270
THEORY OF RELATIVITY
People,
to whom it
is
granted
to contribute
something
to the
progress
of
science,
should not allow
pleasure
in the fruits
of
their
common
work to be clouded
by
such matters.
Die
Leute,
denen
es vergönnt
ist,
zum
Fortschritt der
Wissenschaft
etwas
beizutragen,
sollten sich die Freude über die Früchte
gemeinsamer
Arbeit
nicht
durch
solche
Dinge
trüben
lassen.[107]
Einstein returned to the
relationshp
between inertial
mass
and
energy
in 1906 and in
1907,
giving more general arguments
for their
complete equivalence,[108]
but
he did not
achieve the
complete generality
to which he
aspired.[109]
In his
Salzburg
talk,
Einstein
strongly emphasized
that inertial
mass
is
a property
of
all forms
of
energy,
and
therefore
electromagnetic
radiation must have
mass.
This conclusion
strengthened
Einstein's belief
in the
hypothesis
that
light quanta
manifest
particlelike
properties.[110]
In
1905,
Einstein
proposed a
number
of
other
experimentally
testable
consequences
of
his
theory,
in
particular
the
equations
of
motion
of
the
electron.[111]
His
use
of
the terms
"transverse mass"
and
"longitudinal
mass"
indicates that he
was
familiar
with
some
of
the
earlier
work
on
this
topic.[112]
The
following
year
he
suggested
another
experimental
test
of
these
equations employing
cathode
rays.[113]
In this
paper,
Einstein first mentioned
Kaufmann's
experimental investigations
of
the
motion
of
electrons in
ß-rays.[114]
Starting
in
1901,
Kaufmann had
carried out
a
series
of
experiments on
the deflection
of
ß-rays by
electric and
magnetic
fields. In 1905 he
asserted
that
his recent
experiments
yielded
data for the
dependence
of
the transverse
mass on
velocity
that
were incompatible
with the
(identical)
predictions
of
the Lorentz and
Einstein
theories, but
were compatible
with those
of
the Abraham and
Bucherer
electron
models.[115]
Kaufmann's
work occasioned considerable discussion. Lorentz
was
disheart-
ened
by
the
apparent
refutation
of
his
theory.[116]
Planck
subjected
the
experiment to
a
[107]
Einstein
to Johannes
Stark,
22
February
1908.
[108]
See Einstein 1906e
(Doc. 35)
and Ein-
stein 1907h
(Doc. 45).
[109]
See Einstein 1907h
(Doc. 45),
pp.
371-
372,
for
Einstein's
dissatisfaction with
argu-
ments
based
on special cases.
[110]
See
Einstein 1909c
(Doc. 60),
p.
490.
[111]
See Einstein
1905r
(Doc. 23),
§
10.
Ein-
stein restricted
himself
to
slowly
accelerated
electrons.
[112]
The
concepts were
introduced
by
Lorentz
(see
Lorentz
1900).
The
terms
were apparently
introduced
by
Abraham
(see
Abraham
1902a),
and
commonly employed
thereafter
in
discus-
sions
of
proposed equations
of
motion
for
the
electron
(see,
e.g.,
Abraham
1903,
Bucherer
1904).
[113]
See Einstein
1906g
(Doc
37).
[114]
See
Kaufmann
1905,
1906a,
which
in-
clude references to
Kaufmann's
earlier work.
For citation of
contemporary
reviews
of
attempts
to
determine the variation
of
electron
mass
with
velocity, see
note
124. For
recent accounts
of
Kaufmann's
experiments
and the
subsequent
discussion
of
them,
see
Miller
1981b,
pp.
334-
352; Cushing
1981.
[115]
See
Kaufmann
1905. For
Lorentz's
the-
ory, see
Lorentz 1904a.
For
Abraham's
model,
see
Abraham
1902a, 1902b,
1903. For Bucher-
er's
model
see
Bucherer
1904,
pp.
57-58. Lan-
gevin
had
independently proposed
the
same
hy-
pothesis as
Bucherer about the
shape
of
a
moving
electron. See
Langevin
1905c.
[116]
See the letter
of
Hendrik Lorentz
to
Henri
Poincare,
8
March
1906,
reproduced
in
Miller
1980,
pp.
83-84; and
Lorentz's
comment in
his
1906 lectures
at
Columbia
University, printed
in
Lorentz
1909b,
p.
213.