Magnetic Monopoles

Type of physical science: Elementary particle (high-energy) physics

Field of study: Unified theories

Magnetic monopoles are particles of magnetic charge that have been predicted but have not yet been confirmed by unambiguous observation in nature. There is no reason in classical physics to preclude the existence of magnetic monopoles, and several modern quantum field theories require their existence and predict unusual properties for the particles, including ultra-high mass, extremely small size, and particle cores which exhibit characteristics of the extreme physical conditions during the first moments after the big bang.

Overview

Magnetic monopoles are elementary particles that are predicted to exist and are characterized by having a single, isolated magnetic pole. Yet, single, isolated magnetic charges have yet to be convincingly observed in nature.

All permanent magnets have both a north magnetic pole and a south magnetic pole, a configuration known as a dipole. Early investigations of the magnetic properties of matter demonstrated that this dipole arrangement exists in all naturally occurring magnets. For example, in the year 1269, the French investigator Petrus Peregrinus de Maricourt observed that the lines of force around lodestones (natural magnets) always converged at two separate points, or poles.

Since one of the poles always pointed in the approximate direction of geographic north, it was dubbed "north" while the other pole, attracted in the opposite direction, was named "south."

Positive and negative electric charges can also be arranged as a dipole in which the positive charge is separated from the negative charge by some distance. These electric charges can, in principle, be separated into individual monopoles; that is, the positive and negative charges can be separated and isolated from each other. A single electric charge is known as an electric monopole.

When one attempts, however, to separate the poles of a magnet (for example, by breaking a bar magnet in half), the poles are not separated. Instead, two smaller magnets are created, each with a north and a south pole. The possibility of separating and isolating electric poles but not magnetic poles is a fundamental and historic distinction between the sciences of electricity and magnetism, a distinction that is present in the synthesis of electricity and magnetism, known as electrodynamics.

The classical unified theory of electrodynamics is described by four equations, known as Maxwell's equations after James Clerk Maxwell, that link the electric and magnetic fields.

Electric monopole charges are present in these equations and represent the source charges for the electric field. Yet, there are no corresponding magnetic monopole charges as source charges for the magnetic field. Rather, it is the electric current (flow of electric charges) that generates the magnetic field. Since Maxwell's electrodynamics has been exhaustively tested and has never failed within its extensive domain of validity (for dimensions greater than the atomic scale, which is the domain of quantum electrodynamics) there appears to be no observed macroscopic phenomenon that requires the existence of magnetic monopoles. Maxwell's electrodynamics is successful in describing observed electromagnetic phenomena without magnetic monopoles.

Nevertheless, classical electrodynamics neither rules out nor represents an impediment to the possible existence of magnetic monopoles. The apparent absence of magnetic monopoles still remains an empirical fact rather than a logical necessity. Although classical electrodynamics by itself does not require the existence of magnetic monopoles, efforts at formulating grand unification theories of the fundamental forces of nature do require their existence. Hence, one must look at the nature of unified field theories.

There are four fundamental forces of nature: the electromagnetic force, which is responsible for all interactions involving electric charges; the strong nuclear force, which is responsible for holding nuclei together; the weak nuclear force, which is responsible for beta decay processes; and the gravitational force, which is responsible for the interactions between all gravitational masses. The unification of these fundamental forces has been a long-time goal of physics. Indeed, one of the spectacular unification successes is Maxwell's electrodynamics, which represents the merging of electricity, magnetism, and light into a single unified theory involving the electromagnetic force. The work of Sheldon L. Glashow, Steven Weinberg, Abdus Salam, and others culminated in the electroweak theory, which unified the electromagnetic force and the weak nuclear force into a single theory. Grand unification theories (GUTs) attempt to unify three of the four fundamental forces--the electromagnetic force, the weak nuclear force, and the strong nuclear force--into a single unified theory.

