340
DOC.
17
THE THEORY OF RELATIVITY
Doc.
17
The
Theory
of
Relativity1
by
A.
Einstein
[Naturforschende
Gesellschaft
in Zürich.
Vierteljahrsschrift
56
(1911): 1-14]
[1]
The
one
basic
pillar upon
which
the
theory designated as
the
"theory
of
relativity"
rests
is
the so-called
principle
of
relativity.
First
I will
try
to make
clear
what
is
understood
by
the
principle
of
relativity.
Picture
to
yourself
two
physicists.
Let both
physicists
be
equipped
with
every physical
instrument
imaginable;
let
each
of them
have
a laboratory.
Suppose
that the
laboratory
of
one
of the
physicists
is arranged
somewhere
in
an
open
field,
and that of the second
in
a
railroad
car traveling
at constant
velocity
in
a
given
direction. The
principle
of
relativity
states
the
following:
if,
using
all
their
equipment,
these
two
physicists were
to
study
all
the
laws
of
nature,
one
in his
stationary laboratory
and the other
in his
laboratory
on
the
train, they
would discover
exactly
the
same
laws
of
nature,
provided
that the train
is not
shaking
and
is
traveling
in
uniform motion.
Somewhat
more abstractly, we can
say:
according
to
the
principle
of
relativity,
the
laws
of
nature
are
independent
of the translational motion of the reference
system.
Let
us
consider the role that
this
principle
of
relativity plays
in classical mechanics.
Classical mechanics
is
based
first
and foremost
on
Galileo's
principle, according
to which
a body
not
subjected
to
the
influences
of other bodies
finds
itself
in
uniform,
rectilinear
motion. If
this
principle
holds for
one
of
the
laboratories mentioned
above,
then
it holds
for the other
one as
well. This
we can
deduce
directly
from
intuition;
however, we can
also
deduce
this
from the
equations
of Newtonian
mechanics if
we
transform these
equations
to
a
reference
system
that
moves uniformly
relative to
the
original
reference
system.
All I have
been
talking
about
is
laboratories.
However,
in
mathematical
physics,
it
is
customary
to
relate
things
to
coordinate
systems
and
not to
a
specific
laboratory.
What
is
essential
in this
relating-to-something is
the
following:
when
we
state
anything
whatsoever about the
location
of
a
point,
we always
indicate
the
coincidence
of
this
point
with
some point
of
a specific
other
physical system.
If,
for
example,
I
choose
myself as
this
material
point,
and
say,
"I
am
at this
location
in this
hall,"
then
I have
brought
myself
into
spatial
coincidence with
a
certain
point
of
this
hall,
or
rather,
I have
asserted
this coincidence. This
is
done
in
mathematical
physics by using
three
numbers,
the
so–
called
coordinates,
to indicate with which
points
of
the
rigid system,
called
the coordinate
system,
the
point
whose
location
is
to
be described
coincides.
1
Lecture
given
at
the
meeting
of the Zurich
Naturforschende Gesellschaft on
16 January
1911.