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DOC. 25 WHAT IS THE THEORY OF RELATIVITY
WHAT
IS
THE THEORY OF RELATIVITY?
229
exception
of
gravitation;
the
general
theory
provides
the law of
gravitation
and its relations
to
the other forces of
nature.
It
has,
of
course,
been known since the
days
of the
ancient
Greeks
that
in
order
to
describe the
movement
of
a body, a sec-
ond
body is
needed
to
which the
movement
of the first
is
re-
ferred.
The
movement
of
a
vehicle
is
considered in
reference
to
the earth’s
surface,
that
of
a
planet to
the
totality
of the visible
fixed
stars.
In
physics
the
body to
which
events
are
spatially
referred
is
called the
coordinate
system.
The
laws
of the
mechanics
of
Galileo and
Newton, for
instance,
can
only
be
formulated
with the aid of
a
coordinate
system.
The
state
of motion of the
coordinate
system may
not,
how-
ever,
be
arbitrarily chosen,
if the
laws
of mechanics
are
to
be
valid
(it must
be free from
rotation
and
acceleration).
A
co-
ordinate
system
which
is
admitted in
mechanics
is
called
an
“inertial
system.”
The
state
of
motion
of
an
inertial
system
is
according to
mechanics
not
one
that
is
determined
uniquely
by
nature.
On the
contrary,
the
following
definition
holds
good:
a
coordinate
system
that
is
moved
uniformly
and in
a
straight
line relative
to
an
inertial
system
is
likewise
an
inertial
system.
By
the
“special
principle
of
relativity” is
meant
the
generaliza-
tion
of this
definition
to
include
any
natural
event
whatever:
thus,
every
universal law of
nature
which
is
valid in relation
to
a
coordinate
system C, must
also be
valid,
as
it
stands,
in rela-
tion
to
a
coordinate
system
C',
which
is
in uniform
translatory
motion
relatively
to
C.
The second
principle,
on
which the
special theory
of rela-
tivity
rests, is
the
“principle
of the
constant velocity
of
light
in
vacuo.” This
principle asserts
that
light
in
vacuo always
has
a
definite
velocity
of
propagation (independent
of the
state
of
[5]
motion
of the observer
or
of the
source
of the
light).
The
con-
fidence which
physicists place
in this
principle
springs
from the
successes
achieved
by
the
electrodynamics
of Maxwell and
Lorentz.
Both the above-mentioned
principles
are
powerfully sup-
ported
by experience,
but
appear
not to
be
logically
reconcila-
ble. The
special theory
of
relativity
finally
succeeded
in
recon-