326
DOC.
336 MAY 1917
to be able to
answer
you thoroughly.
Hence
only
a couple
of allusions
regarding
the
relativity postulate.
1)
We
may
be of
one opinion on
the
conditions of
the
“reality”
of
Copernican
coordinate
systems,
as
you
will
see
when
I
send
you
the
next
chapter
(on
“in-
ertial
systems”),
which
is
essentially
finished
but
not
yet
developed enough
for
transcription.
Incidentally,
it
seems
to
me,
the
sense
in which
“real” is
to
be
un-
derstood
has been clarified
long ago by Lange
and
especially by Seeliger
(Munich
Berichte
1906).[3]
Although
the
center
of
gravity
is
not
always known,
it
is
always
traceable,
no
matter
what
displacements
occur
within
the
system.
The
center of
gravity is
unique
in
this
respect
as
well.
2)
I share
your opinion
entirely
and
at
various
places
in
the
paper
have also
stated
that the
legitimacy
of all
Copernican
systems
also entails
a “relativity
postulate,”
but
a
limited
one.
All coordinate
systems
are
“Copernican”; they
differ
only
in
the
range
in which
they
are
“Copernican.”
Thus
it
is
possible
to
proceed
initially
from
any
coordinate
system,
but determinations
of
mass are
possible only
to
the
extent
that the
objects belong
within
the
“inner
region.”
3)
It
is
completely
correct
that
masses,
accelerations,
and
the coordinate
system
are
needed
to
ascertain the
center of
gravity.
But the
crucial
point is
that
one
never
has
the
masses
without the
related
Copernican
coordinate
system.
If,
therefore,
one
wants
to
apply dynamics
to
a
system
of
material
bodies,
thought
experiments
must
be
performed
in which
an
approximation
of
the
Copernican
coordinate
system
is
sought.
The
point
of
origin
of
the
Copernic.
coordinate
system
and
the
possibility
of
examining
the
masses
of
this
system
of bodies
are
found at the
same
time. This becomes
clearly
apparent
in
the
astronomers’ efforts
to
find
the
“Copernican system”
of
the fixed-star
system.
Only once
this
has been
achieved
can
there
be
a
true
dynamics
for
fixed
stars.
A
decisive
counterargument
to
my conception
would be if
a
method
were
successfully
found
according
to
which
the
masses
of the
fixed stars
could be indicated
without
using
the
connected
Copernican system.
I
know
of
no
such
possibility, though.
Cordial
greetings, yours,
Fr.
Adler.