Download by George Alexander The notion of a magnet with only one pole is

by George Alexander
The notion of a magnet with only one pole is difficult
to imagine, but physicists say it ought to exist.
or the past fifty years physicists—like
so many dedicated collectors—have
resolutely tracked down and captured
an impressive number of subatomic particles:
neutrons and neutrinos, positrons and antiprotons, pi mesons a n d K mesons, m u o n s
and taus, to name but a few. T h e scientists
have found these bits a n d pieces of nature in
the interactions between thin metal targets
and streams of helium nuclei from radioactive sources, b e t w e e n gas molecules a n d
molecule-shattering cosmic rays, and between a variety of targets and electrons or
protons whipped to high energies in particle
accelerators.
sity. If the direction of the magnetic field
were away from the center of the particle,
the monopole would be said to be a north
monopole; if toward the center, it would be
a south.
These characteristics—equal b u t opposite
properties—are basically the same as those
of the positively charged p r o t o n and the
negatively charged electron; the proton's
electric field is depicted as pointing outward
from its center and the electron's as pointing
inward. It was this parallel between electrical
and magnetic fields that intrigued the British
theoretical physicist Paul A. M. Dirac back
in the early 1930s, as he contemplated the
equations that Scottish scientist James Clerk
Maxwell had formulated more than 50 years
earlier to describe the interrelatedness of
electricity and magnetism.
Dirac was delighted by the elegance of
Maxwell's equations (an elegance, say most
scientists, that can be fully appreciated only
by those with s o m e m a t h e m a t i c a l b a c k ground). T h o u g h he was convinced of their
truth, he was nonetheless troubled by what
he perceived to be a blemish, an apparent
Indeed, the scientists have been so successful for so long at this business that it
might seem a simple matter: theoretical physicists hypothesizing the existence of new particles and experimenters seeking and finding
them. The reality, however, is that a few
species of particles have long been predicted
to exist—there is, in fact, widespread accord
a m o n g physicists in the belief of their
existence—anu yet not one example nas so
far been discovered. T h e magnetic monopole
is one such elusive particle.
An arcane particle
A magnetic monopole is precisely w h a t
its name implies: a magnetic particle that
has only one pole, either north or south. T h e
concept is difficult, at least initially, for those
accustomed to envisioning magnetic field
lines as emerging from one end of an ordinary bar magnet and t h e n curling back and
around to the opposite end. This is the pattern of a dipole, in w h i c h the direction of
field lines is considered to be from north to
south.
The field lines of a monopole, conversely,
would extend radially in all directions from
the center of the particle, like pins sticking
out of a pincushion, and continue along
straight lines to some practical limit of inten-
Mn.QAIP. Ma\//.liine1Qft1 1Q
asymmetry, in the equations. T h e equations
did indeed establish the unity of electricity
and m a g n e t i s m (they are a U [ l ] symmetry in the language of gauge theory; see
" G r a n d U n i f i c a t i o n : A n Elusive G r a i l / '
Mosaic, Volume 10, N u m b e r 5). N o n e t h e less, they incorporated no basic unit of magnetic charge as they did the q u a n t u m as the
unit of electric charge.
As D i r a c r e e x a m i n e d the p r i n c i p l e s of
quantum electromagnetism, he could perceive
no good reason why there should not be a
q u a n t u m of magnetic charge. Such a quantum would constitute the unit needed to
balance Maxwell's equation neatly. In 1931,
Dirac predicted the existence of magnetic
monopoles.
Seeking, seeking
Since then, a number of investigators have
looked high and low—literally—for monopoles. T h e physicists of the 1930s looked
through their cloud chamber records for signs
of passage of these particles, but those instruments were not at all suited to such research.
T h e difficulty of the search, and the dim
prospects of reward, discouraged others for
m a n y years from pursuing it further.
In the 1960s, interest revived. A team of
Massachusetts Institute of Technology scientists searched iron ore deposits and meteorites for a n y monopoles that might have
been created in the e a r t h ' s u p p e r a t m o sphere by energetic cosmic rays. T h e premise of the MIT physicists was that the m o n o poles created in the collisions b e t w e e n
cosmic rays a n d molecules in the a t m o sphere would fall to earth at relatively low
speeds; because of their large pole strength,
the particles would lose speed rapidly in interactions with atmospheric matter. Moreover, if they landed on surface deposits of
iron ore, magnetic monopoles would be effectively trapped there by the iron's own
magnetic field. T h e monopoles would not
be so tightly b o u n d to the ore, however, that
they could n o t be extracted b y a more
powerful magnetic field.
