THEORY
OF RELATIVITY 255
the
principle
of
relativity
and the Maxwell-Lorentz
theory
of
electrodynamics.[11]
While the
relativity principle
asserts the
physical equivalence
of
all inertial frames
of
ref-
erence,
the Maxwell-Lorentz
theory implies
the existence
of
a privileged
inertial
frame.
The
principle
of
relativity originated
in
classical mechanics.[12]
Assuming
Newton's
laws
of
motion and
central
force
interactions,
it
can
be demonstrated that it
is
impossible
to determine the state
of
motion
of
an
inertial
frame
by means
of
mechanical
experiments
carried out within
a
closed
system
with center
of
mass
at rest in this frame. This conclu-
sion,
well known and
empirically
well confirmed
by
the end
of
the nineteenth
century,
was
sometimes called the
principle
of
relative
motion,
or principle
of
relativity.[13]
The
introduction
of
velocity-dependent
forces
between
charged particles
led to doubts
about the
validity
of
the
relativity principle
for
magnetic
interactions. The
wave theory
of
light
appeared
to
invalidate
the
principle
for
optical
phenomena.
The
theory
seemed to
require
an all-pervading
medium,
the so-called luminiferous
ether,
to
explain
the
propa-
gation
of
light
in
the absence
of
ordinary
matter.
The
assumption
that the ether
moves
together
with
matter
seems
excluded
by
the
phenomenon
of
aberration and
by
Fizeau's
results
on
the
velocity
of
light
in
moving
media.
If
the
ether
is
not
dragged
with
matter,
it
should be
possible
to
detect
motion
relative
to
a
reference frame fixed in the ether
by means
of
optical experiments.
However,
all
attempts
to
detect the motion
of
the earth
through
the
ether
by optical experiments
failed.[14]
Maxwell's
electromagnetic theory was
intended to
provide a
unified
explanation
of
elec-
tric,
magnetic,
and
optical
phenomena.
With its
advent,
the
question arose
of
the status
of
the
principle
of
relativity
for such
phenomena.
Does
the
principle
follow from the funda-
mental
equations
of
electrodynamics?[15]
The
answer
to this
question depends on
the form
of Maxwell's
equations postulated
for bodies in motion. Hertz
developed an
electrody-
namics
of
moving
bodies,
based
on
the
assumption
that the ether
moves
with
matter,
in
which the
relativity principle
holds.[16] In
addition
to
its
inability
to account
for
the
optical
phenomena
mentioned
above,
Hertz's
theory was
unable to
explain
several
new
electro-
magnetic
phenomena,
and it
soon
fell out
of
favor.[17]
[11]
See Einstein
1905r
(Doc. 23), pp.
891-
892;
Einstein
1907j (Doc. 47), pp.
411-413;
and
Einstein 1909c
(Doc. 60), pp.
485-487.
See
Miller
1981b for
a
detailed
study
and
an English
translation
of
Einstein
1905r
(Doc.
23).
[12]
See Torretti 1983
for
a
discussion
of
the
Newtonian
principle
of
relativity,
with
some
historical references.
[13] See,
e.g.,
Violle
1892,
p.
90,
and Poin-
care
1902, pp.
135-137,
281, to
name
two
sources
that Einstein had read
by
1905
(see
Vol.
1,
"Albert
Einstein-Beitrag
für sein Lebens-
bild,"
p.
lxiv,
for
Violle;
and Solovine
1956,
p.
viii,
for
Poincare).
[14]
For
a survey
of
ether
theories,
centering on
attempts
to detect the motion of the earth
through
the
ether,
see
Hirosige 1976;
for
a sur-
vey focusing on
the
Michelson-Morley experi-
ment, see
Swenson
1972. For
a survey
of
work
on
the
optics
of
moving
bodies in the nineteenth
century,
see
Sesmat 1937.
[15]
For discussions
of
the
problem
of
relative
motion in
current
theories
of
electrodynamics,
see
Cohn
1900, 1902,
and
Abraham/Föppl
1904,
pp.
430-436,
which
speaks
of
the
"axiom
of
relative
motion"
("Axiom
der Relativbewe-
gung").
[16]
See Hertz,
H.
1890b. For
a
review
of
Hertz's
theory, see
Hirosige 1966.
[17]
It
appears
that Hertz
was aware
of
the dif-
ficulties
optical phenomena present
for his the-
ory.
He
explicitly
stated that it
was
meant
to
ex-
plain electromagnetic phenomena
in the
narrower
sense (see Hertz,
H.
1890b).
How–