INTRODUCTION TO VOLUME
6
xxiii
1916,
and the
manuscript
is
related
to
that lecture. His
goal
was
to bring
out
the fundamental
aspects
of the
ways
in
which Sackur and Tetrode had used
the
concepts
of
the
quantum theory,
in order
to
gain more insight
into the sub-
ject
"without
offering anything
substantially new."[33]
Three of the four
published papers on
the
quantum
theory
in
this volume
are
concerned,
at
least
in
part,
with
a
recurrent
theme in Einstein's work:
what conditions and
presuppositions
are
needed
to
derive
Planck's radiation
law? This
is
the
primary subject
of Einstein 1914n
(Doc.
5),
one
of the first
papers
Einstein
wrote in
Berlin. Here he avoids the
use
of Boltzmann's
prin-
ciple relating entropy
and
probability
and derives
the Planck distribution
by
purely thermodynamic arguments
with the
help
of the fundamental idea
of
quantum theory.
His
argument
is
particularly interesting
for the
way
it
brings
out
the basic
similarity
between
physical
and chemical
changes
when these
are
both considered from the
point
of view
provided by
the
quantum theory.
In
the second
part
of
this
paper
Einstein
returns
to
another
topic
he
had been
thinking
about for several
years
and had discussed
on
previous
occasions: the
validity
of Nernst's heat
theorem.[34]
During
the
summer
of
1916,
less than
a year
after
he
had
completed
the
general theory
of
relativity,
Einstein made
a new, major
contribution
to
the
quantum theory.
The
two
papers
he wrote then,
Einstein
1916j
(Doc.
34)
and
Einstein 1916n
(Doc.
38),[35]
deal with the
quantum theory
of radiation
by
ar-
guments
that
do
not
depend on
the classical
electromagnetic theory,
as
had all
earlier
treatments
of Planck's
radiation
law.
The
new
theory
also led
to
a
star-
tling
conclusion: radiation emitted
or
absorbed
by
atoms in
a
radiation
field
has
a specific
direction,
and does
not
consist of
spherical waves.[36]
In the
first
of
these
papers,
Einstein
1916j
(Doc. 34),
Einstein considers
a
system
of
atoms
in
equilibrium
with
an
external radiation
field. An
atom
can
change
its internal
energy
state
by absorbing
or
emitting
radiation. Einstein
introduces three basic
assumptions
about these
exchanges
of
energy
between
matter
and
field. First,
the
probability
of
absorption
of radiation
is
propor-
tional
to
the radiation
density.
Second,
there
are
two
kinds of emission
pro-
cesses:
one-spontaneous-following
a
law like that of radioactive
decay;
[33]"ohne
etwas
materiell
Neues
zu
bieten."
Doc.
26,
[p.
1].
[34]The validity
of Nernst's theorem had been
a
topic
of discussion between Einstein
and
Nernst
ever
since the first
Solvay Congress
in 1911.
See Einstein 1914n
(Doc.
5),
note
3,
for
more
details.
[35]The
latter
appeared
in
an
issue of the
Mitteilungen
der
Physikalischen Gesellschaft
Zürich
dedicated
to
Einstein's former
mentor
and
colleague
Alfred
Kleiner,
who had died
on
3
July.
The
paper
was
reprinted
the
next
year
as
Einstein
1917c.
[36]See
also Klein
1964
and Pais
1982, chap. 21,
for historical discussions of these
papers.