DOC. 40 ON KOTTLER'S PAPER 237
Doc. 40
5.
On Friedlich Kottler's
Paper:
"On Einstein's
Equivalence Hypothesis
and Gravitation"1
by
A. Einstein
[p.
639]
Especially noteworthy among
the
papers
that take
a
critical look
at
the
general theory
of
relativity are
those
by
Kottler
because this
colleague
has
truly penetrated deeply
into
the
spirit
of
the
theory.
I
want
to
discuss here the most recent
of
his works.
[2]
Kottler claims
I
had abandoned in
my
later
papers
the
"principle
of
equivalence"
which I did introduce in order to
unify
the
concepts
of "inertial mass" and
"gravitational
mass." This
opinion
must be based
upon
the fact that
we
both do not
denote the
same thing as
"the
principle
of
equivalence";
because
in
my opinion my
theory
rests
exclusively upon
this
principle.
Therefore
I
repeat
the
following:
[3]
1.
The
Limiting
Case of
the Special Theory
of
Relativity.
Let
a
finite
space-time-
like domain be without
a
gravitational
field; i.e.,
let it be
possible
to
introduce
a
system
of reference K
("Galilean
system")
relative to which the
following
is
true
for
the domain. As
is
usually presupposed
in the
special theory
of
relativity,
let
the
coordinates be
directly
measurable in known
manner by means
of
a
unit
measuring
stick,
and the times
by
a
unit clock. Relative to this
system an
isolated material
point
shall
move uniformly
in
a straight
line,
just
as
it
was postulated by
Galileo.
2.
The
Principle
of
Equivalence. Starting
from the
limiting case
of
the
special
theory
of
relativity, one may
ask if in the domain under consideration
an
observer,
who is
uniformly
accelerated relative to
K,
must
necessarily
judge
his state
as [p.
640]
accelerated,
or
whether he has
an option
left-according
to the
(approximately)
known laws of nature-to
interpret
his state
as
"at rest."
Or,
to
phrase
it
more
precisely:
Do the laws
of
nature,
known to
us
in
some approximation,
allow
us
to
consider
a
reference
system
K'
as
being
at
rest
if
it is in uniform acceleration with
respect
to K?
Or,
somewhat
more
generally:
Can the
principle
of
relativity
be
extended such
as
to
encompass
reference
systems
that
are
in
(uniform)
accelerated
motion relative
to
one
another? The
answer
is:
insofar
as we really
know the laws
of
nature, nothing prevents us
from
considering a system
K'
as
at
rest,
provided
we
assume
a
gravitational
field
(homogeneous
in first
approximation)
relative
to
K'.
Because
in
a homogeneous gravitational
field, as
with
regard
to
our system
K',
all
bodies fall
with the
same
acceleration
independent
of
their
physical
nature. I call
"principle
of
equivalence"
the
assumption
that K'
can
be treated with all
rigor as
lAnn.
d.
Phys.
50
(1916),
p.
955.
[1]