DOC.
394 MAY
1912
295
394.
From Paul Ehrenfest
[Kanuka, Estonia,
after
16
May
1912][1]
Dear
Mr. Einstein:
Sommerfeld
wrote
me
that,
in view
of
some personnel shifts,
he
now
wants
me
to
habilitate
in
Munich.[2]
In
view
of the
very pessimistic
information that
Prof. Weiss
gave
me
about
Zurich,[3]
I
immediately agreed.
I must confess
that
I
had
lost
myself
very
deeply
in
the dream of
being
able to work
near
you,
and
that
it has
by no
means
been
easy
for
me
to cut
myself
loose from this
thought.[4]
Regarding
your
remark about the
Ritz-Doppler effect[5]
I
have the
following
to
say:
When
an
ion
that
can
oscillate
only
about
a
fixed
point
in
space
is
struck
by "moving"
plane
light
waves,
then
despite
the
fact
that
it "stands," it emits
"moving"
spherical
waves, i.e.,
the
center
of
the
induced
spherical
waves moves
in
the
same way
as
the
primary
light source
moves;
this
very ugly
assumption
is-as
it
seems
to
me-unavoidable
if
one
wants to
remain
in
agreement
with
the
energy
principle.-
But the
application
of
this
remark
shows
that
for
Ritz, too,
the
canal
rays-Doppler
effect
shows
up
in
the
grating.
D'c'
B'
0
V
\A
0
DCBA
0"
DCBA
Resting light
Moving light
At
the
instant of
time
t
=
0
let the
consecutive
wave
fronts
AA'
. . .
DD' of
the
"resting"
as
well
as
of
the
moving light
(after all, they
have the
same
wavelength, so
it's
possible)
coincide with the consecutive lines
of
the
grating
A',
B',
C', D'.
These
lines send out
induced
spherical
waves
that
have
identical
phases.
If
R
is
the radius of
the
grating,
then
we
wait
for the
length
of
time
R-c.
The induced
spherical
waves
will
then
have
assumed
precisely
the radius
R,
and all will
intersect
in
one
point,
which
is