268 THEORY OF RELATIVITY
still controversial,
and often not
clearly distinguished
from
Lorentz's
electron
theory,
was
a major
topic
of
discussion
among leading German-speaking physicists.[92]
V
The
topic
"electrodynamics
of
moving
bodies,"
as
understood at the time
Einstein
wrote
his 1905
paper, usually
included not
only
the
microscopic
electron
theory
discussed
in
that
paper,
but also the
macroscopic theory, involving
conduction currents in
polarizable
and
magnetizable moving
media. The field
equations
of
such
a theory
could
either
be
postu-
lated
phenomenologically, as
Cohn
did;[93]
or they
could be derived
from
an
underlying
microscopic
theory,
as
in
Lorentz's
electron
theory.[94]
Einstein did not turn to the
mac-
roscopic
theory
until 1908. His work with Laub
on
that
subject
is
discussed
in the
editorial
note,
"Einstein
and Laub
on
the
Electrodynamics
of
Moving
Media,"
pp.
503-507.
Since the
theory
of
relativity grew
out
of Einstein's
long-standing
concern
with
electro-
dynamics,
and
his
applications
of
the
theory were
primarily
in this
field,
the
theory
was
often looked
upon
as
essentially
another
version
of Lorentz's
electron
theory (see
section
II,
pp.
256-257).
Einstein
soon
felt the need to make clear
the distinction between the
kinematical
results
of
the
theory,
deduced from the two
principles
of
the
theory,[95]
and
the
application
of
such kinematical results
to
the
solution
of
problems
in
the
optics
and
electrodynamics
of
moving
bodies,
to the derivation
of
the
equations
of
motion
of
a
charged
particle[96]-or
indeed
to
any physical theory.
He
pointed
out that
the
postulates
of
the
theory
do not constitute
a
"closed
system" ("abgeschlossenes System"),
but
only
a
"heuristic
principle,
which considered
by
itself alone
only
contains assertions about
rigid
bodies,
clocks,
and
light
signals" ("heuristisches
Prinzip,
welches
für
sich allein betrach-
tet
nur
Aussagen
über starre
Körper,
Uhren und
Lichtsignale
enthält").
Beyond
such
as-
sertions,
the
theory
could
only
establish "relations between otherwise
apparently
indepen-
dent
laws"
("Beziehungen
zwischen
sonst voneinander
unabhängig
erscheinenden
Gesetzmäßigkeiten")
of
physics.[97]
A few
months after first
publishing
the
theory
of
relativity,
Einstein discovered
a
rela-
tion that
particularly intrigued
him,
the relation between inertial
mass
and
energy.
He
wrote Conrad Habicht:
One
more consequence
of
the
electrodynamical paper
has also occurred
to
me.
The
principle
of
relativity, together
with
Maxwell's
equations, requires
that
mass
be
a
direct
measure
of
the
energy
contained in
a body; light
transfers
mass.
A noticeable decrease
of
mass
should
occur
in
the
case
of
radium. The
argument
is
amusing
and
attractive;
but
I
can't
tell whether the Lord
isn't
laughing
about
it
and
playing a
trick
on me.
[92]
For the
reception
of
the
theory
of
relativity
in
Germany
and several other
countries, see
Goldberg
1984, part II;
and Glick 1987.
[93]
See Cohn 1900,
1902, 1904a,
1904b.
[94] See, e.g.,
Lorentz 1904c.
[95]
See Einstein
1905r
(Doc. 23),
"Kinema-
tischer
Teil,"
pp.
892-907.
[96]
See Einstein
1905r
(Doc. 23),
"Elektro-
dynamischer
Teil,"
pp.
907-921.
[97]
Einstein
1907g
(Doc.
44), pp.
206-207.
The article is
a reply
to
Ehrenfest
1907.