7 6 D O C . 9 4 A U G U S T 1 9 1 9
Therefore, from (2) and (3), .
You are making the same analogous error for clocks as for measuring rods. The
rotating observer notices very well that, of both his identical clocks, the one
positioned at the circumference runs more slowly than the one positioned at the
center. We also prove this by again assessing the whole process from . Let
be the clock at the center, the one at the periphery. Observed from , runs
more slowly than . An observer positioned next to and rotating along with
it thus also sees running more slowly than . For it is clear that—judged from
—the interval between the occurrence of a position of the clock hands and the
observer’s perception of it is constant (independent of time).–
I hope this explanation will suffice. I am glad that the theory of gravitation meets
with your approval. The photographs from the English expedition supposedly came
out well, but no evaluation has been made yet. They may wait about half a year in
order to take comparison exposures of the relevant sky region with the same instru-
ment.[7]
With amicable greetings and best wishes for the remainder of the vacation, I am
sincerely yours,
A. Einstein
P.S. Was my recommendation any help?[8]
94. To Auguste Hochberger
[Berlin,] Wednesday. 20 August
[1919][1]
Dear Guste,
Back from Switzerland, I discover your letter (unluckily under a mound of cor-
respondence). I know very well that this trip has its difficulties.[2] But first, as con-
cerns the entry permit (permit on the part of the Swiss), this will be easily and
promptly obtained. For, the brother of Mama’s doctor is an official at the Bern Po-
lice Bureau for Alien Registration.[3] So if you still have the courage to take this
trip, immediately direct an application to the Police Bureau for Alien Registration
in Bern and at the same time write Mama a postcard about whether she would re-
quest her doctor, Fräulein Tobler, to take up the matter.
The main difficulty is money, though. How much I would like to make Swiss
money available! But my financial powers are inadequate. I have to come up with
8000 francs annually for my family over there and besides that some more for
Mama (most of it is being borne by Mama’s brother Jakob).[4] Of course you can
U
r
--- -
U0
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1
1
v2
c2
---- -–
------------------ or from (1)

1
v2
c2
---- -–
------------------ = =
K0 Uz
Up K0 Up
Uz Uz
Up Uz
K0
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