2 0 2 D O C U M E N T 6 8 J U N E 1 9 2 0
Your letter was again a masterpiece of
and I let myself be
quite completely convinced by you, especially as far as the fundamental role of the
repetition of similar events is
I really could not quite see the forest for
the trees there. Only on the question of Newton’s law of motion-causality, which
treatment you, on your part, are not quite satisfied with either, do we still not quite
see eye to
How nice a private conversation about it would have been. It is a
complicated affair. If I take the equation
mass · acceleration = force,
then “force” is something “absolute” ([independent] of the frame of reference),
likewise with mass, if only the units (also of length) are fixed. Thus an absolute
meaning must be assigned to acceleration too. This itself is defined by length and
time, by the expression ; so one is not permitted, on the other hand, to define
acceleration by the law of inertia either. One rather has to decide to define x and t
themselves as absolute, or physically meaningful quantities. For t this succeeds
with a clock if one ignores the problem of simultaneity, c = practically ; but for
x it won’t work. One has to resort to ascribing a mysterious, i.e., empirically inac-
cessible reality to space. But the special principle of relativity in mechanics again
speaks against this.
Besides, even according to the gen. theo[ry] of r., physical space has reality, but
not an independent one, in that its properties are fully determined by matter.[5] It is
incorporated into the causal nexus without playing a one-sided role in the causal
series. It is to the credit of his logical conscience that Newton decided to create ab-
solute space (and absolute time, which, however, was less necessary). He could just
as well have called the absolute space the “rigid ether.”[6] He needed such a reality
in order to give objective meaning to acceleration. Later attempts to do without this
absolute space in mechanics were (with the exception of Mach’s) only “playing
With cordial regards, also to your wife, and in wishing (nonetheless) to see you
soon again, I am yours,
A. Einstein.
68. From Edouard Guillaume[1]
Berne, 30 June 1920
Dear Einstein,
I gather that you are in possession of my latest paper[2] and that you have read
it—which is probably not the case . . . ![3]
Previous Page Next Page