DOCS.
182,
183
JANUARY
1916 179
obviously
since
the
energy
tensor should
surely
enter
linearly.
So
that the
outlined
consideration
yields
the
conservation
law of matter
((2)
with
Aa
=
0),
A
=
-1/2
must
necessarily
be chosen. Otherwise
a
contradiction arises.
It
was
precisely
this
that
I
had
not noticed in
my
first
communication.[13]
De Sitter
has asked
me
for
a copy
of
my
last
year’s
paper.[14]
Unfortunately
I
have
none
left. Please lend it
to
him and
relay
my
best
regards.
Imagine my delight
at
realizing
that
general
covariance
was
feasible and
at
finding
out
that
the
equations yield Mercury’s perihelion
motion
correctly.
I
was
beside
myself
with
joy
and excitement for
days.
Is
Tetrode at
Leyden?
His
papers on
the
entropy
constants
are superb.[15]
Congratulate
him for
me,
if
you
see
him.
Recently
I
gave
a
presentation
on
it
at
the
German
Phys.
Society.[16]
Accept,
with
your
wife
and
the little
ones,
my warm
greetings, yours,
Einstein.
Greetings
to Fokker.[17]
183. To Hendrik A. Lorentz
[Berlin,]
17 January 1916
Dear and
highly
esteemed
Colleague,
I
am
in
possession
of
your
three letters
and
very happy
about
your
concur-
rence,
especially
since
I
see
that
you
have considered
the
principal
parts of
the
theory thoroughly
and have taken to
the
idea
that
all
of
our
experience
in
physics
relates
to coincidences.[1]
This
point
of view
quite
consequently requires
the for-
mulation
of
generally
covariant
equations.
I
had taken
this
view
together
with
Grossmann
already
three
years ago
but
had then
come
to
the
false notion
that
it
was
in
contradiction
to
the
requirement
of
unique
causal
dependence.
I
had hit
upon
this
notion,
which
corresponds
to
the
standpoint
held
by you
in
the
first of
your
letters,[2]
after
all
my
efforts at
that
time
to find
a
link between covariant
gravitation equations
and Newton’s
theory
had failed.
My
series of
gravitation
papers
are a
chain of
wrong
tracks,
which nevertheless did
gradually
lead closer
to
the
objective.
That
is
why
now
finally
the
basic formulas
are
good,
but the
derivations
abominable;
this
deficiency
must still be eliminated.
In
both
your
letters
you presented
the
sense
of
the
general
covariance
require-
ment in
an
exemplarily
clear fashion.
It
would
certainly
be of tremendous benefit
to
the
issue if
you
made
your
considerations available to
other
physicists
as well,
by
writing
an
article
on
the
foundations of
the
theory,
as
you kindly proposed
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