14 DOC.
8
MAY
1914
8. To Paul Ehrenfest
[Charlottenburg, 18
May
1914]
Dear
Ehrenfest,
I
would have
had the
devil to
answer
to
long ago,
had
he done his
duty,
for not
answering
such
a
dear
person
as
you
for
so
long.
The
photographs
are charming.
I
was delighted
with
them.
The business
about
rotating
bodies
in
a
magnetic
field is
not
quite
clear
to
me.[1]
Mutual
energy certainly
does exist between
two
electrical
circuits.
But
there
is
no
mutual
energy
between
a
current
and
a
magnet;
should
this
fiction be
unpermissible
in
principle?
Visualize the state of
affairs
clearly!
magnet
fixed plane
rotating
electic
charge
rod.
Depending
on
the
position
of
the
magnet,
the
probabilities
of
the
various
rotation
velocities should be different
a
priori,
even
though
the
electrodynamic
forces
are
constantly
being compensated by
the
rod. This
necessarily follows
if
you
assume
that the
“probabilities
a
priori”
are
not
altered
by
adiabatic
change
(infinitely
slow
movement
of
the
magnet).
I
have considered
your
last letter
thoroughly.
You
are
inclined
to
the
view
that
lg
W deviates from
thermodynamic
entropy.
You
prove
that this
is
really
the
case
if
the
probability
function
G(q, p,
a) depended
on
the
a
parameters.[2]
I
believe, however,
that the
assumption
of such
a
dependency
is
not
permissible,
that
it
is
even
completely
contrary to
Boltzmann’s
conception.
For if
you
just
have
a
look
at
the
states
the
system
assumes
of
its
own
accord in
the
course
of
time,
it
is
evident
that then
variable
parameters
a are
altogether
inconceivable
as
independent
entities. Therefore
I
am
of the
opinion
that
G
can
only
be
dependent
on
p
and
q,
but
not
on a.
The scale
by
which
the
probabilities
of the
system
states
are
evaluated
may
not itself be made
dependent
on
the
system
state. Otherwise
a
comparison
of
probabilities
calculated
under such
varying
states
would be futile.