THEORY
OF RELATIVITY 257
approach
to the
explanation
of
failure
of
attempts
to
detect motion
through
the
ether,
thus,
was
to show that the basic
equations
of the electron
theory,
in
spite
of
the fact that
they
single
out the ether rest
frame, can
still be
used to
explain
the failure
of
all
optical
and
electromagnetic attempts
to detect the
earth's
motion
through
the ether.
Einstein's work
was
based
on a
new
outlook
on
the
problem.
Instead
of
regarding
the
failure
of
electromagnetic
and
optical experiments
to detect
the
earth's
motion
through
the
ether
as something
to be deduced from the
electrodynamical equations,
he took this failure
as empirical
evidence for the
validity
of
the
principle
of
relativity
in
electrodynamics
and
optics.
Indeed,
he asserted the universal
validity
of
the
principle, making
it
a
criterion
for
the
acceptability
of
any physical
law. In this
respect
he
gave
the
principle
of
relativity a
role similar to that
of
the
principles
of
thermodynamics, an example
that he later stated
helped
to
guide
him.[24]
Rather than
being
deductions from other
theories,
such
principles
are
taken
as postulates
for deductive chains
of
reasoning, resulting
in
a theory formulating
general
criteria that
more specialized
theories
must
satisfy.[25]
Einstein
now
confronted the
problem
of
making
Maxwell-Lorentz
electrodynamics
compatible
with the
principle
of
relativity.
He did
so
by
means
of
a
principle
drawn from
electrodynamics,
the
principle
of
the
constancy
of
the
velocity
of
light.
That the
velocity
of
light
is
independent
of
that
of
its source,
and has
a
constant value
in
the
ether
rest frame,
can
be deduced from the Maxwell-Lorentz
theory.
Einstein
dropped
the ether from
con-
sideration,
and took the
constancy
of
the
velocity
of
light as a
second
postulate,
supported
by
all the
empirical
evidence
in
favor
of
the Maxwell-Lorentz
theory.
When combined
with the
relativity principle,
this leads to
an apparently paradoxical
conclusion: the veloc-
ity
of
light
must be the
same
in
all inertial frames. This result conflicts
with the Newtonian
law
of
addition
of
velocities, forcing
a
revision
of
the
kinematical foundations
of
electro-
dynamics.
Einstein showed that the
simultaneity
of
distant events
is
only
defined
physi-
cally
relative
to
a
particular
inertial
frame,
leading
to kinematical
transformations
between
the
spatial
and
temporal
coordinates
of
two
inertial
frames that
agree formally
with the
transformations introduced
by
Lorentz
in
1904.
Einstein next considered the
implications
of
the
new
kinematics for
electrodynamics
and mechanics.
By eliminating
the
concept
of
the
ether,
he in effect
asserted that electro-
magnetic
fields do
not
require an underlying
substratum.[26] He showed that the
Maxwell-
Lorentz
equations
for
empty
space
remain invariant
in
form under the
new
kinematical
velocity
that
would
make the
equations
with
sources fully
invariant. These laws
were sup-
plied by
Poincare,
who
recognized
that Lo-
rentz's
transformations form
a group,
under
which the Maxwell-Lorentz
equations
remain
invariant in form. Poincare
regarded
these
re-
sults
as an explanation
of the
apparent
universal
validity
of
the
principle
of
relativity (see
Poin-
care 1905b, 1906).
[24]
Einstein first made the
comparison
be-
tween the
relativity principle
and the
principles
of
thermodynamics
in Einstein
1907g
(Doc. 44).
He stated that these
principles
had served
as
his
"example"
("Vorbild") in Einstein
1979, p. 48.
[25]
Einstein later formulated the distinction
between
"theories of
principle" ("Prinziptheo-
rien") and
"constructive theories"
("konstruk-
tive Theorien") to
explain
the
relationship
be-
tween
the two
types
of
theories
(see
Einstein
1919).
See the
Introduction,
pp.
xxi-xxii,
for
further discussion
of Einstein's
use
of
such
prin-
ciples.
[26]
This
concept
of
electromagnetic
fields
is
first stated
explicitly
in
Einstein
1907j (Doc. 47),
p.
413;
but
it is
implicit
in Einstein 1905i
(Doc.
14), as
well
as
in Einstein
1905r
(Doc. 23).
Previous Page Next Page