SUPERLUMINAL VELOCITIES 57
Sommerfeld[4]
and Emil
Wiechert[5] it
appeared
that for
rigid
electrons-the model
championed
by
Max
Abraham-stationary
superluminal
motion
was
only possible
for
volume-charged
electrons under
constant
application
of
an
external force. For
particles
with
a
surface
charge
such motion
was
excluded. The rival model of
a
de-
formable
electron,
defended
by H. A. Lorentz,
did
not allow
superluminal speeds
at
all:
it
would need
an
infinite
amount
of
energy
to accelerate
such
an
electron
to
the
speed
of
light.
Wilhelm Wien's
participation
in the debate started in
1904 in
an
exchange
with
Abraham in the Annalen der
Physik.[6]
His
position was
firmly
on
the side of the
de-
formable electron and thus
on
the
impossibility
of
superluminal speeds.
He stressed
the
successes
the model of the
deformable
electron had
achieved,
in
particular
through
the work
of
Lorentz.
In
a
lecture
at
a
meeting
of the Gesellschaft Deutscher
Naturforscher und Ärzte the
next
year
he
reaffirmed his belief and called the
conse-
quences
drawn from the
rigid
electron model
"physically
of little
probability."[7]
The
appearance
of Einstein's
1905
relativity
paper,
Einstein 1905r
(Vol. 2,
Doc.
23),
added
a new
aspect
to
the
debate,
as
the
theory
did
not
allow
superluminal speeds
at all.[8] By
the time of the Wien
correspondence,
Einstein had
developed
an
argu-
ment
against superluminal
velocities that is
quite
distinct from those featured
in
the
earlier
paper.
First
appearing
in Einstein 1907h
(Vol.
2,
Doc.
45),
this
argument
uses
the relativistic
velocity
addition formula
to
show that
a
signal propagating superlu-
minally
from
cause
to
effect in the
rest
frame of those
two events
will
propagate
from
effect
to
cause
in another frame
moving relatively to
the
first.[9]
Einstein concludes:
Even
though,
in
my opinion,
this result contains
no
contradiction from
a
purely logical point
of
view,
it conflicts
so
absolutely
with the character of
all
our
experience,
that the
impossibility
of the
assumption W V
is suffi-
ciently proved by
this
result.[10]
The fundamental
impossibility
of
superluminal speeds
became all the
more con-
troversial,
because it had become clear that
in
dispersive
and
absorptive
media the
phase velocity
of
a
plane
wave
and
even
the
group velocity
of
a
superposition
of
waves
could exceed the
speed
of
light.[11]
Thus,
not only
some
electron
theories, but,
even more
fundamentally, pure
Maxwell
theory
seemed
to
contradict
relativity.
It
is
on
this latter
point
that the
debate with Wien
centers.
It
is not
unlikely
that Wien's
interest
in these
matters
was
stimulated
by
his
reading
of Einstein 1907h
(Vol. 2,
Doc.
45),
which
had
appeared
in June.
II
The
exchange
between Einstein and Wien
starts
with
a
letter
by
Einstein which
responds
to
an inquiry
made
by
Wien,
presumably
on
the
discrepancy
between the
occurrence
of
superluminal
velocities in
electromagnetic theory
and Einstein's
con-
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