EINSTEIN'S EARLY WORK ON THE
QUANTUM
HYPOTHESIS
I
From 1905
through
1909 Einstein
published
five
major
papers
on
the
hypothesis
of
energy
quanta,
its theoretical
implications,
and its
use
in the
explanation
of
various
phenomena:
Einstein
1905i
(Doc. 14),
Einstein 1906d
(Doc. 34),
Einstein
1907a
(Doc. 38)
with
a
correction
in Einstein 1907d
(Doc.
42), Einstein 1909b
(Doc.
56), and
Einstein
1909c
(Doc.
60).
In
addition,
he reviewed
Planck's
lectures
on
thermal radiation[1] in
Einstein
1906f
(Doc.
37),
and with Walter Ritz summarized their differences
on
the
radiation
prob-
lem in Ritz
and
Einstein 1909
(Doc.
57).
In
describing
four
of
his 1905
papers,
Einstein characterized
only
the
one on
the
quan-
tum
hypothesis
as revolutionary.[2]
It
is
now regarded as revolutionary
in
challenging
the
unlimited
validity
of Maxwell's
theory
of
light
and
suggesting
the existence
of
light
quanta.
The
paper
shows
that,
at
a sufficiently high frequency,
the
entropy
of
equilibrium
thermal
(or
"black-body")[3]
radiation behaves
as
if
the radiation consists
of
a
gas
of
independent
"quanta
of
light
energy" ("Lichtenergiequanten,"
or simply
"light quanta,"
"Lichtquanten"),
each with
energy proportional
to the
frequency.
Einstein showed how
to
explain
several otherwise
puzzling phenomena by assuming
that the interaction
of
light
with matter
consists
of
the emission
or absorption
of
such
energy quanta.
In
subsequent papers,
Einstein examined
the tacit
assumptions underlying
Planck's
der-
ivation
of
the
energy
distribution law for
black-body
radiation,[4] explored
several further
implications
of
the law
itself,
and showed
that the
concept
of
energy
quantization is
appli-
cable not
only
to radiation but also to material
oscillators. Einstein
1906d
(Doc. 34)
dem-
onstrates that
Planck's
derivation
presupposes quantization
of
the
energy
of
charged
oscil-
lators in
interaction with thermal radiation. Einstein 1907a
(Doc. 38) explains
the
anomalous
behavior of
specific
heats with
decreasing temperature by treating
the atoms
or
ions in
a
solid
as a
lattice
of
quantized
oscillators.[5] Einstein 1909b
(Doc. 56)
shows
that
Planck's
law
gives
rise to fluctuations in the
energy
and
pressure
of
black-body
radiation
that
appear
to arise from
two
independent causes: light waves
and
light
quanta.
In
a
masterful address to the
Salzburg meeting
of
the
Gesellschaft Deutscher Natur-
forscher und
Ärzte,
Einstein
1909c
(Doc. 60),
Einstein summarized his views
on
radia–
[1]
Planck
1906c.
[2]
See Einstein to Conrad
Habicht,
18
May-8
June 1905. The
paper on
the
quantum
hypothe-
sis is Einstein 1905i
(Doc. 14);
the other three
papers
mentioned
are on: relativity theory,
Ein-
stein
1905r
(Doc. 23);
molecular
dimensions,
Einstein
1905j
(Doc. 15);
and Brownian
motion,
Einstein
1905k
(Doc. 16).
[3]
The term
"black
body" ("schwarzer
Kör-
per")
was
in
general
use
by
the
turn
of
the
cen-
tury
(see,
e.g.,
Rayleigh
1900,
p. 539,
and
Planck
1900d,
p.
764).
Einstein first used the
term in
Einstein
1904
(Doc.
5), p.
354,
but
re-
ferred
to
the radiation
itself
as
"temperature
ra-
diation"
("Temperaturstrahlung"),
p.
361.
Einstein
1905i
(Doc.
14)
refers to it
as
"black
radiation"
("schwarze Strahlung").
[4]
Planck
presented
this law in
Planck
1900c.
He
gave
derivations
of
it
in
Planck
1900e,
1901a, and 1906c.
[5]
Einstein
1907d
(Doc. 42)
considers both
charged
and
uncharged
lattice oscillators.
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