DOCS.
183,
184
JANUARY
1916 181
It
is important
that the
equations initially
be
given
in
a
generally
covariant
form because
only
in
this
setup
for the
equations
is
all arbitrariness avoided. For
if
you
confine
yourself
from
the
start
to
the
case
y-g
=
1,
then
a
factor
(V-g)n
could be added to scalar
G
without
disturbing
the
thus
restricted
covariance.
The
same
is
valid
analogously
to
the
equations covering
matter.
It
would
undoubtedly
be
a
step
forward if
later the
frame of reference could
be
specialized
even
further
in
a
natural
way. My
efforts in this direction have
been unsuccessful
up
to
now,
though.
In
any
case,
the
problem
must
obviously
be
arranged
so
that
dx1, dx2, dx3
is of
a
spatial nature throughout
and
dx4
of
a
temporal
one.
But
this
is
a
specialization solely by
means
of
inequalities,
not
equalities.
Your
remark
on
extinction
is
very
convincing.[8]
If
only light
were
finally
shed
on
the
absorption
process!
But the
proof
of
the
existence
of
zero-point
energy
shows
us
how far
away
we are
from
a genuine
understanding
here.[9]
In
thanking
you again
for
your burning
interest and
even more
for
your
inten-
tion
to
place your
efforts in
the
service
of
this
problem,
I
remain
with heartiest
greetings
to
you
and
yours,
in
friendship,
yours truly,
A.
Einstein.
Cordial
thanks
for
sending
your
lectures
on
statistical
mechanics.[10]
Feb.
23.
I
admire
very
much Tetrode’s
splendid analyses
on
the
entropy
constant.[11] I
recently gave a
report
on
it.[12]
184. To
Hendrik
A. Lorentz
[Berlin,]
19 January 1916
Dear and
highly
esteemed
Colleague,
Only
too
well
do
I
understand
your
attempt
also to derive
gravitation
from
the
field
equations
in
the
manner
of
Hamilton’s
principle.
I
myself
am
compelled
to
derive
the
Hamiltonian function
retroactively,
in
order
to
derive
the
expression
for
the
conservation laws
conveniently.[1]
Nevertheless,
I must
admit that
I
actually
do not
see
in Hamilt.
princip. anything
more
than
a means
toward
reducing
a
system
of
tensor
equations
to
a
scalar
equation
for which
the
conservation laws
are
always
satisfied and
easily
derived. I
definitely
believe
that it
is
possible
to
find
a
Hamiltonian form also for
the
generally
covariant form of
the
equations,
as
I
already
indicated
in
yesterday’s
letter.[2]
I
must
emphasize again
that
my
field
equations (2a) given
in
my
contribution,
“The Field
Equations of
Gravitation”[3]
Gim=k(Tim-1/2gimT)