DOCS.
480,
481
NOVEMBER
1913 359
the
wavelengths
of
Röntgen
rays.
Some
metals
(e.g.,
radium)
display very narrow
emission
ranges,
so
that
it
is
really
possible
to
produce
monochromatic
Röntgen
rays.
One
can safely say
that Laue's
figures
are now
totally
explained.[6]
At the
same
time,
Debije's
work
on
the
effect
of
temperature is
also
of
great
importance.[7]
One
hopes
that
Debije
will
soon
demonstrate the incorrectness of the
hypothesis
of
zero-point
energy,
the theoretical
untenability
of
which
became
glaringly
obvious to
me
soon
after the
publication
of
the
paper
I
co-authored
with Mr.
Stern.[8]
It
is
settled that the
momentum
&
energy
fluctuations reside
in
the
radiation.[9]
With kindest
regards
to
you
and
your
wife,
yours truly,
Einstein
481. To
Paul Ehrenfest
[Zurich,
before
7
November
[1913][1]
Dear
Ehrenfest,
At that
point
in time
I
was
much
too
busy
to
come along
to
Leiden,[2]
but
I
most
solemnly
promise
that
I will
make
up
for
it
immediately
after
my move
to Berlin.[3]
I
am
tremendously
excited
about
it
already.
The
gravitation
affair has
been
clarified to
my
complete satisfaction (namely
the circumstance
that the
equations
of the
gr.
field
are
covariant
only
with
respect
to
linear transformations. For
it
can
be
proved
that
generally
covariant
equations
that determine the
field
completely
from
the
matter tensor
cannot
exist at
all.[4]
Can
there
be
anything
more
beautiful than
this,
that the
necessary
specialization
follows from
the conservation
laws?[5] Thus,
the conservation
laws
determine the
surfaces that, from
among
all
the
surfaces,
are
to be
privileged
as
coordinate
surfaces. We
can
designate
these
privileged
surfaces
as
planes,
since
we are
left with
the
linear
substitutions
as
the
only
ones
that
are
justified.
There remains
one
difficulty,
the
solution of
which will
surely
be difficult from
a
mathematical
point
of
view,
namely
the
differentiation of the
time
axis
from
the
space
axes.
What this amounts to
is
that the
gßv
would have to
satisfy
certain
inequalities
in the whole
space
for
one
to
be
able
to
differentiate
the
temporal
from
the
spatial
in
general.
I
am very glad
that
Mr.
Fokker
is
coming.[6]
I
hope
that
we
will
manage
to
accomplish something
useful
together.[7]
I
cannot
get your
idea of adiabatic transforma-
tions off
my
mind.[8]
This
may
be
our
most valuable
resource
in
our
state
of
general
hopelessness,
especially
since
the
zero-point
energy
is
now
as
dead
as a mouse.
Mr.
Keesom
has
badly aggravated
its
condition,
even
though
he
took
great pains
to
improve
it.[9]
With best
regards,
also
familiaribus, your
Einstein