140
ELECTRODYNAMICS OF MOVING
BODIES
Doc. 23
ON
THE ELECTRODYNAMICS OF MOVING
BODIES
by
A.
Einstein
[Annalen
der
Physik
17
(1905):
891-921]
It is well
known
that
Maxwell's electrodynamics-as usually
understood
at
present-when
applied to
moving
bodies,
leads
to
asymmetries
that
do not
seem
to
attach
to
the
phenomena.
Let
us
recall,
for
example,
the electro-
dynamic
interaction
between
a
magnet
and
a
conductor.
The
observable
phenome-
non
depends
here
only
on
the relative
motion of
conductor
and
magnet,
while
according
to
the
customary
conception
the
two
cases,
in
which,
respectively,
either
the
one or
the
other
of
the
two
bodies is the
one
in
motion,
are
to
be
strictly
differentiated
from each
other.
For
if the
magnet
is in motion
and
the conductor is
at
rest,
there arises
in
the
surroundings
of
the
magnet
an
electric field
endowed
with
a
certain
energy
value that
produces
a
current
in
the
places where parts of
the conductor
are
located.
But
if the
magnet
is
at
rest and
the conductor is in motion,
no
electric field arises in the
surroundings
of
the
magnet,
while in the conductor
an
electromotive force will
arise,
to
which
in itself there
does not
correspond
any
energy,
but
which,
provided
that the relative
motion
in the
two
cases
considered is the
same,
gives
rise
to
electrical
currents
that
have
the
same
magnitude
and
the
same
course as
those
produced
by
the electric forces in the first-mentioned
case.
Examples
of
a
similar kind,
and
the failure of
attempts
to
detect
a
[2]
motion
of the earth relative
to
the
"light
medium",
lead
to
the
conjecture
that
not
only
in
mechanics, but
in
electrodynamics
as
well,
the
phenomena
do
not
have
any
properties
corresponding
to
the
concept
of
absolute
rest,
but
that in all coordinate
systems
in
which
the
mechanical
equations
are
valid,
also the
same
electrodynamic
and optical laws
are
valid,
as
has already been
[3] shown
for quantities
of
the first order.
We
shall raise this conjecture
[4]
(whose
content
will
be
called "the principle
of
relativity"
hereafter)
to
the
status of
a
postulate
and
shall introduce, in addition, the postulate,
only
seemingly
incompatible
with the former
one,
that in
empty
space
light
is
always propagated
with
a
definite velocity
V
which
is
independent of
the
[5]
state of motion
of the
emitting
body.
These
two
postulates suffice for
arriving at
a
simple
and
consistent
electrodynamics
of
moving
bodies
on
the
[1]