DOC.
19
LECTURE
ON
FLUCTUATIONS
455
sion for the
energy density
p
of
black-body
radiation
and
derives
from this
expression its
entropy density
s
(and
hence the
entropy
S),
using
the relation
dp/T
=
ds
(T
being
the
tempera-
ture).
In the first
approach,
which
he deleted, he
chose the
temperature
T
as
the
integration
variable, while in
a
second
approach, starting
at
the bottom of
[p. 2]
and carried
through
to
the
end of the
calculation, he
chose
p
as
the
integration
variable. In the second
approach
the
temperature is
expressed
by
the relation
hv/kT
=
lg(1
+
f/p),
where
h is
Planck's
constant,
K
Boltzmann's
constant,
v
the
frequency,
and
f
an
abbreviation for
8nhv3-c3,
c
being
the
speed
of
light.
[7]Einstein
discussed the
subject
of thermal fluctuations of
a
body
immersed
in
radiation with
H. A.
Lorentz after the lecture and
in
a
letter
to
Lorentz
some days
later;
see
Einstein
to H.
A.
Lorentz,
15 February
1911.
He
presented
a
detailed
account at
the
Solvay Congress; see
Einstein
1914
(Doc.
26),
pp.
342-348. For
his
earlier
analysis
of
a
mirror
moving
in
a
radiation
field,
see
Einstein
1909b
(Vol.
2,
Doc.
56),
pp.
189-190.