244
DOCS.
332,
333 DECEMBER
1911
As
for the Elster
&
Geitel
instrument,[7]
it
breaks
down all the
time,
which
pleases
me a
great
deal. A
little tear in
the
thread,
a speck
of dust
on
the
thread,
a speck
of
dust
on
the
condenser,
a
small
over-voltage,
and it
goes
out.
Prof.
Haber
has
written
to
request
a
Maschinchen for his
inspection.[8]
The
man
is
very
capable
and
will
surely
come
up
with
something,
which will
then
be
a
good
advertisement.
I
wrote
to Mme
Curie[9]
about
3
weeks
ago,
but
have not received
an answer.
I
am
delighted
that
we
will
again
be able
to
work
together
in Zuri!
There
will still be much
testing
to be
done
then,
and
many things
to be
found
out
about the
ultra-resistor[10] and
the
Maschinchen.
Cordial
greetings
to
you your
wife
and
Bujo
from
your
P.
H.
333.
From Max Laue
Munich,
Bismarkstr. 22
27
December
[1911][1]
Dear
Professor Einstein:
I
have
now
carefully
studied
your paper
on
gravitation[2]
and have also
lectured about
it
at
our colloquium.[3]
I do
not
believe in this
theory
because
I cannot
concede the
full
equivalence
of
your systems
K
and
K'.[4]
After
all,
a
body causing
the
gravitation
must be
present
for the
gravitational
field in
system
K,
but
not
for
the
accelerated
system
K'.
Thus,
the
presence or, else,
the
absence
of
such
a
body
will decide
immediately
whether
we are
dealing
with
a
real
gravitational
field
or
only
an
accelerated
system.
In
spite
of
that,
I found the
paper
very
interesting
because
it shows
very clearly
the
consequences
of
equating
the inertial
and
the
ponderable
mass.
It
also
seems
to
me
singularly
characteristic that the
gravitational
potential
obtains here
a physical
meaning
that the
electrostatic
potential
completely
lacks. For, in
principle,
the former
could be
determined
directly by
measuring
the
velocity
of
light.
Your
question,
whether the
gravitational
field
strength
is to
be
represented
as
a
four-vector
or a
six-vector,
thus takes
care
of itself
completely.
Accordingly,
the
potential,
rather than
the
field
strength,
seems
to
me
to be
the
primary concept
whose
four-dimensional
representation
must be
sought.
For
all that, I do not believe in
the
correctness
of
the
theory;
I do not know
whether
the astronomical
test
that
you
proposed is
easy
to
carry
out,
but
I
fear that
if
a
deflection
were
to
be
observed,
it could
always
be
blamed
on
the
variation of the refractive
index
in
the
sun's
atmosphere,
about the
nature
of
which,
after
all, many
hypotheses
are
possible.[5]
A test
exposed
to fewer
objections,
though
no
doubt
only
a
Michelson[6]
could
carry
it
out,
seems
to
me
to
be
a
terrestrial interference
experiment
in which two
coherent
rays
would have to be
made
to traverse two
paths
that
are
essentially
horizontal
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