258
THEORY
OF RELATIVITY
transformations
when the transformation laws for the electric and
magnetic
fields
are
ap-
propriately
defined.
He deduced
appropriate
transformation laws for
charge
densities
and
velocities from
the
requirement
that
Maxwell's
equations
remain
invariant when
convec-
tion currents
are
added.
Finally, by
assuming
that
Newton's
equations
hold
for
a charged
particle
at rest, he
was
able to
use
a
kinematical transformation to deduce the
equations
of
motion
of
a charged particle
("electron")
with
arbitrary velocity.
The
problems
connected
with the formulation
of
an electrodynamics
of
moving
bodies
consistent
with all the
experimental
evidence
were
discussed
frequently during
the
years
Einstein
was working on
his
theory.
Statements
similar
to
many
of
the individual
points
made in
Einstein 1905r
(Doc. 23)
occur
in the
contemporary
literature, and
Einstein
may
well have been familiar with
some
of
the books and articles in which
they
do.[27]
But his
approach
to the
problem, leading
to the
peculiar
combination
of
these ideas in his
paper,
is
unique-particularly
the
recognition
that
a new
kinematics
of
universal
applicability is
needed
as
the
basis for
a
consistent
approach
to the
electrodynamics
of
moving
bodies.
III
Einstein's
work
on relativity grew
out
of
his
long-standing
interest in the
electrodynamics
and
optics
of
moving bodies.[28]
His
first
scientific
essay,
written in
1895,
discussed the
propagation
of
light
through
the ether.[29]
The next
year, as
he later
recalled,
the
following
problem
started to
puzzle
him:
If
one
were
to
pursue
a light wave
with the
velocity
of
light, one
would be
confronted
with
a time-independent wave
field. Such
a thing
doesn't
seem
to
exist,
however! This
was
the first childlike
thought-experiment
concerned with
the
special theory
of
relativity.
Wenn
man
einer Lichtwelle mit
Lichtgeschwindigkeit
nachläuft,
so
würde
man
ein
zeitunabhängiges
Wellenfeld
vor
sich haben. So etwas scheint
es
aber doch
nicht
zu geben!
Dies
war
das erste kindliche
Gedanken-Experiment
das mit der
speziellen
Relativitätstheorie
zu
tun
hat.[30]
By
this time Einstein
presumably was
familiar with the
principle
of
relativity
in classical
mechanics. While
preparing
for the ETH entrance examination in
1895,
he had studied
the German edition
of Violle's
textbook.[31]
Violle
actually
based
his
treatment
of
dynam–
[27]
Some
of
these
are
cited in the editorial
notes
to
Einstein 1905r
(Doc. 23) (see
notes 5,
6, 9, 10, 12).
Two
papers
by
Emil
Cohn,
Cohn
1904a,
1904b,
published
in
response
to
Lorentz
1904a, deserve
special
mention.
They are
"re-
markable
...
because in
some
respects they re-
mind
us
of
the
point
of
view taken in the
theory
of
relativity"
(Hirosige
1966,
p.
35).
For evi-
dence
of Einstein's later
familiarity
with
Cohn's
work, see
§
V.
However,
there
is
still
no
evi-
dence
indicating
when
he
first read Cohn.
[28]
See
Vol.
1,
the editorial
note,
"Einstein
on
the
Electrodynamics
of
Moving
Bodies,"
pp.
223-225.
[29]
See
"On
the
Investigation
of
the State of
the
Ether
in
a Magnetic
Field"
(Vol.
1,
Doc.
5).
[30]
Einstein
1955,
p.
146. See also Einstein
1979,
pp.
48, 50.
[31]
Violle
1892,
1893. For
a
reference
to
the
evidence about
when Einstein studied
Violle's
textbook,
see
note
13.