DOCS.
51,
52
JULY-AUGUST
1907 35
In
any
case,
extremely
complicated
investigations
would be
needed
in
order
to
clearly
define and calculate the
propagation
velocities
of
optical signals
in metals.
Further,
since
according
to
Wiechert's
results[2]
it
is
beyond
doubt that
our
electromag-
netic
theories of
dispersion
can never yield a
superluminal velocity
for the
propagation
of
an
optical
signal,
the
equation
signal velocity velocity
of
light
L
cannot
contain
anything new
for
us.-
It
was
only
due to
an
error
in calculation
that
the
possibility
of
a
negative velocity
for the
planes
of
equal phase
of
wave
fronts entered
my
mind.
In the
hope
that
you
will
not
too
badly
misconstrue
my "writing frenzy,"
I
remain
very respectfully yours
A.
Einstein
52.
To Wilhelm
Wien
Lenk, 7
August
1907
Highly
esteemed Professor
Wien:
Thank
you
so
much for
your
last
letter.
As
I
now
have
two weeks
of
vacation,[1]
I
have
been
able
to
work in
peace
and
quiet
on
the
problem
at
hand,
and
I
think that
I
have
now
succeeded
in
achieving clarity.
First of
all,
I
cannot
agree
with
your
assertion
concerning
the
range
of
validity
of
Wiechert's
equations.[2]
The latter
are
equivalent
with Maxwell's
two
systems
of
equations
which connect
the relations
between
the curl
of
the
electric
force
or
the curl
of the
magnetic force,
and the
magnetic
flux
or
the electric
current,
respectively.
They
have
therefore
absolutely general validity according
to Maxwell's
theory,
and
they permit
the
calculation of the
magnetic
and
electric
forces if
the motions of the electric
masses
in all
of
space
and
at
all times
can
be viewed
as
known. I
cannot
therefore understand
why (aside
from
reasons
of
expediency) they
should
not
be
applied
in
the
theory
of
dispersion
and
in the
theory
of electrons
moving
arbitrarily
fast.
That
Maxwell's
equations
do
not
exclude the
possibility
of
a
motion of
rigid
electrons
with
superluminal
velocity[3]
is
no
proof against
the correctness
of
my
argumentation,
according
to
which
an
optical
signal
cannot
propagate
with
superluminal
velocity.[4]
That
is to
say
that
this
argumentation applies
only
in
the
case
where
the
electric
masses
in
a
volume
element
cannot be set
into motion
by any
other
cause
except
the
electromagnetic field
in
this volume
element,
while in the
case
of
a
rigid
electron,
rigid
bonds
are
also
assumed
to be
causes
of motion. For
owing
to
the
rigid
bonds between
the
electric
masses
of the
electron,
the
electric
forces
acting
in
one
volume element of
the electron
can
act
as
the immediate
motive
cause
for the
electric
masses
situated
in
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