2 6 2 D O C . 3 1 6 F E B R U A R Y 1 9 2 0 only one solution: the nature of this course as a private lecture series be retained students who have accredited themselves receive reserved seats the remaining au- ditors gain admission as space allows. Although this solution does not follow the regulations, I do not see any other possibility that satisfies all legitimate wishes. In case that this is not acceptable, I will discontinue the course. In this latter case, I ask you kindly to issue directions in the sense that the course fees be reimbursed. Requesting prompt notification about the decision reached, I send you kind re- gards, yours sincerely. P.S. In the event of approval, I would place a notice in the newspaper.[2] 316. From Edouard Guillaume Bern, 15 February 1920 Dear Einstein, I am very glad to find that you have kept your good humor despite the difficult times.[1] But you shouldn’t make it so easy for the rascal to reply![2] “A concept,” says Einstein with full justification, “exists for a physicist only when there is a possibility of finding out in a concrete case whether or not the con- cept applies.”[3] I am therefore going to ask you to tell me what you mean by a “clock (?) judged (?)” from K.[4] The statement has no physical meaning then it is of no interest to a physicist and you are not allowed to draw any conclusions from it about spectrum lines. The statement has physical meaning but then you must specify exactly what kind of experimental arrangement is needed in order to see anything. One more remark: that a clock’s carrier is at rest with reference to does not say that no motion is occurring within the interior and surroundings with reference to . For otherwise, how would the clock function? No motion: no clock. The train of light waves moving along a plane, which you mention in the second exercise, surely originates from a light source. When you introduce it explicitly, the apparent discrepancy between the two cases disappears, in that you are likewise in- troducing a light source as a “clock” into the first case. Are you still not convinced? Fine! Examine closely the following systems of equations:[5] K′ K′
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