DOCS. 49-50 JULY
1907 33
a
superluminal
velocity
in
the
case
of
purely
electromagnetic
signals
of
stationary
bodies
is not
possible.
4.
In
general,
it
can
be said that
no
process
in
the
optics
of
stationary
bodies that
is
consonant with the Maxwell-Lorentz
theory
can
relativity principle,
because
the
foundation of the
Maxwell-Lorentz
theory is
consistent
with
the
relativity
principle.
All in
all,
the
question
as
to
whether the
case
U
L
really
occurs
seems
to
me
of the
greatest
interest. From
Maxwell's
theory
there
follows
with
necessity
the relation
U
L, or,
if
n
denotes
the refractive
index,
for
an arbitrary
substance,
dX
Thus,
if for
some
X
we
have
n
1,
then,
according
to
the
formula,
there
exists
necessarily
a
larger
X
for which
n
=
0;
for still
larger
X
there
do
not
exist
any
waves
that
propagate
in
the
body
(n2
0).
Very
respectfully
yours,
A.
Einstein
50.
To
Wilhelm
Wien
Bern, 25
August [July]
1907[1]
Highly
esteemed Professor
Wien:
As I have
now
recognized,
the letter
I sent
you
yesterday[2]
is
in
need of
an
amendment and
a
correction.
We
must stick with
the relation
n
-
X-
1,
as
well
as
with
the
assumption
that
dX
the existence
of
a
X1
for
which
n1
1
requires
the
existence
of
a
X2
for
which
n2
=
0.
On the
other
hand, it
is not
cor-
rect
that
no wave
exists for
larger
X
;
rather, for
larger
X we
have
X 0.[3]
That
this
is
so,
and what
it
means,
can
easily
be
seen
in
this
way: Sup
pose
that
a
plane
wave
is
re-
flected
perpendicularly
at
the
me-
dium,
with
the
Z-axis
being
the normal directed
toward
the interior of the medium.
Then,
we
can
set[4]
in
the
medium,
X
=
Aejw(t-az).
Aether
Medium
positive
Z-axis
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