the velocity of light in its custom-
ary version holds only for spaces with constant gravitational potential. [Ell]Evidently he did not quite know yet what to believe of his most recent work.
However, he was certain that something new was needed. A few months later, he
wrote to his friend Heinrich Zangger, director of the Institute for Forensic Med-
icine at the University of Zurich: 'Just now I am teaching the foundations of the
poor deceased mechanics, which is so beautiful. What will her successor look like?
With that [question] I torment myself incessantly' [E12].
I conclude this section by paying my respects to the German geodete and astron-
omer Johann Georg von Soldner, who in 1801 became the first to answer New-
ton's query on the bending of light [S3]. 'No one would find it objectionable, I
hope, that I treat a light ray as a heavy body.... One cannot think of a thing
which exists and works on our senses that would not have the property of matter,'
Soldner wrote.* He was motivated by the desire to check on possible corrections
in the evaluation of astronomical data. His calculations are based on Newton's
emission theory, according to which light consists of particles. On this picture the
scattering of light by the sun becomes an exercise in Newtonian scattering theory.
For small mass of the light-particles, the answer depends as little on that mass as
Einstein's wave calculation depends on the light frequency. Soldner made the scat-
tering calculation, put in numbers, and found a = 0''84!!
In 1911 Einstein did not know of Soldner's work. The latter's paper was in fact
entirely unknown in the physics community until 1921. In that turbulent year,
Lenard, in one of his attempts to discredit Einstein, reproduced part of Soldner's
paper in the Annalen der Physik [L2], together with a lengthy introduction in
which he also claimed priority for Hasenohrl in connection with the mass-energy
equivalence.** Von Laue took care of Lenard shortly afterward [L3].
*I have seen not his original paper but only an English translation that was recently published
together with informative historical data [J2].
**See Section 7b.