66
DOCS.
53,
54 FEBRUARY
1915
53. To
Erwin Freundlich
[Berlin,
ca.
3
February
1915][1]
Dear
Freundlich,
As
a
result
of
our
conversation
yesterday,
the
following
idea
occurred
to
me
that
may
not be
new
to
you.
If
a
spectroscopic binary
star
exists,
both
compo-
nents
of
which
provide
sharply
observable
separate spectral
lines,
then the
mass
of
both
components
can
be obtained to
a
certain
degree
of
accuracy.
The
wave-
lengths
A1
(of
the
1st
component)
and
A2
(of
the
second
component)
oscillate
around
each other.
Both
components possess
the
same
average velocity
within
the radius
of
vision,
however.
Thus,
if
no
influence
were
present
other than
motion,
then the
temporal
mean
values
A1
and
A2
must
be
equal.
The variation
between
them
may
be determined
by
the influence of
gravitation;[2] however,
that
is
also
dependent
on
the
star’s
radius,
which
can
be
estimated
only very roughly.
What do
you
think
of
this matter?
I
am
writing
to Planck under
separate
cover
about the
observer
affair.[3]
Best
regards, yours,
A.
Einstein.
54.
To
Erwin
Freundlich
[Berlin,]
Friday.
[5
February
1915][1]
Dear
Freundlich,
Yesterday
Planck
spoke
with Struve
about
you.[2]
Struve indicated
that
he
would not
oppose
a
habilitation
petition
submitted
by
you.
Otherwise he railed
about
you thoroughly.
He
said
you
do
not
do what he asks of
you,
etc.[3]
Planck
thinks the
only
way
for
you
would be
to
aspire
for
a
teaching position
in theoretical
astronomy,
and
he
thinks that
you
have
good
chances
in
this
respect.
He
favors
habilitation
as soon as
possible,
with
a
thesis
preferably
in
the
field
of
theoretical
astronomy.
I
think
he
is
right
to
the
extent
that
we
cannot
place everything
on
the
one
observer-position
card.[4]
With best
regards, yours,
A.
Einstein.
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