CHAPTER FIVE
what share has every mass in the determination of direction and velocity
in the law of inertia? No definite answer can be given to this question
by our experience. We only know that the share of the nearest masses
vanishes in comparison with that of the farthest.”
7
Mach’s suggestion that the distribution and motion of masses may
determine the inertial behavior of test particles was soon tested exper-
imentally by the brothers Benedict and Immanuel Friedlaender. They
tried to find out whether particles at the center of a huge rotating
flywheel are subject to centrifugal forces, an effect that they referred
to as the “inversion of centrifugal forces” (“Umkehrbarkeit der Cen-
trifugalkraft”) and which can be regarded as an anticipation of the
“Thirring-Lense effect.” Although they failed to detect this effect they
declared prophetically: “A correct formulation of the law of inertia will
be obtained only if the relative inertia qua mutual interaction of masses
and gravitation, which is likewise an interaction between masses, will
be reduced to one and the same law.”
8
Einstein had probably never read the Friedlaenders’ essay. But like
them, he was greatly influenced by Mach, whom he had read avidly
in his student years. Like them he devised an experiment to study the
effect of moving masses on a test particle at rest.
9
It differed from the
Friedlaender experiment insofar as it was a thought experiment, made
use of a massive hollow rotating sphere instead of a flywheel, and—
most importantly—was designed to study not the inertial motion of the
7
“Was w
¨
urde aus dem Tr
¨
agheitsgesetz, wenn der ganze Himmel in Bewegung k
¨
ame
und die Sterne durcheinandergingen? . . . Allein im Falle einer Weltersch
¨
utterung . . .
erfahren wir, dass alle K
¨
orper in dem Tr
¨
agheitsgesetz jeder mit seinem Antheil, . . . von
Wichtigkeit sind. . . . Welchen Antheil hat nun jede Masse an der Bestimmung der Rich-
tung and Geschwindigkeit im Tr
¨
agheitsgesetze?” E. Mach, Die Geschichte und die Wurzel
des Satzes von der Erhaltung der Energie (Prague: Calve, 1872), pp. 49–50; History and Root
of the Principle of the Conservation of Energy (Chicago: Open Court, 1911), pp. 78–79.
8
“Die richtige Fassung des Gesetzes der Tr
¨
agheit [wird] erst dann gefunden . . . , wenn
die relative Tr
¨
agheit als eine Wirkung von Massen auf einander und die Gravitation, die ja
auch eine Wirkung von Massen auf einander ist, auf ein einheitliches Gesetz zur
¨
uckgef
¨
uhrt
sein werden.” B. and I. Friedlaender, Absolute oder Relative Bewegung (Berlin: L. Simion,
1896), p. 17. B. Friedlaender, “Absolute or Relative Motion?,” in J. B. Barbour and H. Pfister,
Mach’s Principle (Boston: Birkh
¨
auser, 1995), pp. 114–118.
9
A. Einstein, “Gibt es eine Gravitationswirkung, die der elektrodynamischen In-
duktionswirkung analog ist?,” Vierteljahrsschrift f
¨
ur gerichtliche Medizin und
¨
offentliches
Sanit
¨
atswesen 44, 37–40 (1912); “Is There a Gravitational Effect Which Is Analogous to
Electrodynamic Induction?,” Collected Papers, vol. 4, pp. 175–178.
146