With the advent of modern quantum field theories, many candidate theories for grand unification not only allow for the existence of magnetic monopoles but also require it. The three fundamental forces would have appeared as a single unified force during the first moments after the big bang when the temperature of the universe exceeded 1030 degrees Kelvin. During this epoch, GUTs predict that magnetic monopoles should have been created (and annihilated) in prodigious quantities and that some should be left over as remnants of the big bang.

Applications

In classical physics, there seems to be no need based on experimental observations for the existence of magnetic monopoles. Classical electrodynamics, however, has been unable to shed any light on the question of why the electric charge--the all-important source for both electric and magnetic fields--comes in a fundamental unit (e = 1.6 x 10-19 coulombs). Yet, it has been observed that all electric charge, positive or negative, comes in multiples of this fundamental unit or quantum of charge.

In 1931, the theoretical physicist Paul Adrien Maurice Dirac proposed the existence of magnetic monopoles in an attempt to explain the existence of the fundamental quantum of electric charge, e. The Dirac monopole can be viewed as the end of a semi-infinite solenoid or tube through which magnetic field lines emerge. In order for this model to be consistent with observation, Dirac predicted that the magnetic charge g and the fundamental unit of electric charge e are related such that their product is a constant--specifically, Planck's constant multiplied by the speed of light divided by 4π.

This relationship between the electric and magnetic quanta of charge leads to two predictions regarding magnetic monopoles: First, the minimum magnetic charge is about seventy times the quantum of electric charge e, and second, the astonishing obverse of the Dirac condition. This second prediction states that if a magnetic monopole charge g exists anywhere in the universe, then the electric charge must be quantized everywhere in the universe.

If the Dirac magnetic monopole were to be observed, then its existence would explain the quantization of electric charge. Furthermore, Dirac predicted that the magnetic monopole should have an antiparticle associated with it, but he did not make any predictions regarding the mass or size of the monopole or of its abundance in the present day or early universe.

In 1974, Gerard 't Hooft of the University of Utrecht and A. M. Polyakov of the Landau Institute for Theoretical Physics independently showed that certain classes of gauge theories in elementary particle physics that spontaneously undergo a breakdown of scale have solutions corresponding to Dirac magnetic monopoles. Their work showed that magnetic monopoles need not be postulated on an ad hoc basic. Rather, their existence was a natural consequence of the new unified theories.

Consequently, some efforts to construct GUTs of the fundamental forces of nature have required the existence of magnetic monopoles. These theories predict that magnetic monopoles should be extremely massive, on the order of 1016 gigaelectronvolts. That mass represents approximately 1016 times the mass of a proton or, equivalently, 10-8 grames--the mass of an amoeba. In spite of this gargantuan mass (relative to that of other elementary particles), the magnetic monopole is expected to be quite tiny with an "onionskin"-like structure surrounding a core which is about 10-30 centimeters in diameter. In contrast, a proton is about 10-13 centimeters in diameter.

Furthermore, the core of the magnetic monopole is predicted to resemble the physical properties of the universe as they were during the first 10-35 seconds after the big bang.

Since the extraordinarily large mass of a magnetic monopole cannot be produced in a particle accelerator, the only possible source for the production of magnetic monopoles was during the first moments of the universe. Consequently, some magnetic monopoles should exist as relics of the big bang. The temperature requirement for the production of a monopole is greater than 1030 degrees Kelvin--a condition that existed during the first 10-35 seconds after the big bang. Although most monopoles would have annihilated one another as a result of particle-antiparticle collisions during this brief epoch, some would have escaped this fate during the expansion and subsequent cooling of the universe. If this scenario is accurate, then two questions naturally arise: how abundant are magnetic monopoles and how can they be detected?

In 1982, Blas Cabrera of Stanford University observed an event in a detector composed of a series of superconducting coils that he built specifically for the search for magnetic monopoles. The event that he recorded resembled the signal that would have been expected as a result of the magnetic flux produced by a magnetic monopole passing through his detector. For a brief period of time, the report of this event initiated a flurry of interest in the scientific community. The lack of further detection of monopole events by his apparatus or by others, however, left the scientific community unconvinced that magnetic monopoles have been observed unambiguously. Even Cabrera was reluctant to view the recorded event as definitely caused by a magnetic monopole. Nevertheless, this event was the strongest evidence yet found for the existence of magnetic monopoles.

