40
DOCS.
54,
55
AUGUST
1907
54.
To
Paul and Conrad Habicht
[Bern] Friday
[16
August 1907]
Dear
Habichts,
I
am
not
a
little
astounded
at
the
the
lightning
speed
with which
you
built the
Maschinchen.[1]
I'll
show
up
on
Sunday.
Looking
forward
to
seeing you again, yours,
Einstein
55.
To
Wilhelm
Wien
Bern,
26
August
1907
Highly
esteemed Professor
Wien:
Of
course, I
completely
agree
with
you
that
by signal velocity
and
by group
velocity
one can
understand whatever
one
wants.
I
now
designate
the
kind
of
velocity
that,
according
to
the
theory
of
relativity,
cannot be
greater
than
the
velocity
of
light
in
a
vacuum
as
"signal velocity."
This
is
a velocity
with which
a
one-time
(not regularly
recurring) influence,
which
is not
yet
determined
by
past electrodynamic
processes,
is
propagated;
thus,
we are
dealing
here
with
the
propagation
of
an
influence
that
could,
for
example,
be used for
sending
an
arbitrary
signal.
The
propagation
velocity
of
the
amplitude
minimum in
your analysis
is not
a
"signal velocity"
because,
according to
your
derivation, this
velocity
refers
to
a
periodical process (periodical amplitude change,
not
an
amplitude
change
of
the
most general
kind).
Further, I
must retract
my
own
derivation of
a
signal velocity.
For
if
I-as
I
explained
in
my
last
letter[1]-let the
light
vector
on
the other
side
of the
plane
propagating
with
the
velocity
V,[2]
in which the
amplitude
vanishes,
if
I
let
the
light
vector vanish, I
violate the
continuity
condition
for
the
magnetic
force.
Hence
also
a
treatment
by
means
of Fourier
integrals
seems
to
me
to
be called
for.[3]
Regarding
the
application
of
Wiechert's
results,[4]
there
seems
to
be
a
misunder-
standing
between
us,
which
I
shall
now
try
to
clarify.
By
the
pure
Maxwell
theory
I
understand the relations
electric
current
=
curl
H
-dH
=
curl E.
dt
These relations
do
not
yet
determine
any
material
process,
since
they
require
supplementary stipulations regarding
the
manner
in which
the motions of
the electric
masses
proceed.