146
DOC.
12
EXPERT OPINION
to
moving charged particles,
the
magnetic
molecules would
act
as
small
gyroscopes.
Sudden reversal of
the
magnetization
of
a
magnetized macroscopic body
would then
be noticeable
as a
change
in
angular
momentum
of the
body. Conversely, setting
an
unmagnetized body
into rotation would tend
to
orient the small
gyroscopes
in the
same
direction,
which would then show itself
as a
macroscopic magnetization.
The
latter effect
was
also cited
as a
possible explanation
of terrestrial
magnetism.[2]
According to
a
later
recollection,
Einstein became interested
in
the
problem
of
Ampère's
molecular
currents
when he
was
asked
to act
as an expert
in
a
court
case
initiated
by
the German firm Anschütz
&
Co.
against
the
American
Sperry Gyroscope
Company.[3]
The
complaint
was
patent infringement
on
a
patent
Anschütz held
on
the
design
of
a
gyrocompass.
At the
request
of
the court,
Einstein
wrote two
expert opin-
ions;[4]
the
case was
eventually
decided in
favor of
the
Anschütz
firm.[5]
The
concept
of molecular
currents is
also
connected
to
several
problems
in
early
twentieth-century physics.
One of
these,
the
possible
existence of
a
zero-point
ener-
gy,
is
explicitly
mentioned in the first section of Einstein and De Haas 1915a
(Doc.
13).
The authors
argue
that the existence of
a
molecular
magnetic
moment
at
zero
temperature,
as
is
implied by
the
Curie-Langevin
law of
paramagnetism, suggests
the
presence
of
a
zero-point energy
of
the
rotational motion
of
the intra-atomic electrons.
But
they
hasten
to
add that the
hypothesis
of
a
zero-point energy
meets
with resis-
tance
on
the
part
of
many
physicists.[6]
In fact,
Einstein himself
had
explored
the
con-
sequences
of
a
zero-point energy
in 1913 in
a
joint paper
with Otto
Stern,
finding
re-
sults for the
specific
heat of
hydrogen
that seemed
to
support
the
hypothesis.[7]
Not
long
afterwards, however, he
changed
his mind and
rejected
the idea of
zero-point
energy.[8]
Another connection between molecular
currents
and
contemporary physics
is
re-
lated
to
the
apparent discrepancy
between the existence of
rotating
electrons and
Maxwell's
theory, according
to
which electrons should radiate and thus
quickly
lose
their
energy.
As
becomes clear from Einstein and De Haas 1915a
(Doc.
13),
its
au-
[2]See
Galison
1987, chap.
2,
for
more
details.
[3]"For
instance,
it
was an
expert opinion
that
I
had
to
write
on a
gyrocompass
that
gave
me
the idea for
demonstrating
the
top-like
character of
paramagnetic
atoms"
("Zum
Nachweis der
Kreiselnatur der
paramagnetischen
Atome wurde ich
z.
B.
angeregt
durch ein
Gutachten,
das
ich uber einen
Kreiselkompass
auszuarbeiten hatte." Einstein
to
Emile
Meyerson,
27
January
1930).
[4]They
are
presented
here
as
Docs.
12
and
19.
[5]See
Lohmeier and Schell
1992
for
more background
on
Einstein's connections with the
Anschütz
firm.
[6]In
Einstein 1915c
(Doc.
15),
p.
237,
Einstein reiterates this
view, saying
that "no theore-
tician
nowadays
utters
the word
'zero-point energy'
without
a
half-embarrassed,
half-ironic
smile
playing on
his
lips" ("Kein
Theoretiker
spricht gegenwärtig
das Wort
"Nullpunktsener-
gie" aus,
ohne daß
in
seinem Gesicht ein halb
verlegenes,
halb ironisches Lächeln
zu
sehen
wäre").
[7]See
Einstein and Stern
1913
(Vol.
4,
Doc.
11).
[8]See Vol.
4,
the editorial note,
"Einstein and Stern
on
Zero-Point
Energy," pp. 270-273,
for
a
discussion.