130
DOC.
8
REPLY TO A COMMENT BY
M.
ABRAHAM
Doc.
8
Relativity
and
Gravitation
Reply
to
a
Comment
by
M.
Abraham
by
A.
Einstein
[Annalen
der
Physik
38
(1912):
1059-1064]
In
a
note
appearing
in these
Annalen,
M.
Abraham has
responded
to
some
critical
remarks that
I
made about his
investigations
on
gravitation,
and
has,
in
turn,
criticized
[1]
my
papers on
this
subject.
In what
follows, I
wish
to
take
up,
one
by
one,
the
points
he touched
upon,
and in
particular,
I
wish
to
contrast
my
views
on
the
current state
of
the
theory
of
relativity
with those
expressed by
him.
Abraham
notes
that
I
have delivered the
coup
de
grace
to
the
relativity theory by
abandoning
the
postulate
of the
constancy
of the
velocity
of
light
and
by
the
therewith connected
relinquishment
of
the
invariance of
the
systems
of
equations
with
[2]
respect
to
the
Lorentz transformations. A
reply
to
this necessitates
a
consideration of
the foundations of the
theory
of
relativity.
[3]
The
theory presently designated
as
"the
theory
of
relativity"
rests
on
two
principles
that
are
totally independent
of
one
another,
namely
1.
the
principle
of
relativity (with
respect
to
uniform
translation),
2.
the
principle
of the
constancy
of the
velocity
of
light.
I want to formulate both
principles
more
precisely,
not
in the belief that
I
am
bringing up
something
new,
but
only
so as
to
be able
to
express myself
more
comfortably
at
a
later
point.
Let
us
contrast two
formulations
of
the
principle
of
relativity
with each
other:
1.
If
we
refer
the
physical system
to
a
coordinate
system
K that is such that the
laws of
nature
become
as simple
as
possible,
then there exist
infinitely many
coordinate
systems
with
respect
to
which these laws
are
the
same, namely,
all of
those coordinate
systems
that
are
in uniform translation relative
to K.
2.
Let E be
a
system
isolated from all other
physical systems (in
the
customary
physical
sense
of
the
term),
and let E be referred
to
a
coordinate
system
K that is
such that the laws
to which
the
spatial-temporal changes
of
E conform be
as
simple
as
possible;
then there exist
infinitely many
coordinate
systems
with
respect
to
which
these laws
are
the
same, namely,
all of those coordinate
systems
that
are
in uniform
translation relative
to K.
It is
easy
to
see
that it
is
only
the
relativity principle
in
form
2
that is
urged upon
us
by
the available
experiments.
For let E
again
denote the "isolated"
system
under
consideration and
U
the
totality
of
all
of
the other
systems
in the world. To
test
the
relativity principle
in form
1,
one
would
have
to
conduct
two
experiments,
in the first