INTRODUCTION
TO VOLUME
8
xlv
papers
on
quantum
theory
from
1916 and
1917.[12]
Was this because
they
did
not
stir
up controversy, as
did the
papers
on
relativity?
It is true that the basic
ideas and
techniques
of
the
two
papers
from
1916[13]
go
back
to earlier work
by
Einstein,
such
as
his
work
on
fluctuations and the
papers
he
wrote
together
with
Ludwig
Hopf,
but
their
results
were striking
nevertheless:
a new
and
simple
derivation
of
Planck’s
law,
characterized
by
Einstein
as
“the
derivation”
(Doc. 250),
and the conclusion
that
radiation emitted
or
absorbed
by
atoms
has
a
directed character.
Perhaps
Ein-
stein’s
readers could not
help
but be
swayed by
the
conceptually simple
and
per-
suasive
arguments
that
made these conclusions
inescapable.
As to the other
important
paper
on
quantum
theory,
Einstein 1917d
(Vol. 6,
Doc.
45),
the
only
information
this volume
provides
is that
Einstein
had turned
to
the
mathematician
Constantin
Carathéodory (Docs.
255 and
285)
and
to
Planck
(Doc. 295)
for
clari-
fication
of
Hamilton-Jacobi
theory,
one
of
the tools used in
the
paper.
In
addition,
a
fair
amount
of
other
topics
that
occur
in the
correspondence
have
no
or
hardly any
connection
with Einstein’s
published
papers. Examples
are
dis-
cussions
on quantization
with Ehrenfest in
May
1914 and
an exchange
with
Edgar
Meyer
and Lise
Meitner
in the fall
of
1918
on experiments
to
determine the nature
of
the
absorption process
of
y-rays.
Another
important
topic
which merits
special
attention is the fundamental
matter
of
the
validity
of
Walther Nernst’s
heat
theorem
and the related issue
of
the
entropy
constant in the framework
of
quantum theory.
In
one
of
the
first
papers
published during
his Berlin
years,
Einstein 1914n
(Vol.
6,
Doc.
5),
Einstein
challenges
his readers to
provide
a strictly
thermodynamic
proof
of Nernst’s heat
theorem,
promising
that
he will refute all
of
them. Einstein
had been concerned with the heat theorem
for
several
years already, as
becomes
clear,
for
instance,
from
a
discussion
with
Nernst
during
the First
Solvay Congress
in
1911
(Vol. 3,
Doc.
25, sec. 5),
and from
a paper
criticizing
Nest,
submitted
to
Physikalische Zeitschrift,
but
eventually
retracted.[14]
In
the fall
of
1913,
at the Sec-
ond
Solvay Congress,
Einstein
again spoke
out,
this time
against
a
recent thermo-
dynamic
proof
by
Nernst
(Vol.
4,
Doc.
22,
sec.
4).
The
paper
of
1914
can
be
seen
as a logical
next
step
in the
development
of
the
views
put
forward
by
Einstein at the
Solvay meetings.
For
Einstein,
as
he
explains
in
the
paper, quantum theory was
needed
as an
essential
ingredient
in
any
derivation
of
the heat theorem. The
same
point
is made in
a
manuscript
from 1916
on
the
theory
of
Otto Sackur and
Hugo
Tetrode
for
the
entropy
constant
(Vol.
6,
Doc.
26),
in which Einstein reexamines
the issue
of
the value
of
the
entropy
at
absolute
zero,
and reaffirms
a
conclusion
already
drawn in Einstein 1914n
(Vol. 6,
Doc.
5):
the heat
theorem,
in the formu-
lation that at absolute
zero
the
entropy vanishes,
is valid for
pure
crystals
only,
not
for mixed
crystals.