530 
more probable than any other. In Newron’s theory on the other 
hand the value 4 =O is prescribed, and this has led to the belief 
that zero is the “true” value of &, and that any other value it 
may have in some system of reference must be “explained” by 
hypothetical masses. 
If we inquire how the conviction that 4 =O is the “true” value 
has arisen, we find the following. Observations made during long 
centuries befores those referred to above had shown that all celestial 
bodies have, superposed upon their mutual relative motions, a 
rotation —w relatively to the earth. Already long before Newton 
it had become usual to regard this apparent revolution of the stars 
round the earth as unreal and as caused by a real rotation + w of 
the earth round its axis. One of the reasons why it was considered 
unreal is the difficulty encountered in conceiving real linear velo-. 
cities of many million times the velocity of Jight. Another is that 
the hypothesis that it were real would, in an absolute space, assign 
a quite exceptional place to the earth in the universe. Both reasons 
of course have no force in a theory of relativity. 
Relatively to the system of coordinates which has just been con- 
sidered, an average star has a rotation w,—w = o,,. If we transforin 
to axes relatively to which this star has no rotation, we find 
‘ng = (ko). 
Observations of stars *) require very approximately the value (1), 
i.e. g",, = 0. It is assumed that zero is the exact value, and con- 
sequently 
— 
Le. we find. the same value as above in (4). This is certainly 
remarkable fact, and a confirmation of Einstein's theory *). Now 
if we accept an absolute space, and if we also assume that the 
system of the stars has no real rotation, i.e. no rotation relatively 
to the absolute space, then the “true” value of w, is zero, 
O, =O, =O ee OS ac EN 
2 
1) We. need not consider. the fixed stars at all. The moon will give exactly 
the same result through KepLer’s third law, if we do not neglect the mass of 
the earth. 
2) Or of Newron’s theory of inertia, which up to the approximation here used 
(and also in the next) is equivalent to EiNsrTEIN’s theory. It may be pointed out 
that in Newron’s theory inertia and gravitation are two entirely different things, 
while in Ernsrern’s theory they are one, and may be called by either name. 
Erstein’s theory is generally called a theory of. gravitation, and this of course 
is correct. But it might also be called a theory of inertia, denying the separate 
existence of gravitation, and this would also be.correct.. To the approximation 
here used, what Newron called gravitation is not involved at all, it-only- comes 
in in the next approximation through the mass of the earth. 
