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-