DOC.
460
FEBRUARY
1918 469
2)
Its
boundary
conditions
at
infinity
are
not
covariant under
arbitrary
transfor-
mations, i.e., not
relativistic.
The
inertial
phenomena
and
metric properties
of
the
space-time
continuum
are
basically
determined
by
these nonrelativistic
boundary
conditions,
not
by
the
bodies.
These deficiencies
are
eliminated with
the
A-hypothesis. According
to
it,
there
can
be
no
singularity-free
guv-world
without
matter.[11]
The
guv's
rather
seem entirely
defined
by
matter,
obviously up
to
the
4
arbitrary
functions
which
correspond
to
the
physically
meaningless
vacuous
choice
of coordinates.
You
should not
say
that
I
prefer
the
curved world
(over
the
flat
one)
but that
I
prefer
the
matter-filled
world
over
the
essentially empty
one.
Matter
necessarily
causes
curvature.
I
do not
agree
at all with
your
reflection
about the bent
(flapping) rod.[12]
All
physical descriptions
that
yield
the
same
observable relations
(coincidences)
are
equivalent
in
principle,[13]
provided
that both
descriptions
are
also based
on
the
same
laws
of nature.
The
choice of
coordinates
can
have
great
practical
importance
from
the
point of
view of
clarity
of
description;
in
principle,
though,
it
is
entirely insignificant.
It
means
nothing
that
“arbitrary gravitational fields”
occur, depending
on
the
coordinate
choice;
it
is not
the
fields
themselves
that
lay
claim
to
reality. They
are
merely analytical
auxilliaries in
the
description
of
realities;
in
principle, one can
actually only
learn
something
about the latter
by eliminating
the
coordinates. The
ghost
of absolute
space
haunts
your
rod
example;
the
argument
works "ad hominem”
but
does
not
hit the mark
at
all
as
I interpret
it.
It
is
not
a
question
of
a
violation of
logic.
That
a
rod
must
either be
straight
or
bent but
cannot be
both
corresponds exactly
to
the
objection
advanced
by philosophers against
the
special theory
of rel.
that the
same
body
(at
the
same
instant
space-time
point)
cannot
be
simultaneously
at rest
and in
motion.
Are
my sketchy
remarks clear
enough,
I
wonder? Once
the
world has
wo-
ken
up
again
from its miserable
nightmare,
once
the
railways
can
again
be used
with
pleasure,[14]
and
provided we
both
are
still
alive,
let
us
make
our
planned
conversation
a reality.
With best
regards,
I
am
yours truly,
A.
Einstein
Read
Schlick’s
booklet
on
space & time).[15]
Dr. Berliner
is
sending
it to
you![16]