DOCS.
664,
665
DECEMBER
1918 705
completely
healthy
again;
but the
main
thing is
that
one
is
content,
and
that
I
really
am.
I
am
particularly
thankful
to
fate
that
my great
and
ardent
wish has
now
been
fulfilled.[9] The flu is less
virulent here
than
elsewhere.
We
have
been
spared
until
now.
What
do
you say
to
Ehrenhaft’s
negative light-pressure?[10]
Nobody
has been able
to
raise
any
sound
objection
to
the
interpretation
of
the
experiments yet.
Cordial
greetings
to all
of
you,
the
Nordströms,
de
Sitter,
and
Lorentz, yours,
Einstein.
665. To
Arnold Sommerfeld
[Berlin,]
6
December
[1918][1]
Dear
Sommerfeld,
In
my
view
it
is
not at all
arbitrary
for
elementary
domains
of
the
same
size
to be
treated
a
priori
as
equally
probable.[2]
For
the
following
is
evidently
a
neat
consequence
of mechanics. If
a
system
S
is
in
contact
with
a
second
system
of rel-
atively infinitely
great
energy
(interaction
of
the
systems
with additive
energies),
then
dW
=
const. e~krdqi...dpn
is
valid for
the
elementary probability
(frequency)
of the state of S.
The
essential
thing
there
is
that the
factor “const” be
independent
of
q1...pn.
Disregarding
the
temperature
factor, equally large elementary
domains
are
hence
equally
probable
(equally frequent). (In
the
limiting case
T
=
oo,
equally large
domains
in
phase
space
are
in fact
equally probable,
whereas it
really
would be conceivable
a
priori
that another factor
p(q1...pn),
e.g.,
Q(E)
were
added.) Computationally,
this
state
of
affairs is
usually
taken into
account
by
assigning
a
priori
equally
high
probability
to
equally large
domains.
It
must also be mentioned
that the
equation
for dW from
the
Liouville
equation
is deduced for the
(oo
large)
total
system,
so
although
the
usual form of
expression
is
imprecise,
it
is
not
exactly
false.-
It
is
true
that
I
have
hopes
for these
times, despite
the
many ugly
things
it
is
bringing
forth
in
the
details.[3]
I
see
the
political
and economic
organization
of
our
planet
advancing.
If
England
and America
are
sensible
enough
to
agree,
wars
of
some
consequence
will
not be able to
occur
at
all
anymore.
The
military
economy
so
repugnant to
me
will
also
virtually disappear.
If
the
period
of tran-
sition becomes
quite grievous
for
us,
in
particular,[4]
in
my opinion
it
is-frankly
speaking-not
entirely
undeserved.
However,
I
am
of
the
firm conviction
that
culture-loving
Germans
will
soon
again
be able
to
be
as proud
as ever
of
their
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