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a gravitational ghost of the Sun as well. But this one does not coincide with the light
ghost. I have not calculated where it could possibly be found—it would not be so
easy, either, I fear. Maybe it coincides with the current position of the Sun, in which
case it would not be dangerous. Since the Sun’s velocity has certainly not been uni-
form during the period of 500,000,000 years, however, I do not know whether one
can assert anything about the gravity ghost without complicated calculations. There
is no such thing as gravity
I showed (from the motion of the Moon)
that the absorption coefficient of gravity (cgs units) is certainly less than 4 · 10
for a journey around the universe that would yield an absorption of 1/10,000,000.
Nevertheless, it is not merely and not primarily a fear of ghosts that makes your
theory somewhat unpalatable to me; most [predominant] for me is that you make
time absolute again. Your hypothesis violates the principle of special relativity. A
Lorentz transformation is not permitted in your world. We have often quarreled
about this already, and it ultimately remains a matter of taste, which system one
wants to consider most probable.
On p. 13 of your talk you say that even the tiniest positive mean density of matter
in the world must necessarily lead to the assumption of a spatially closed world. I
think that this can be upheld only if one makes the additional hypothesis that the
world is in (statistical) equilibrium. I elaborated a bit more on this in a short article
that I sent to Lorentz to submit to the Amsterdam
I hope you have
nothing against that.
With cordial greetings, yours sincerely,
W. de Sitter.
Give Ehrenfest my regards!
191. From Paul Ehrenfest
Sunday, Maarn, 7 November 1920
Dear Einstein,
Forgive me if I hurriedly write you now, without being asked, about the Wiscon-
—it happens on the basis of a very serious conversation with Mr. de
Ridder (Kernhem) [father of Carl de

We beg you earnestly to follow this advice strictly:
1. You calmly hear out
(Wisconsin agent) and ask him to summa-
rize the main points of his proposal briefly in writing as well.
2. You do not agree to anything either in writing or
[Not even any
“contingent consent,” such as, e.g., the following: “In case I go to Columbia Univ-
ers., I shall come and see you too.] Absolutely no consent!! Instead, you say:
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