THEORY OF RELATIVITY 265
relative motions
of
ponderable
bodies
are significant
in
determining
electromagnetic
and
optical
phenomena;
at
some point,
this conviction led him to abandon the
concept
of
the ether.
(2)
He
temporarily
abandoned
Lorentz's
theory
of
electrodynamics,
which
appears
to attach
physical significance
to absolute motion
(i.e.,
motion with
respect
to
empty
space or
the
ether).
(3)
He
explored
the
possibility
of
an
alternative
electrodynamical theory,
which
would
justify
the emission
hypothesis
about the
velocity
of
light
relative to its
source.
(4) Abandoning
such
attempts,
he
reexamined
Lorentz's
theory,
at
some
point
fo-
cusing
his
concern on
the conflict
of
his ideas
on
relative motion with
a
particular
consequence
of Lorentz's
theory:
the
independence
of
velocity
of
light
of
the
velocity
of
its
source.
(5)
He
recognized
that this conflict involves
previously tacitly accepted
kinematical
assumptions
about
temporal
and
spatial
intervals,
leading
him to examine the
mean-
ing
of
the
concept
of
the
simultaneity
of
distant events.
(6)
He defined
simultaneity physically,
and constructed
a new
kinematical
theory
based
on
the
relativity principle
and the
light
principle,
thus
resolving
the
apparent
conflict between
them.
There have been
a
number
of
attempts
at
a
detailed reconstruction
of Einstein's
devel-
opment
of
the
theory
of
relativity, attempts
which often differ
significantly
in
their
conclu-
sions.[69]
Such
a
reconstruction has
to
take into account other strands in
Einstein's
work at
this
time. In
particular,
by
the time he wrote the
relativity paper,
he
no longer
regarded
Maxwell's
electromagnetic theory as universally valid,
and had
already proposed
his
light
quantum hypothesis.[70]
He had also shown that the
equipartition
theorem,
which
his work
on
the foundations
of
thermodynamics
convinced him
is
valid for the most
general
classi-
cal-mechanical
system,
combined
with
Maxwell's
theory,
leads to
an
incorrect law
for
black-body
radiation. Thus, he
already challenged
the unlimited
validity
of
both
classical
mechanics and
of Maxwell's
theory.
For further discussion
of
his doubts about classical
theories
of
matter
and radiation,
see
the
Introduction,
pp.
xvi-xxix, and the editorial
note,
"Einstein's
Early
Work
on
the
Quantum
Hypothesis,"
pp.
134-148.
Einstein later recalled
that,
uncertain how to
proceed
in
the search for
better
theories
of
the structure
of
matter and
radiation,
he became convinced that
"only
the
discovery
of
a
universal formal
principle
could lead
...
to
assured
results"
("nur die
Auffindung
eines
allgemeinen
formalen
Prinzips
...
zu gesicherten Ergebnissen
führen
könnte").[71]
Such
principles play a
role
analogous
in this
respect
with the role
played by
the
principles
of
thermodynamics.
The
theory
of
relativity
is
based
on
just
such
principles:
even
though
suggested originally by specific
mechanical and
electromagnetic
theories,
the
principles
[69] See,
e.g.,
Earman
et
al.
1982; Goldberg
1983;
Hirosige 1976;
Holton
1973,
part II;
Miller
1981b;
Pais
1982;
Schaffner
1982.
[70]
See
Einstein
1905i
(Doc. 14).
In
Einstein
1907h
(Doc. 45),
pp.
372-373, Einstein dis-
cussed
why
Maxwell's
theory
could
neverthe-
less be used with confidence within the limits of
its
validity.
[71]
Einstein
1979, p. 48; English translation,
p.
49.