In particle physics, a magnetic monopole is a hypothetical elementary particle that is an isolated magnet with only one magnetic pole (a north pole without a south pole or vice versa). A magnetic monopole would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unified and superstring theories, which predict their existence.
Many early scientists attributed the magnetism of lodestones to two different "magnetic fluids" ("effluvia"), a north-pole fluid at one end and a south-pole fluid at the other, which attracted and repelled each other in analogy to positive and negative electric charge. However, an improved understanding of electromagnetism in the nineteenth century showed that the magnetism of lodestones was properly explained not by magnetic monopole fluids, but rather by a combination of electric currents, the electron magnetic moment, and the magnetic moments of other particles. Gauss's law for magnetism, one of Maxwell's equations, is the mathematical statement that magnetic monopoles do not exist. Nevertheless, it was pointed out by Pierre Curie in 1894 that magnetic monopoles could conceivably exist, despite not having been seen so far.
The quantum theory of magnetic charge started with a paper by the physicist Paul Dirac in 1931. In this paper, Dirac showed that if any magnetic monopoles exist in the universe, then all electric charge in the universe must be quantized (Dirac quantization condition). The electric charge is, in fact, quantized, which is consistent with (but does not prove) the existence of monopoles.
Since Dirac's paper, several systematic monopole searches have been performed. Experiments in 1975 and 1982 produced candidate events that were initially interpreted as monopoles, but are now regarded as inconclusive. Therefore, it remains an open question whether monopoles exist. Further advances in theoretical particle physics, particularly developments in grand unified theories and quantum gravity, have led to more compelling arguments (detailed below) that monopoles do exist. Joseph Polchinski, a string-theorist, described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen". These theories are not necessarily inconsistent with the experimental evidence. In some theoretical models, magnetic monopoles are unlikely to be observed, because they are too massive to create in particle accelerators (see § Searches for magnetic monopoles below), and also too rare in the Universe to enter a particle detector with much probability.
Some condensed matter systems propose a structure superficially similar to a magnetic monopole, known as a flux tube. The ends of a flux tube form a magnetic dipole, but since they move independently, they can be treated for many purposes as independent magnetic monopole quasiparticles. Since 2009, numerous news reports from the popular media have incorrectly described these systems as the long-awaited discovery of the magnetic monopoles, but the two phenomena are only superficially related to one another. These condensed-matter systems remain an area of active research. (See § "Monopoles" in condensed-matter systems below.)
All matter ever isolated to date, including every atom on the periodic table and every particle in the standard model, has zero magnetic monopole charge. Therefore, the ordinary phenomena of magnetism and magnets have nothing to do with magnetic monopoles.
Instead, magnetism in ordinary matter comes from two sources. First, electric currents create magnetic fields according to Ampère's law. Second, many elementary particles have an intrinsic magnetic moment, the most important of which is the electron magnetic dipole moment. (This magnetism is related to quantum-mechanical "spin".)
Mathematically, the magnetic field of an object is often described in terms of a multipole expansion. This is an expression of the field as the sum of component fields with specific mathematical forms. The first term in the expansion is called the monopole term, the second is called dipole, then quadrupole, then octupole, and so on. Any of these terms can be present in the multipole expansion of an electric field, for example. However, in the multipole expansion of a magnetic field, the "monopole" term is always exactly zero (for ordinary matter). A magnetic monopole, if it exists, would have the defining property of producing a magnetic field whose monopole term is non-zero.
A magnetic dipole is something whose magnetic field is predominantly or exactly described by the magnetic dipole term of the multipole expansion. The term dipole means two poles, corresponding to the fact that a dipole magnet typically contains a north pole on one side and a south pole on the other side. This is analogous to an electric dipole, which has positive charge on one side and negative charge on the other. However, an electric dipole and magnetic dipole are fundamentally quite different. In an electric dipole made of ordinary matter, the positive charge is made of protons and the negative charge is made of electrons, but a magnetic dipole does not have different types of matter creating the north pole and south pole. Instead, the two magnetic poles arise simultaneously from the aggregate effect of all the currents and intrinsic moments throughout the magnet. Because of this, the two poles of a magnetic dipole must always have equal and opposite strength, and the two poles cannot be separated from each other.
Maxwell's equations of electromagnetism relate the electric and magnetic fields to each other and to the motions of electric charges. The standard equations provide for electric charges, but they posit no magnetic charges. Except for this difference, the equations are symmetric under the interchange of the electric and magnetic fields.[notes 1] In fact, symmetric Maxwell's equations can be written when all charges (and hence electric currents) are zero, and this is how the electromagnetic wave equation is derived.
Fully symmetric Maxwell's equations can also be written if one allows for the possibility of "magnetic charges" analogous to electric charges. With the inclusion of a variable for the density of these magnetic charges, say ρm, there is also a "magnetic current density" variable in the equations, jm.