T h e MIT team took a 60,000-gauss electromagnet (the earth's field is about one
g a u s s ) to an i r o n - o r e f o r m a t i o n in N e w
York State's Adirondack M o u n t a i n s . Using
the instrument in much the same way as
they'd have used a v a c u u m cleaner, they
s w e p t o u t an a p p r o x i m a t e l y t e n - s q u a r e meter surface area. Any monopoles in the
ore would have been sucked o u t of the formation by the electromagnet and accelerated
u p w a r d t h r o u g h a pair of emulsion plates.
Their passage would have been marked in
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writer
20 MOSAIC Mav/.lunfi1Qft1
for the Los
the plates by holes t h r o u g h the emulsion as
far as an iron b a c k s t o p . T h e MIT physicists
found not a hint of a monopole in either the
A d i r o n d a c k iron ore or in a n u m b e r of
meteorites made available for the experiment by Harvard University.
Then in 1973, with the availability of more
sophisticated equipment, scientists at the
Fermi National Accelerator Laboratory in
Batavia, Illinois, processed 2,000 liters of
Atlantic Ocean water, h o p i n g to find any
monopoles that might have wandered into
the earth's h y d r o s p h e r e and that might still
be there, bobbing along like some sort of
exotic flotsam.
T h e seawater was exposed, in small, continuous quantities, to the field of a superc o n d u c t i n g e l e c t r o m a g n e t , a device that
would surely have attracted (or repelled)
any monopoles present and directed them
against a stacked series of carefully calibrated counters. T h e counters would have
stopped any such particle and provided an
estimate of its magnetic charge. None was
detected.
Efforts to create monopoles in the highenergy beams of accelerators were no more
successful, and more than one physicist began
to wonder if (1) the search simply h a d n ' t
been directed to the right places or (2) there
were something in n a t u r e forbidding m o n o poles' existence.
Hope springs eternal
T h e n in the s u m m e r of 1975, a team of
physicists from the University of California
at Berkeley and the University of Houston
startled their c o l l e a g u e s e v e r y w h e r e by
claiming—tentatively—to have found the first
hard evidence of a monopole. The evidence
was a particle track in a sandwiched stack of
plastic sheets, film, and photographic emulsion. Back in 1973 the team—P. Buford Price
of Berkeley; E d w a r d K. Shirk, n o w an
Oakland-based consultant; W. Zack Osborne,
n o w at the University of Oklahoma; and
Lawrence S. Pinsky of Houston—attached
the package to a helium balloon and flew it
to an altitude of 40 kilometers above Iowa
for several days. T h e scientists were looking
for ultraheavy cosmic rays and were richly
rewarded. As a b o n u s , it appeared that they
had also caught a monopole.
Cosmic rays, which range from protons
to the heavy nuclei of elements like iron or
oxygen, riddle materials like plastic and photographic film as easily as shotgun pellets
p e n e t r a t e s t y r o f o a m . T o identify cosmic
ray strikes, scientists are able to make the
tracks in the plastic sheets visible by a chemical etching process. T h e y can them determine, from the width, d e p t h , and angle of
etched-out tracks, m a n y of the properties of
the particles responsible.
Of the 75 etched pits that the BerkeleyHouston team found on their detector, 74
were clearly the marks of cosmic rays. (Tostate the finding that simply belies the painstaking perusal of each track; the elapsed
time between the experiment and the report
was two years.)
The remaining track was strikingly different. Its area of d a m a g e was not only
much larger than the other pits, but it had
punched its way through all the layers of
plastic and film w i t h o u t any a p p a r e n t
change in speed. Price concluded—prematurely, as he was later to concede—that the
track seemed to have been the product of a
massive particle having an ionizing charge
137 times that of an electron. It also appeared to be relatively slow moving, having
a velocity roughly half that of the speed of
light.
