132
DOC.
8
REPLY TO A COMMENT BY
M.
ABRAHAM
There exists
a
reference
system
K in which
every light ray propagates
in
vacuum
with the universal
velocity
c,
regardless
of
whether the
light-emitting body
is in
motion
or
at rest
relative
to
K.
From these two
principles
it
is
possible
to
develop
the
theory currently
known
as
the
"theory
of
relativity."
This
theory
is
correct to
the
extent to
which the
two
principles
upon
which it is
based
are
correct.
Since these
seem
to
be correct to
a great
extent,
the
theory
of
relativity
in
its
present
form
seems
to
represent
an
important
advance; I
do
not
think that it has
hampered
the further
development
of theoretical
[7] physics!
But what about the limits of
validity
of
the
two
principles?
As
I
have
already
emphasized,
we
have
not
the
slightest
reason
to
doubt the
general validity
of
the
principle
of
relativity.
On the other
hand,
I
am
of the view that the
principle
of
the
constancy
of
the
velocity
of
light can
be maintained
only
insofar
as one
restricts
oneself
to
spatio-temporal regions
of
constant
gravitational potential.
This is
where,
in
my opinion,
the limit of
validity
of the
principle
of the
constancy
of
the
velocity
of
light-though
not
that of the
principle
of
relativity-and
therewith the limit of
validity
of
our
current
theory
of
relativity
lies.
I
have
come
to
this
opinion
on
the
basis of the
arguments
indicated below.
One
of
the
most
important
results of the
theory
of
relativity
is
the realization that
every type
of
energy
E
possesses
an
inertia
(E/c2)
proportional
to it. Since,
as
far
as
our
experience goes, every
inertial
mass
is
at
the
same
time
a gravitational
mass,
we
cannot
avoid
also
ascribing
a
gravitational
mass E/c2
to
every type
of
energy E.1
From this it
follows
immediately
that
gravity
has
a
stronger
effect
on a
moving body
than
on
the
same
body
when it
is at
rest.
If the
gravitational
field
can
be
interpreted
in
terms
of
our present
theory
of
relativity,
this
can probably
be
done
in
only
two
ways.
One
can
conceive of the
[9]
gravitational
vector
either
as a
four-vector
or as a
six-vector. For each of these
two
[10]
cases one can
obtain transformation
formulas for the transition
to
a
uniformly moving
reference
system.
With the
help
of
these
transformation formulas and the transformation
formulas for the
ponderomotive
forces,
one can
then find the forces that
act
on
material
[8]
1In
a
conversation,
Mr.
Langevin
has
drawn
my
attention to the fact that
one comes
into conflict with
experience
if
one
does not make this
assumption. Namely,
since
great
quantities
of
energy
are
given
off
during
radioactive
decomposition,
the
inertial
mass
of
the
matter must
diminish in this
process.
If
the
gravitational mass were
not
to
diminish
proportionally,
bodies
composed
of
different elements would have to have
demonstrably
different
gravitational
accelerations in the
same
gravitational
field.