414
DOCS.
404,
405
DECEMBER
1917
to
me
for
the
good
of
the
research.
Well, one
cannot
think
of
everything.
A
good
part of such
a
contract
must
be left
to
trust and faith.
I
hope
that,
essentially,
everything
in
the
contract
appears
to
you
carefully
thought-out.
With best
regards,
Erwin Freundlich.
405. To
Hans
Thirring
[Berlin,]
7 December
1917
Esteemed
Colleague,
First
to
your
second
question.[1]
Your
equations
of
motion
are
correct.
They
read,
if
negligible
terms
are
omitted:[2]
d2xi
_
1
dgu
ds2
2
dxi
d2x
4
dgu
.
dxA
(
dgu
ds2
dx\
Xl
+
•
+
ds
-
-
\
dx-t
¿i
+
•
+
However,
one
should
not set
d2x/ds2
equal
to
the
time derivative of
the
kinetic
energy.[3]
A
point’s negative
momentum
and
energy,
of
mass 1
in
natural
measure,
with
a
coordinate
choice
according
to
g
=
-1,
form
a
covariant four
vector
of
the
components[4]
E^is
dxa
(
(act
dxj,
ds
The
energy
is
thus
\
dxa
Ç"4“ ds
or
accurately
enough
for
us,
dx
4
3i~d7'
If
this
is
derived for
s,
then
dgu
dx4
d2xi
ds ds
+
gtt7^