630 DOC.
604 AUGUST
1918
Then
Weyl’s
basic
assumption
is not correct
on
the
molecular
level
in
any
case.
As
far
as
I
can
see,
not
a
single physical
[reason]
speaks
for
it
being
valid for
gravitational
fields. The
[argument
against
it], however,
is
that the
gravitational
field equations
become of
the fourth
order,[8]
for
which
there
has been
absolutely
no
indication
in
experience up
to
now,
also
that
a
somewhat
plausible
formulation
for
the
energy
law does
not
exist
as soon as
the
Hamiltonian of
the
gravit.
field
contains
derivatives
higher
than
of
the
first order for
the
guv’s.
This
leads
me
to
the
energy question.
Your
suggestion
reveals
to
me
that
you
also
are
of
the
opinion
that
an energy
tensor
for
gravitation
could be
dispensed
with.[9]
But then the
energy
law
immediately
loses
all value.
The
matter does
satisfy
[the]
“energy
law”
dxv
+
2
dx.
^
=
0.
But the
second term
causes
this
equation
to have
no
consequence
of
the
form
d_
dt
{fdV} =
0.
It
is
also
intuitively
immediately apparent
that without
a
stress tensor
for the
static
gravitational
field,
the
Newtonian forces cannot be derived from
an energy
tensor.
If
the
energy-momentum
conservation
concept
[is
not
also]
applied
to
the
guv-field,
it loses
all
physical
value.-
What
I
wrote
you
about
A
is
worthless.[10]
The
reasons
remain
the
following:
Either the
world has
a
center point,
has
on
the
whole
an
infinitesimal
density,
and
is
empty
at
infinity,
whither
all
thermal
energy eventually dissipates
as
radiation.
Or: All
points
are
on average
equivalent,
the
mean
density
is
the
same
throughout.
Then
a
hypothetical
constant
A
is
needed,
which indicates at which
mean
density
this
matter
can
be
at
equilibrium.
One
definitely gets
the
feeling
that the
second
possibility
is
the
more
satis-
factory
one, especially
since it
implies
a
finite
magnitude
for
the
world. Since
the
world
just
exists
as a
single
specimen,
it
is
essentially
the
same
whether
a
constant is
given
the
form of
one
belonging
within
the natural
laws
or
the
form
of
an “integration
constant.”-[11]
A
priori,
irreversible
elementary
laws
can
very
well be
expected.
But
closer
observations
up
to
now
do not
speak
for
it
(particularly
the
quantum
laws), no
more so
the fact
of
thermal
equilibrium. Judging
from all
I
know,
I
believe in
the
reversibility
of
elementary
events. All temporal
bias
seems
to
be based
on
“order.”
You
will
counter
with
radioactivity.
But
I
am
convinced
that
the
inverse
process
is
only impossible practically.
Warm
regards, yours,
Albert.