2 1 0 D O C U M E N T 7 7 J U L Y 1 9 2 0
77. To Edouard Guillaume
[Berlin,] 19 July 1920
Dear Guillaume,
In the formula for the light vectors,
t means the standard clock reading at the specified location relative to the resting
coordin. system, the progression of readings of such a clock as a complete wave
passes by it (period of oscillation). Hence, both quantities are numbers that are ob-
tained as measurement results with standard clocks at rest. Both are obtained by
counting the periods by means of a standard clock.[1] I do not understand what you
conceive as a “period” in your letter; with me, it is not a number but the designation
for a cyclic process that repeats itself.
Your deduction of the relation seems to me, quite frankly, completely
crazy. In the consideration that leads to the relations [2]
, etc.,
x, y, z, t are variables, for whose total values the equations must be satisfied identi-
cally. In the second part of the consideration, x, y, z, t are the coordinates of a plane
moving with the velocity of light (x = ctl, y = ctm, z = ctn, , etc.), thus
something quite different from the first consideration. It makes absolutely no rea-
sonable sense to compare the period length with the arrival time t of that plane
moving with the velocity of light, which passes through the origin of the coordi-
nates at time t = 0.– Your relation t = invariant does not relate to all world points
but only to a three-dimensional
My above remark about the meaning of and t as results of measurements taken
from standard clocks is just valid for the special theory of relativity, whereas in the
general theory of relativity only ds is defined as a measurement result. In the gen.
th. of r., dt initially has a purely conventional meaning. In the consideration about
line displacements, however, t again receives an absolute meaning in that the 4 co-
ordinates are chosen so as to have the field of an isolated mass-point become static;
thus the number of wavelengths that are traveling between the sun and the observer
cannot depend on t.–
In any case, I must emphasize that it seems absolutely senseless to establish a
relation between the quantities and t appearing in your consideration. Upon clos-
er reflection you will also find it so. If Hadamard and Levi-Cività are capable of
------ t

t t =
1 l +
------------------------- =
x ct l =
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