EARLY WORK ON QUANTUM
HYPOTHESIS
141
quanta,
Einstein
1906d
(Doc. 34).
He returned
to this
question
in
Einstein
1907a
(Doc.
38), showing
that,
if
the structure function
in
phase space
he had
introduced
earlier[48]
is
assumed
to
restrict the oscillators
to
orbits with
energies
that
are integral multiples
of
hv,
then the
average
oscillator
energy
in
a
canonical ensemble
yields
Planck's
law,
when sub-
stituted in
eq.
(1).
This
assumption
about the oscillator
energies
is
inconsistent
with
Planck's
derivation
of
eq. (1),
which
assumes
that the
energy
of
a radiating
oscillator
varies
continuously.
So Einstein also had
to
assume that,
in
spite
of
this
inconsistency,
Maxwell's
theory gives
the correct
average energy
of
an
oscillator
in
a
radiation field.
Except
for
his
paper on specific
heats,
Einstein 1907a
(Doc. 38),
submitted in Novem-
ber
1906,
Einstein did not write another
major paper on
the
quantum hypothesis
for
over
two
years. Although
his
publications
concentrate
on relativity theory during
this
period,
he did not
cease
to wrestle with the
problem
of
quanta.
He wrote Laub
in
April?
1909
about his intense
struggle
with the
problem, especially as
it
pertains
to
light:
I
am ceaselessly occupied
with the
problem
of
the constitution
of
radiation.
. . .
This
question
of
quanta
is
so extraordinarily important
and difficult that
every-
one
should be concerned with it.
Ich
beschäftige
mich
unablässig
mit der
Frage
der Konstitution der Strah-
lung.
. . .
Diese
Quantenfrage
ist
so ungemein wichtig
und
schwer,
dass sich
alle darum bemühen
sollten.[49]
Einstein
presented
the results
of
those
two
years
of
reflection
in
Einstein 1909b
(Doc.
56),
and
in
his
Salzburg
lecture,
Einstein 1909c
(Doc. 60),
which
are
discussed in sections
V
and VI below.
IV
In addition to their contributions
to
theory,
each
of Einstein's
first three
papers on
the
quantum hypothesis
also
provides ingenious explanations
of
observed
phenomena or pre-
dictions
of
new ones.
Einstein 1905i
(Doc.
14)
examines three interactions
of
light
with
matter,
treated
"as
if
light
consisted
of
such
energy quanta"
("wie
wenn
das
Licht
aus
derartigen Energiequanten bestünde"):[50]
Stokes's
rule for
fluorescence;
the ionization
of
gases by
ultraviolet
light;
and the
photoelectric
effect. In Einstein 1906d
(Doc. 34),
he
deduced
a relationship
between the electromotive force series for metals and
their
photo-
electric
sensitivity.
Einstein 1907a
(Doc. 38)
offers
an explanation
of
the anomalous de-
crease
of
specific
heats with
decreasing temperature.
Einstein's
explanations
of
the
photo-
electric effect and
of
the behavior
of
specific
heats
proved
to
be
especially significant.
Einstein 1905i
(Doc.
14) proposes
what later became known
as
Einstein's
photoelectric
equation,
[48] See,
in
particular,
Einstein 1904
(Doc.
5),
p.
359.
[49]
Einstein
to
Jakob
Laub, April?
1909.
[50]
Einstein
1905i
(Doc. 14),
p.
144.