These two factors—its slow speed and the
ionization charge 137 times that of a singly
charged particle—seemed to be compatible
with the expected behavior of a very massive monopole and incompatible with any
k n o w n atomic nucleus, Price says. However
random the n u m b e r 137 might sound, It
happens to be exactly twice the value (68.5)
that Dirac had calculated as the smallest
q u a n t u m of pole strength that a monopole
could possess.
The charge of an electron has been shown
to occur in Integral multiples (1, 2, 3, 4, etc.)
of its basic value (1.60 X 10~19 coulombs); so
magnetic charges should also occur in integral multiples of some basic value. W h a t ever it was that tore through the BerkeleyHouston a p p a r a t u s , it certainly appeared to
have two units of magnetic charge; Price declared it "a monopole—two plus."
Unfortunately for the team, the ionization charge later came down from 137 to
114, plus or minus 4, when Price recalibrated
what might be termed the damage index for
the plastic sheets. While this is certainly a
whopping charge, it is not an integral multiple
of that of a monopole; the revision seriously
weakened the claim that the penetrating particle was the elusive monopole. Nor, other
scientists w h o s t u d i e d the m e a s u r e m e n t s
made by the team concluded, was there any
reason to assume that the particle was traveling at about half the speed of light. It
could have been half, or it could have been
nine-tenths; either assumption was valid.
These two revisions wiped out the claim
for a found m o n o p o l e . W h e n the BerkeleyHouston scientists published their final analysis, they concluded that, though there were
no grounds to attribute the anomalous track
to a monopole, it appeared to represent the
only example to date of a nucleus with unusual
properties. Perhaps its ionization charge of
114 signified that it was the first example
of a long-sought "superheavy" element with
an anticipated atomic number of approximately 114.
" U n t i l further examples of such a particle
can be f o u n d , " says Price, "it will remain in
the scientific literature as a mystery, one of
several cases of observations that have not
been confirmed by further evidence."
Undaunted
Has Price's experience discouraged him
or others from seeking monopoles? No more
t h a n archaeologists were put off by the discovery that most of the pyramids and tombs
of Egyptian pharoahs had been plundered
long before the scholars arrived at the site;
after all, England's Howard Carter did happen u p o n the largely intact tomb and treasures of T u t a n k h a m u n , didn't he? Perhaps a
similar piece of luck will strike some physicist.
" M a y b e m o n o p o l e s d o n ' t e x i s t , " says
David Fryberger, a Stanford Linear Accelerator Center physicist who at present, along
with Price, is mounting a search for the
elusive particle. "Or maybe they're very well
hidden, or maybe there just aren't very many
of them. M y betting is that they're scarce in
nature and very well concealed."
It could also be that monopoles exist only
as colossi, at least in the realm of elementary
particles. According to some grand unification theories, monopoles would have been
made as gargantuan objects during the first
few seconds of the Big Bang. At that time,
when the universe began and everything was
a near-homogeneous blend of high temperature, high-density radiation and fundamental
particles, monopoles would have been fashioned with mass equivalents of 10 25 electron
volts. (For comparison, the most energetic
particles k n o w n are the rare, highest-energy
cosmic rays, at 10 20 electron volts.)
A mass equivalent to 10 25 electron volts—
for a particle—borders on being i n c o m p r e hensible. And if monopoles exist only as
such objects, then the approximately fortybillion electron-volt (4 X 10 10 ) detector that
some accelerator physicists are n o w setting
out for them stands about as much c h a n c e
of catching one as a mousetrap does an
African elephant. Fryberger, however, b e lieves that monopoles may exist in a wide
M O S A I C Mav/June 1981
21
range of sizes and energies; he is working collaboratively with Price and Kay
Kinoshita, a Berkeley graduate student, in
an effort to find some in the mouse-sized
range.
The three scientists have designed a detector for the Positron-Electron Project ring
at the Stanford Linear Accelerator Center, a
facility s u p p o r t e d by the D e p a r t m e n t of
Energy. T h e detector is a polyhedral structure less than 30 centimeters wide between
any opposing two of its twelve faces. Each
face is a sandwich of plastic sheets; the
resulting dodecahedron envelops a length
of the accelerator's main ring—a thin-walled,
f i v e - c e n t i m e t e r - d i a m e t e r stainless steel
pipe, inside of which beams of highly energetic p o s i t r o n s and electrons circulate in
opposite directions.