Cabrera and collaborators subsequently built a detector fifty times more sensitive than the one that recorded the 1982 event. As of 1991, this new detector had not observed the magnetic flux corresponding to the passage of a magnetic monopole through or near the detector.

Consequently, this experiment placed an upper limit on the relative abundance of magnetic monopoles at no more than one for every 1017 protons.

Grand unified theories predict another effect that is coupled with the predicted existence of magnetic monopoles. Since monopoles are relics of the early universe, during which three of the four fundamental forces were unified, they should catalyze proton decay. According to GUTs, proton decay does not readily proceed unless the temperature exceeds 1030 degrees Kelvin. Hence, GUTs predict that, in a modern environment, a proton will have a very small probability of decaying and therefore an expected lifetime of greater than 1031 years. Experimental observations suggest that, if the proton is unstable, then its lifetime must be greater than 3.5 x 1032 years. In the early universe (10-35 seconds after the big bang), however, proton decay was quite prodigious. The influence of a monopole's core (with characteristics of the early universe) passing by a proton today would catalyze its decay mode. An expected decay mode would be a monopole and a proton interacting to create a neutral pion, a positron, and the original monopole. Magnetic monopoles, which are predicted to be slowly moving particles (one-thousandth of the speed of light), would be expected to signify their passage through a detecting apparatus by leaving a chain of decayed protons in their paths.

The predicted abundance of magnetic monopoles, as well as their ability to catalyze proton decay, are still far from reaching general agreement in the scientific community. Predicted cross sections (the probabilities of such encounters between magnetic monopoles and protons) from various theoretical models for interactions differ from one another by as much as four orders of magnitude (a factor of ten thousand). On the other hand, upper limits on the abundance of monopoles have been proposed based on observations of such diverse phenomena as X rays (or lack of) given off by neutron stars, the strength of the galactic magnetic field, and the Cabrera experiment, a direct detection technique. Observations by the orbiting X-ray telescope Einstein of a particular neutron star set an upper limit on the monopole particle flux at 10-21 monopoles per square centimeter per year, the equivalent of one monopole per year passing through an area the size of Chicago. If the limits that are determined by using observed neutron star emission are valid, then the Cabrera monopole detector should see about one monopole every trillion years. On the other hand, Cabrera's reported event (which is not yet accepted to be a monopole) occurred 185 days after he began the experiment.

Context

The observation of magnetic monopoles would have a profound impact on theoretical physicists' understanding of the universe. For example, some predicted effects of magnetic monopoles are not time reversal-invariant; that is, if an experiment involving magnetic monopoles is reversed in time, then the result is not likewise reversed in time. This contradiction is one reason that some physicists continue to doubt the existence of the magnetic monopole.

Even though time reversal invariance is a cherished principle of physics, an experiment performed at Brookhaven National Laboratory in 1964 showed a violation of time reversal invariance in the decay of subatomic particles known as neutral kaons. The significance and implications of the results of that experiment are still not well understood.

Because of their large mass, magnetic monopoles would possibly have a significant impact on the "missing mass" question regarding the observed expansion rate of the universe.

Furthermore, the observed rotation rate of galaxies suggests that more mass must be present than observed within the galaxies. Even with their predicted low relative abundance, if magnetic monopoles exist then they could contribute positively to both of these missing mass problems.

In spite of significant efforts to observe proton decay, neither the decay of the proton nor the magnetic monopole has been convincingly observed in nature. This null result has potentially significant implications for the success of present GUTs but does not preclude the existence of the monopole.

