DOC.53 DEVELOPMENT OF RELATIVITY 407
782
NATURE
[February
17,
1921
A
Brief Outline of the
Development of
the
Theory
of
Relativity.
By
Prof. A. Einstein.
[Translated
by
Dr. Robert
W. Lawson.]
THERE
is
something
attractive
in presenting
the evolution of
a
sequence
of ideas
in
as
brief
a
form
as possible,
and
yet with
a
complete
ness
sufficient
to preserve throughout
the
con
tinuity of
development.
We shall endeavour
to
do this
for the
Theory
of
Relativity,
and
to
show
that the whole
ascent
is
composed
of
small,
almost
selfevident
steps of thought.
The entire
development
starts
off from,
and
is
dominated
by,
the
idea of Faraday and
Maxwell,
according
to
which
all
physical processes
involve
a continuity
of action
(as opposed to
action
at
a
distance), or,
in the
language
of
mathematics,
they are
expressed by partial
differential
equa
tions.
Maxwell succeeded
in
doing
this for
electromagnetic processes in
bodies
at rest
by
means
of the
conception
of the
magnetic
effect
of
the
vacuumdisplacementcurrent, together with
the
postulate of the
identity of
the
nature
of
electrodynamic fields
produced by
induction,
and
the
electrostatic
field.
The
extension of
electrodynamics to
the
case
of
moving
bodies
fell
to
the lot of Maxwell’s
suc
cessors.
H.
Hertz
attempted to
solve the
problem
by
ascribing
to
empty space
(the aether) quite
similar
physical properties to those
possessed
by
ponderable matter;
in
particular,
like
ponderable
matter, the
aether ought
to
have
at
every point a
definite
velocity.
As
in
bodies
at
rest,
electro
magnetic or
magnetoelectric
induction
ought to
be
determined
by
the
rate
of
change
of the elec
tric
or magnetic flow respectively, provided
that
these
velocities of alteration
are
referred
to
sur
face
elements
moving
with the
body.
But the
theory
of Hertz
was opposed to
the fundamental
experiment
of
Fizeau
on
the
propagation
of
light
in
flowing liquids.
The
most
obvious
extension
of Maxwell's
theory to
the
case
of
moving
bodies
was incompatible
with the results of
experiment.
At
this
point,
H.
A.
Lorentz
came
to
the
rescue.
In view of his
unqualified
adherence
to
the atomic
theory of matter, Lorentz felt unable
to
regard
the latter
as
the
seat
of continuous electro
magnetic fields.
He thus conceived of
these
fields
as being
conditions
of the
aether,
which
was
regarded as
continuous. Lorentz considered the
aether
to
be
intrinsically
independent
of
matter,
both from
a
mechanical and
a physical point
of
view. The æther did
not
take
part
in
the motions
of matter,
and
a reciprocity
between aether and
matter
could be assumed
only
in
so
far
as
the
latter
was
considered
to
be the
carrier
of attached
[2]
electrical
charges.
The
great value of the
theory
of
Lorentz
lay
in the
fact that the entire electro
dynamics
of bodies
at
rest
and of bodies
in
motion
was
led back
to
Maxwell's
equations
of
empty
space.
Not
only
did this
theory surpass
that
of
Hertz
from
the
point
of
view of
method,
but with
NO. 2677, VOL. 106]
its
aid
H. A. Lorentz
was
also
preeminently
successful
in explaining
the
experimental
facts.
The
theory appeared
to be
unsatisfactory only
in
one point
of fundamental
importance.
It
appeared to give preference
to
one
system
of
co
ordinates of
a
particular state
of motion
(at
rest
relative
to
the
æther)
as
against
all other
systems
of coordinates
in
motion with
respect to
this
one.
In this
point
the
theory
seemed
to
stand in
direct
opposition to
classical
mechanics,
in which all
inertial
systems
which
are
in
uniform motion with
respect
to
each other
are
equally justifiable
systems
of coordinates
(Special Principle
of
Rela
tivity).
In this
connection, all experience
also
in
the realm
of electrodynamics (in
particular
Michelson's
experiment) supported
the idea of the
equivalence
of all
inertial
systems,
i.e.
was
in
favour of the
special principle
of
relativity.
The
Special Theory
of
Relativity owes
its
origin
to
this
difficulty,
which,
because of
its fundamental
nature, was
felt to
be intolerable. This theory
originated as
the
answer
to
the
question
:
Is
the
special principle
of
relativity
really
contradic
tory to
the
field equations
of
Maxwell for empty
space?
The
answer
to
this
question appeared
to
be in the affirmative.
For if those
equations are
valid
with reference
to a system
of coordinates.
K,
and
we
introduce
a new
system
of coordinates.
K'
in conformity
with
theto
all
appearances [3]
readily
establishableequations
of
transformation
zr'xv/l
(Galileo transformation),
/W J
then Maxwell’s field equations are no
longer
valid
in the
new
coordinates (x',
y',
z',
t').
But
appearances are
deceptive.
A
more
searching
analysis
of the
physical significance
of
space
and
time rendered
it evident
that the Galileo
trans
formation is
founded
on
arbitrary assumptions,
and
in
particular on
the assumption
that the state
ment
of
simultaneity
has
a
meaning
which
is
independent
of the
state
of
motion of the
system
of
coordinates
used. It
was
shown that the
field
equations
for
vacuo
satisfy
the
special
principle
of
relativity, provided
we
make
use
of
the
equa
tions of transformation
stated below:
,
xvt
\
(Lorentz
transformation).
V
I

v*¡c1'
In
these
equations x, y, z
represent
the coordi
nates
measured with
measuringrods
which
are
at rest
with reference to
the
system
of coordi
nates,
and t
represents
the time measured
with
suitably adjusted
clocks
of identical
construction
which
are
in
a
state
of rest.
[1]