W h e n the high-energy beams collide and
particles of opposite charge annihilate each
other, they disappear in a burst of electromagnetic energy. The burst, after the briefest of brief instants, is transformed into a
spray of new particles. (Those particles are
not thought to be buried deep inside the
annihilating positron or electron as seeds
are in a melon; instead, the resulting particles are formed out of the energy as Einstein's special theory of relativity-™-E = mc2—
dictates.)
The mousetrap. Kay Kinoshita and Buford Price with the polyhedral detector which, installed in Stanford's accelerator (lower left), will
show a monopole as a track like that in plastic mockup. The researchers can derive the ratio of the ionization charge
to the velocity of the penetrating particie from microscopic measurements of the pits' dimensions. Certain proportions could
indicate the existence of monopoles.
Lawrence Berkeley Laboratory; Buford Price, by permission.
22 MOSAIC Mav/June1Qft1
The experiment
N o one really k n o w s the a n n i h i l a t i o n
energies at which monopoles might appear
among the resultant particles. Fryberger,
Price, and Kinoshita plan to look for them at
energies up to the energy limit of the colliding beams: 36 billion electron volts. If the
experiment does p r o d u c e monopoles, Fryberger says, he w o u l d expect them to be
made in pairs. These pairs would fly off in
opposite directions, penetrate the thin walls
of the steel v a c u u m pipe, and p r o d u c e
tracks in the plastic sheets of the detector.
The detector is expected to remain a r o u n d
the vacuum pipe for several m o n t h s at a
time, recording the tracks of ionizing particles. T h e n Fryberger will pull the plastic
sheets and send them to Price and Kinoshita
at Berkeley. Using a technique that Price
developed, the Berkeley scientists will dip
the sheets in a caustic solution of sodium
h y d r o x i d e . T h e s o l u t i o n eats a w a y the
material in a particle-caused track at a rate
proportional to the a m o u n t of ionization
that went into the m a k i n g of the track. T h e
result is a conical pit at the top and b o t t o m
of every plastic sheet traversed by the particle. From microscopic measurement of the
pits' dimensions, the researchers can derive
the ratio of the i o n i z a t i o n c h a r g e to the
velocity of the p e n e t r a t i n g particle. Particles with ratios of certain proportions will
be analyzed m o r e closely and carefully;
these are the ones that could be monopoles.
Dirac's number—68.5—is one to be checked.
Fryberger has his o w n number—about 25—
which he'd like to be able to reach. A determining factor will be the e x p e r i m e n t e r s '
ability to eliminate " b a c k g r o u n d noise" in
the detectors. " W e ' d like to get d o w n to 20
or even to 10," the Stanford scientist says.
The three California scientists operated
the detector on the Stanford machine for
three months in mid-1980, b u t the machine
was producing colliding beams of positrons
and electrons for only a week or two during
that span. The accelerator was just starting
up then, and the experimenters consider those
runs as only b a c k g r o u n d studies. T h e y
weren't really looking until the machine
went into p r o d u c t i o n - r u n n i n g — c o l l i d i n g
beams most of the time—early in 1981.
" W e won't find t h e m if they exist only as
10 16 GeV [billion-electron-volt] particles,"
Price says, "because the machine can't reach
those levels. But if there are what we might
call not-so-grand monopoles, with masses
no more than some 18 billion electron volts
each [1.8 X 10 10 ] , m a y b e we'll find some.
But I wouldn't w a n t to pin the whole experiment on just monopoles alone—what if
we don't find any?—so we're using plastic
sheets that could also detect electrically
charged particles less highly ionizing than
monopoles but which could be discovered
by the bigger detectors at PEP."
Elsewhere
Another niche being searched for the elusive magnetic monopoles is in pulsars, those
extremely compact n e u t r o n stars that rotate
rapidly and emit lighthouse-like beams of
radio noise. M. Bonnardeau of the National
Center for Scientific Research near Paris
and A. K. Drukier of the Institute for Low
T e m p e r a t u r e Research in M u n i c h have suggested that the very violent collisions between electrons and the surface of a pulsar
might produce monopoles in the mass range
being explored in California.
