290 DOC. 26 COMMENTS ON DOC.
13
enable
us
to
define
the
straight
line
physically
even
though
no
object
or
process
exists
in
our theory
that could
serve directly as a
model of the
straight
line,
as
for
example
the
light ray
does
in
the
customary theory
of
relativity.
Regarding §4
and
§5.
The fundamental
equations
of the
theory acquire
an
especially
clear form if
one
introduces mixed
tensors.
If
one
sets
Eov
=
fiSa,t0pv~
=
£
^gga
one
obtains,
instead of
(10),
^
dx
2^
dx
TV'
Instead of
(19),
one
has
^
(2av
+
tav)
~
0
v
a*v
and
instead
of
equations (18)
for the
gravitational field,
dy £
\f~gyap8
aßn
äXa
/*v
dx

K
($ov + tov).
Regarding
§7.
The
objection
raised in
§7 against
the scalar
theory
of
gravitation
(Nordström's
theory)
has turned
out to
be invalid. One
escapes
it
by letting
the
expanse
of
the bodies
be
dependent
on
the
gravitational potential
in
an
appropriate
manner.
Further details
can
be found in
a
lecture
given by
the author
on
this
subject
(Naturforscherversammlung
in
Vienna),
which will be
published
in the
Phys.
Zeitschrift
at the end of
1913.
[5]
[6]
[7]