DOC.
31
ON THE RELATIVITY PROBLEM 309
themselves
to
be
applicable
to
cathode
rays
and
ß-radiation
(motion
of free
particles
of
electricity).
Moreover,
in the
application
of
the
theory
of
relativity
neither
a
logical
contradiction
nor
a
conflict with
experimental
results has
yet
occurred.
Only
one
result of
the
theory
of
relativity
shall be cited here
specifically,
because
this result is
important
for the considerations that follow.
According
to
Newtonian
mechanics,
the inertia of
a
system consisting
of
an
aggregate
of material
points
(inertial
resistance
against
the acceleration of the
center
of
gravity
of the
system)
is
independent
of the
state
of the
system. By contrast,
according
to
the
theory
of
relativity,
the inertia of
an
isolated
system (one floating
in
a
vacuum) depends
on
the
state
of
the
system,
in such
a
way
that this inertia increases with the
energy
content
of
the
system. Thus, according
to
the
theory
of
relativity
it
is,
in the last
analysis,
the
energy
to
which the attribute of inertia attaches. It
is this,
and
not
the inertial
mass
of material
points,
to
which
we
have to ascribe
indestructibility;
thus,
the law of
conservation of
mass
is
incorporated
into the law of
conservation
of
energy.
It
was
noted above that it would be
a
great
mistake
to
view the
theory
of
relativity
as a
universal method that
permits
the
establishing
of
an
absolutely
correct
theory
for
a
domain
of
phenomena regardless
how little it has been
investigated
empirically.
The
theory
of
relativity merely
reduces
to
a
considerable
extent
the
number
of
empirical
observations needed for
setting up a theory.
There is
only
one
area
of fundamental
importance
about which
our
empirical knowledge
is
so
small
that,
in
conjunction
with the
theory
of
relativity,
it
does
not suffice,
by
a
wide
margin,
for
a
univocal formulation of the
general theory.
This
is
the
area
of
gravitational phenomena.
The
only way
we can
reach
our
goal
here
is
by adding
physical hypotheses
to
what is
empirically
known
in
order
to
complete
the basis of
the
theory.
The
following arguments
shall show how
one
arrives
at what
are
in
my
view the most natural such
hypotheses.
When
we
speak
about the
mass
of
a
body,
we
associate with this word
two
definitions that
are
logically completely independent
of each other.
By
mass we
understand,
on
the
one
hand,
the
constant
associated with the
body
that
measures
the
resistance
of
the
body
to
its
own
acceleration
("inertial mass"), and,
on
the other
hand,
the
constant
of
the
body
that determines the
magnitude
of
the
force
experienced
by
the
body
in
a
gravitational
field
("gravitational
mass").
It
is
by
no means
self-
evident
a
priori
that the inertial
mass
and the
gravitational mass
of
a body
must
be
identical;
we are
only
accustomed
to
assuming
their
identity.
The
belief in this
identity
comes
from the observation that the acceleration
experienced by
diverse
bodies in
a
gravitational
field is
independent
of
the material of which
they
are
constituted. In
any
case,
Eötvös has shown that the inertial and the
gravitational
mass
coincide with each other with
great
precision,
in
that, by
means
of
experiments
with
the torsion
balance,
he ruled
out
the existence of
a
relative deviation of the
two
[6]
[7]
[8]
[9]