If magnetic monopoles do exist and do catalyze proton decay, then a dramatic consequence would be that they will hasten the disintegration of baryonic matter in the universe, that is, protons and neutrons that are the core of atoms and molecules. Without the catalysis of proton decay, GUTs predict the instability of the proton to be on the order of or greater than 1031 years. Based on this prediction, the existence of neutron stars, white dwarfs, planets, or any object that is composed of baryons will cease to exist after approximately 1032 years (or within a factor of ten greater than the lifetime of the proton). If magnetic monopoles do exist and do catalyze proton decay, however, then the resulting accelerated decay of protons would have a definite impact on the long-range future of the universe. For example, the minuscule monopole flux, as suggested by the Einstein X-ray telescope observations, would result in the complete vaporization of the earth in approximately 1018 years.

Principal terms

BARYONS: a class of particles in elementary particle physics, the most common and well-known examples of which are protons and neutrons

DIPOLE: an arrangement of two equal but opposite charges separated by a fixed distance, such as a bar magnet

ELECTRIC CHARGE: the intrinsic property of matter that is responsible for all electromagnetic phenomena

ELECTRIC FIELD: the influence that a distribution of charges has on the space around it, such that the force experienced by a small test charge q is given by the value of that charge times the electric field that is attributable to the charge distribution

ELECTRIC MONOPOLE: a single, isolated electric charge which is either positive or negative

GRAND UNIFIED THEORIES: theories of modern physics that attempt to unite three of the four fundamental forces of nature (the electromagnetic, and the weak and strong nuclear forces) into a single unified force, such that these three forces would have been essentially identical during the early moments after the big bang

MAGNETIC CHARGE: a postulated property of matter which would give rise to magnetic fields in the way in which electric charges give rise to electric fields

MAGNETIC FIELD: the influence that a distribution of electric currents has on the space around it

MAGNETIC FLUX: the value of a magnetic field multiplied by the surface area that it crosses

MAGNETIC MONOPOLE: a particle predicted to exist in nature but not yet observed; it would have a magnetic charge of only one kind (either north or south) and would be a source charge for the magnetic field

QUANTUM OF CHARGE: the smallest unit of charge that exists in nature, of which all other charges are integer multiples; the quantum of electric charge is 1.6 x 10-19 coulombs, and the quantum of magnetic charge has yet to be observed

Bibliography

Carrigan, Richard A., Jr., and W. Peter Trower. "Superheavy Magnetic Monopoles." SCIENTIFIC AMERICAN 246 (April, 1982): 106-114. This article is an excellent source on the topic of magnetic monopoles for the general reader. Describes the properties and importance of magnetic monopoles and the reason for their rarity if they do exist. May be considered the major reference for the general reader.

Lubkin, Gloria B. "Cabrera Counts Flux Quanta to Find a Dirac Monopole." PHYSICS TODAY 35 (June, 1982): 17-19. This article highlights Blas Cabrera's apparatus and the magnetic monopole event that he recorded.

Preskill, John. "Magnetic Monopoles." ANNUAL REVIEW OF NUCLEAR AND PARTICLE SCIENCE 34 (1984): 461-530. This review article may be a bit rough for the general reader, but it is included for two reasons: First, the introductory chapter is relatively accessible to the lay reader, and second, it is an appropriate synthesis of a field which is almost exclusively documented in technical literature.

"Science and the Citizen: Cosmic Catalysis." SCIENTIFIC AMERICAN 250 (May, 1984): 66-66B. This brief article discusses the consequences of proton decay catalyzed by magnetic monopoles.

Waldrop, M. Mitchell. "Do Monopoles Catalyze Proton Decay?" SCIENCE 218 (October 15, 1982): 274-275. This brief article discusses magnetic monopoles and their predicted capacity to catalyze proton decay.

Waldrop, M. Mitchell. "In Search of the Magnetic Monopole." SCIENCE 216 (June 4, 1982): 1086-1088. Discusses the event observed by Blas Cabrera in 1982 and his apparatus that is designed to detect the passage of a magnetic monopole.

X-ray telescope

Grand Unification Theories and Supersymmetry

Essay by Stephen Huber