224
ELECTRODYNAMICS
OF MOVING BODIES
p.
27).[3]
In addition
to
ponderable
matter
or
in its absence,
the ether
provides
such
a
medium.[4]
Electricity
and
magnetism,
rather than
being
the substantive entities
that
produce
such
states,[5] are
only
"Namen" for certain derived
concepts
(ibid.,
p.
223).
They
are
convenient and colorful embellishments of the
theory
that
add
nothing
to its
physical
content
(ibid., pp.
223,
225).
Einstein
expressed
doubt
about the role of
"des
Namens
'Äther'"
in
electromagnetic
theory.
His
skepticism is
directed
primarily
at
Hertz's
concept
of the ether
as a me-
dium
with
a
state
of motion
(see, e.g.,
Hertz
1892, pp. 256-257);[6]
it
is
not
clear
whether it extended to the ether
concept
itself.
Einstein attributed fundamental
sig-
nificance
to
the
concept
of "elektrische
Massen."
Hence, he
regarded
electric
cur-
rents
as
motions of such
charges
in
empty space,
rather than
as
the
"Verschwinden
elektrischer Polarisation in der Zeit." His
views
show
his
acquaintance
with "sub-
stantive" theories of
electricity
and
magnetism
(see
note
5),
as
well
as
with
current
investigations
of
charged
particles.[7]
In
many respects,
the
view
of
electrodynamics
sketched
in
this letter
(Doc.
52)
is strikingly
similar
to
that of Lorentz
(see
Lorentz
1892,
1895).[8]
Einstein concluded that
"Strahlungsversuche"
were
needed
to
decide
between the
two
viewpoints
he
contrasted. His
next
letter
(Doc. 54)
mentions
an
idea for
experi-
mentally investigating
the influence of motion relative
to
the ether
on
light prop-
agation
in
transparent
bodies and
a
related
theory
he
was
developing.
His
physics
professor
showed
no
enthusiasm
for
this
work,
but
a
reading
of Wien 1898
encouraged
Einstein
to
write
to
Wilhelm
Wien
about
his
ideas
(see
Doc.
57).
In this
paper,
Wien
discussed thirteen
experiments bearing
on
"die translatorische
Bewegung
des
Licht-
äthers,"
including
the
Michelson-Morley
experiment.[9]
About this
time,
Einstein also
mentioned
his
intention
to
read Helmholtz
1897
(Docs.
57,
58).
[3] In
his Introductory
Survey (ibid.,
pp.
1-31),
Hertz used "Polarisationszustand"
to
denote the
same
concept
he
had denoted
by
"Kraft"
in his
earlier
papers
(see p.
30).
[4]
While Hertz sometimes used the
phrase
"Polarisationszustand
des
Raumes"
(e.g., pp.
28,
30),
he
made it clear
that, if
space
could
be
emptied
of
the ether,
it would
be
incapable
of
sustaining
such
states
(ibid., p. 28).
[5] This "substantive"
concept
of
electricity,
particularly as developed by
Wilhelm Weber
(1804-1891), was
widely
accepted by
German–
speaking
physicists.
It
was
taken
for
granted
in
the lectures
on
electricity
by
H.F. Weber,
as
indicated
by
Einstein's
notes
(see
Doc.
37,
e.g.,
pp.
178-179).
[6]
For
a
much later
comment
on
Hertz's
electrodynamics,
see
Einstein
1979,
p.
32.
[7] Docs. 52
and
79
indicate Einstein's
fa-
miliarity
with the
theory
of
ions
("ion" was
often
used
generically
at
this time
to
denote
all
charged particles).
Doc.
111
shows
his
strong
interest
in
Lenard's
experiments
on
cathode
rays. See
also the
editorial
note,
"Einstein
on
Thermal,
Electrical,
and Radia-
tion Phenomena."
[8]
Lorentz
is
not mentioned, however,
until
Doc.
131.
For
a
much later discussion
by
Ein-
stein of the historical role of Lorentz's
theory,
see
Einstein
1953a, pp.
35-39.
[9]
Einstein later stated
that, while
a
stu-
dent but before
he
became familiar with the
Michelson-Morley experiment,
he
devised
an
experiment
that
was
similar
to it in
principle
(see
the
report
of
his 1922
lecture
at
Kyoto
University
in
Ishiwara
1971,
p.
79;
see
Hirosige
1976,
p.
54,
for
an
account in
English).
A light
ray
is
split
into
two
by
means
of
mirrors,
one
ray traveling
in
the direction of the earth's
motion and the other
one
in
the
opposite
di-
rection.
Any
difference in
the
energies
of the
two
rays is
detected
by
thermoelements.
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