D O C U M E N T 2 1 6 D E C E M B E R 1 9 2 0 3 1 9
ing inertia as an interaction between bodies?”
It certainly is so, and I also have always been very inclined not to assume the -
term. However . . . the apparent repulsive force that results out of my interpretation
of the universe really does seem to exist! When I wrote about
it in 1917, the radial velocities of only 3 spiral nebulae had been measured by
then.[4] Today, 25 have. And, with 3 exceptions, all are positive. The average, if one
excludes the two brightest and thus probably the closest ones, is +631 km/sec. The
largest observed velocity is 1,200 km/sec. The velocities are radial; the nebulae are
distributed unevenly throughout the whole sky.[5]
You say that the universe is too inhomogeneous (optically [speaking?]) to allow
ghost suns to come into any focus.[6] Yet the universe is incredibly empty. It is an
observational result, whose accuracy can scarcely be doubted, that outside of the
Milky Way up to a distance of about 100,000 light-years there is still no sign of ab-
sorption or dispersion of light—thus certainly less than 1/20.—It is not excluded,
of course, that over much larger distances (of 100,000,000 light-years) absorption
and dispersion would become noticeable. But we still know nothing about that at
present.
Yours sincerely,
W. de Sitter.
215. From Wander de Haas
Delft, December 1920
[Not selected for translation.]
216. To Hans Mühsam
[Berlin,] 1 December 1920
Esteemed Doctor,
By instruction of Prof. Einstein I enclose for you a short handwritten manuscript
by Prof. Einstein for the benefit of a charitable Jewish cause.[1]
In great respect,
The Secretary,
Ilse Einstein.
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