THE STRUCTURE OF SPACE
137
necessarily finite. In fact, the theory supplies
us with a simple connection
1
between the space-
expanse of the universe and the average density
of matter in it.
1
For the “radius” R of the universe we obtain the equation
ρ
κ
2
2
=
R
The use of the C.G.S. system in this equation gives
27
10
08
.
1
2
⋅
=
κ
;
ρ
is the average density of the matter.
A P P E N D I X I
SIMPLE DERIVATION OF THE LORENTZ
TRANSFORMATION [S
UPPLEMENTARY TO
S
EC
-
TION
XI
]
OR the relative orientation of the co-ordi-
nate systems indicated in
Fig. 2
, the
x-axes of both systems permanently co-
incide. In the present case we can divide the
problem into parts by considering first only
events which are localised on the x-axis. Any
such event is represented with respect to the co-
ordinate system K by the abscissa x and the
time t, and with respect to the system K' by the
abscissa x' and the time t'. We require to find
x' and t' when x and t are given.
A light-signal, which is proceeding along the
positive axis of x, is transmitted according to the
equation
x
=
ct
or
x
−
ct
=
0
Since the same light-signal has to be transmitted
relative to K' with the velocity c, the propagation
139
F
. . . . . . . . . (1).
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