256 THEORY OF RELATIVITY
By
the
turn
of
this
century,
when
Einstein started
working on
the
electrodynamics
of
moving
bodies,
Lorentz's
very
successful version
of Maxwell's
theory
had
gained
wide
acceptance.[18]
Lorentz's
electrodynamics
is
based
on a microscopic theory
that
came
to
be known
as
the electron
theory.[19]
The
theory
makes
a sharp
distinction
between
ordi-
nary, ponderable
matter and the ether.
Ordinary
matter
is
composed
of
finite-sized
material
particles,
at least
some
of
which
are electrically charged.
All
of
space, even
those
regions
occupied
by
material
particles,
is
pervaded by
the
ether,
a
medium with
no
mechanical
properties,
such
as mass.
The ether
is
the seat
of
all electric and
magnetic
fields.
Matter
only
influences the ether
through charged particles,
which create these fields. The
ether
only
acts
on
matter
through
the electric and
magnetic
forces
that
the fields exert
on
charged
particles.
By assuming
such atoms
of
electricity
("electrons"),
the
theory
incorporates an
important
element
of
the
pre-Maxwellian
continental tradition into
Maxwell's
theory,
from which it took the field
equations.
The
parts
of
the ether
are
assumed to be immobile relative to each other.
Hence,
Lo-
rentz's
ether defines
a rigid
reference
frame,
which
is
assumed to be
inertial. It
is
in this
frame that
Maxwell's
equations are
valid;
in other
frames,
the
Galilei-transformed
form
of
these
equations
hold. Hence it should be
possible
to detect the motion
of
the
earth
through
the ether
by suitably designed
terrestrial
electromagnetic or optical experiments.
Lorentz
was
well
aware
of
the failure
of
all
attempts
to detect the motion
of
the earth
through
the
ether,
in
particular
such sensitive
optical
attempts
as
the
Michelson-Morley experiment,[20]
and
attempted
to
explain
this failure
on
the
basis of his
theory.
His basic
approach
to this
problem
in 1895
was
to
use
the theorem
of
"corresponding
states"
("correspondierende Zustände") in
combination
with the well-known
contraction
hypothesis.[21]
The theorem
is
essentially
a
calculational tool
that
sets
up a
correspondence
between
phenomena
in
moving
systems
and
those
in
stationary sytems by introducing
transformed coordinates and fields. On this
basis,
Lorentz
was
able to account
for
the
failure
of
most
electromagnetic experiments
to
detect the motion
of
the
earth
through
the
ether. In 1904 he showed how to
explain
the failure
of
all such
experiments
by
a
general-
ization
of
his
theory.
He introduced
a
set
of
transformations for the
spatial
and
temporal
coordinates
(soon
named the Lorentz transformations
by Poincare)[22]
and
for
the electric
and
magnetic
field
components,
such that
under
these transformations
Maxwell's
equa-
tions in the absence
of
charges
take the
same
form in all inertial
frames.[23]
Lorentz's
ever, even
here it
yields
incorrect
predictions.
For its
rejection
see,
e.g.,
Lorentz
1892c,
p.
6
(reprint
edition);
Cohn
1900, p.
518;
Cohn
1902,
p.
29;
and Abraham/Föppl
1904, pp.
427-428, 435.
[18]
Theories similar to
Lorentz's
electron the-
ory
were
proposed by
Wiechert (Wiechert
1896)
and
Larmor
(Larmor 1894, 1895, 1897).
Emil
Cohn
proposed
a macroscopic electrodynamics
of
moving
media
(Cohn
1900, 1902),
which
re-
ceived
some
attention. For
a
review of these the-
ories,
see Hirosige
1966. Einstein mentioned
Cohn's
theory
in Einstein
1907j
(Doc. 47),
p.
413, and
a copy
of Cohn 1904a
is
in Einstein's
reprint
collection
(now
in
IsReW).
[19]
For Lorentz's
program
see, e.g., Lorentz
1892c, pp.
70-71.
For reviews
of
his
theory, see
Lorentz
1904c,
1909b. For discussions
of
the
theory, see
McCormmach
1970a,
1970b.
[20]
Michelson
1881;
Michelson
and
Morley
1887. For
Lorentz's
first discussion
of
Michel-
son's
experiment, see
Lorentz 1886.
[21]
See Lorentz 1895.
[22]
See
Poincare
1905b.
[23]
See Lorentz 1904a. Lorentz did not have
transformation laws for the
charge density
and