DOC.
8
REPLY TO
A COMMENT
BY
M.
ABRAHAM
131
of which
U
and E
are
brought
into
exactly
the
same
state
relative
to
K
as
in the
second
experiment
relative
to
K'. This has
never
been and will
never
be
possible. By
contrast,
in order
to
test
the
principle
in form
2,
one
need
only
bring
E
into
different
states,
without
having
to
worry
about
U;
one
has
to
conduct
two
experiments,
in the
first of which E alone
is
brought
into the
same
state
relative
to
K
as
it
is
brought
in
the second
experiment
relative
to
K'.
Distinguishing
between these
two
formulations
was
superfluous
until
now,
because
no room
was
allowed for
any
influences of the "residual
system"
on
processes having
a
bearing upon
E.
But
my analysis
of
gravitation,
like that of
Abraham,
does
not
permit
such
a
view.
According
to
this
analysis,
the
course
of
processes
in E
(e.g.,
the
velocity
of
light) depends on
the
state
of
U
(e.g., on
the
mean
distance between E and the individual
systems
that constitute
U).
One has
to
maintain, however,
that the
relativity principle
in form
2
is
supported by
the character
of
our
entire
physical experience,
and
especially by
the Michelson and
Morley [4]
experiment,
to
such
a
degree
that it would take
powerful
arguments
to warrant
our
doubting
that
principle.
The
relativity postulate
in
its
empirically supported
form
2
can
also be
expressed
in
an
abbreviated,
but less
precise way:
"The relative
velocity
of the reference
system
K with
respect
to
the residual
system
U
does
not enter
the
physical
laws."
From the
arguments
indicated
above,
it
follows,
in
my opinion,
that
one
has
to
reject
any theory
that
privileges
one
reference
system
over
the reference
systems
that
are
in uniform
translation relative
to it.
Abraham
even
makes
an
attempt
to
specify
such
a
privileged system
in the
following
way:
"If,
from
among
all
of
the reference
[5]
systems,
the
system
in which the
gravitational
field
is
static
or
quasi-static
is
privileged,
then it
is
permissible
to
call
a
motion referred
to
this
system 'absolute,'
etc." This
seems
incorrect
to
me even
if it
were
possible
to
transform each element
of
a
dynamic
gravitational
field into
a
static
one
by means
of
a
velocity
transforma-
tion. Because it is
out
of the
question
that
a
transformation of
this
kind would
simultaneously
transform
all
elements of
a
dynamic gravitational
field in such
a
way;
thus, no
reference
system
can
be
privileged
with
respect
to all
other
systems
that
are
in uniform translational motion
relative
to
it
by means
of
such
a
stipulation.
It is
common knowledge
that
it
is
impossible
to base
a
theory
of the transforma-
tion
laws
of
space
and time
on
the
principle
of
relativity
alone. As
we
know,
this
is
connected with the
relativity
of
the
concepts
of
"simultaneity"
and
"shape
of
moving
bodies." To fill this
gap,
I
introduced the
principle
of the
constancy
of the
velocity
of
light,
which
I
borrowed
from
H.
A. Lorentz's
theory
of the
stationary
luminiferous
ether,
and
which,
like the
principle
of
relativity,
contains
a
physical assumption
that
seemed to be
justified only by
the relevant
experiments
(experiments by Fizeau,
Rowland, etc.).
This
principle says:
[6]