T h e resulting monopoles would be accelerated by the star's e n o r m o u s l y powerful
magnetic field. One would be driven toward,
or into, the pulsar; the other away from it,
toward interstellar space. Because a monopole d e s t r o y s magnetic fields (it d r a w s
energy from the fields by the act of being
accelerated), the production of monopoles
should be seen initially as a reduction of the
pulsar's rotation rate. A s t r o n o m e r s are now
c h e c k i n g the projections a g a i n s t actual
pulsar observations.
But what if monopoles come only in the
grand variety? Despite the fact that they were
created, with the universe, some 16 billion
or 18 billion years ago, there should still be
some drifting about that have so far managed
to escape an annihilating encounter with an
oppositely charged monopole. Richard A.
Carrigan, jr., assistant head of the Fermi
National Laboratory's research division, believes that some of these massive particles
might have found a home deep in the interiors of the earth and other planets.
Being so massive, these monopoles would
have responded appreciably to gravitational
fields as well as to magnetic forces. Some
could well have been d r a w n into young galaxies like the Milky W a y , and a still smaller
fraction may have found their way into the
solar nebula when it began to condense and
form the solar system.
If this happened, Carrigan has conjectured,
the gigantic monopoles picked up by the
accreting earth (and Venus, M a r s , and other
bodies) would have settled slowly deep into
the planet until, at last, they reached an
equilibrium point with the planet's magnetic, gravitational, and other force fields.
His calculations s h o w that e q u i l i b r i u m
point to be less than 1,600 kilometers out
from the center of the earth—well within
the approximately 3,500-kilometer radius
of the body's molten core.
There the monopoles would separate ac-
cording to their polarity. S o u t h monopoles
w o u l d c o n g r e g a t e all a r o u n d the n o r t h facing side of this equilibrium shell as the
north particles would the south-facing side.
As long as they remained in equilibrium,
the monopoles would pretty much stay put.
But they would still be immersed in a fluid
medium, the molten core, and could move.
Motion could be induced, Carrigan says, if
the earth's magnetic polarity reversed; indeed, this is k n o w n to occur every h u n d r e d
thousand years or so. Suddenly, then, the
monopoles would find themselves in disequilibrium because they would be in magnetic fields of the same, and therefore repulsive, sign. The particles would immediately
begin to migrate toward the opposite pole.
If, in this migration, a s o u t h - b o u n d north
monopole were to come within anything
less than a millionth of a centimeter of a
north-bound south monopole, Carrigan calculates, the two would be attracted to each
other. The outcome: annihilation and the
release of a large q u a n t i t y of heat.
There is much that is either not k n o w n or
is poorly understood about the earth's deepseated heat. Compression and radioactive
decay are usually invoked to explain it. But
if only a fourth of the planet's total heat
o u t p u t were derived from monopole annihilation, Carrigan calculates, the earth's
monopole population could average as high
as 1.5 particles per t h o u s a n d cubic centim e t e r s . T h u s even a relative h a n d f u l of
monopoles could have—or have had—significant impact on planetary evolution.
Although all sorts of science-fiction uses
have been conjectured for monopoles, should
any ever be found and isolated, physicists
would be satisfied initially to k n o w that
they exist and what their properties are.
" D o we need m o n o p o l e s to m a k e m a g netism w o r k ? " Fryberger asks rhetorically.
" T h e answer is ' n o ' ; electric charge and
electric fields can e x p l a i n e v e r y t h i n g we
currently k n o w about electromagnetism. It
really is an aesthetic argument—electric and
magnetic fields are so much alike and yet
one has something the other doesn't. T h a t
doesn't seem right, s o m e h o w . "
If there is a dogma in modern physics, it
might be that in nature anything not expressly forbidden to exist by a specific law
must be presumed to exist. Barring the discovery of some principle that prohibits them,
then, there must be magnetic monopoles.
Physicists are just going to have to get clever
enough to find them. •
The National
tributed to the
in this article
ticle Physics
Science Foundation has consupport of research discussed
through its Elementary ParProgram.
MOSAIP. Mflv/.lune1QR1 93