If magnetic charges do not exist – or if they do exist but are not present in a region of space – then the new terms in Maxwell's equations are all zero, and the extended equations reduce to the conventional equations of electromagnetism such as ∇⋅B = 0 (where ∇⋅ is divergence and B is the magnetic B field).
|Name||Without magnetic monopoles||With magnetic monopoles|
|Gauss's law for magnetism|
|Faraday's law of induction|
|Ampère's law (with Maxwell's extension)|
|Lorentz force law|
In these equations ρm is the magnetic charge density, jm is the magnetic current density, and qm is the magnetic charge of a test particle, all defined analogously to the related quantities of electric charge and current; v is the particle's velocity and c is the speed of light. For all other definitions and details, see Maxwell's equations. For the equations in nondimensionalized form, remove the factors of c.
In SI units, there are two conflicting units in use for magnetic charge qm: webers (Wb) and ampere·meters (A·m). The conversion between them is qm(Wb) = μ0qm(A·m), since the units are 1 Wb = 1 H·A = (1 H·m−1)·(1 A·m) by dimensional analysis (H is the henry – the SI unit of inductance).
Maxwell's equations then take the following forms (using the same notation above):[notes 2]
|With magnetic monopoles|
|Weber convention||Ampere·meter convention|
|Gauss's Law for magnetism|
|Faraday's Law of induction|
|Ampère's Law (with Maxwell's extension)|
|Lorentz force equation|
|Maxwell equations||Gaussian units||SI units (Wb)||SI units (A⋅m)|
|Lorentz force law|
For a particle having only electric charge, one can express its field using a four-potential, according to the standard covariant formulation of classical electromagnetism:
This formula for the fields is often called the Cabibbo–Ferrari relation, though Shanmugadhasan proposed it earlier. The quantity εαβγδ is the Levi-Civita symbol, and the indices (as usual) behave according to the Einstein summation convention.
The generalized Maxwell's equations possess a certain symmetry, called a duality transformation. One can choose any real angle ξ, and simultaneously change the fields and charges everywhere in the universe as follows (in Gaussian units):
|Charges and currents||Fields|
where the primed quantities are the charges and fields before the transformation, and the unprimed quantities are after the transformation. The fields and charges after this transformation still obey the same Maxwell's equations. The matrix is a two-dimensional rotation matrix.
Because of the duality transformation, one cannot uniquely decide whether a particle has an electric charge, a magnetic charge, or both, just by observing its behavior and comparing that to Maxwell's equations. For example, it is merely a convention, not a requirement of Maxwell's equations, that electrons have electric charge but not magnetic charge; after a ξ = π/2 transformation, it would be the other way around. The key empirical fact is that all particles ever observed have the same ratio of magnetic charge to electric charge. Duality transformations can change the ratio to any arbitrary numerical value, but cannot change the fact that all particles have the same ratio. Since this is the case, a duality transformation can be made that sets this ratio at zero, so that all particles have no magnetic charge. This choice underlies the "conventional" definitions of electricity and magnetism.
One of the defining advances in quantum theory was Paul Dirac's work on developing a relativistic quantum electromagnetism. Before his formulation, the presence of electric charge was simply "inserted" into the equations of quantum mechanics (QM), but in 1931 Dirac showed that a discrete charge naturally "falls out" of QM. That is to say, we can maintain the form of Maxwell's equations and still have magnetic charges.
Consider a system consisting of a single stationary electric monopole (an electron, say) and a single stationary magnetic monopole. Classically, the electromagnetic field surrounding them has a momentum density given by the Poynting vector, and it also has a total angular momentum, which is proportional to the product qeqm, and independent of the distance between them.
Quantum mechanics dictates, however, that angular momentum is quantized in units of ħ, so therefore the product qeqm must also be quantized. This means that if even a single magnetic monopole existed in the universe, and the form of Maxwell's equations is valid, all electric charges would then be quantized.
What are the units in which magnetic charge would be quantized? Although it would be possible simply to integrate over all space to find the total angular momentum in the above example, Dirac took a different approach. This led him to new ideas. He considered a point-like magnetic charge whose magnetic field behaves as qm / r 2 and is directed in the radial direction, located at the origin. Because the divergence of B is equal to zero almost everywhere, except for the locus of the magnetic monopole at r = 0, one can locally define the vector potential such that the curl of the vector potential A equals the magnetic field B.
However, the vector potential cannot be defined globally precisely because the divergence of the magnetic field is proportional to the Dirac delta function at the origin. We must define one set of functions for the vector potential on the "northern hemisphere" (the half-space z > 0 above the particle), and another set of functions for the "southern hemisphere". These two vector potentials are matched at the "equator" (the plane z = 0 through the particle), and they differ by a gauge transformation. The wave function of an electrically charged particle (a "probe charge") that orbits the "equator" generally changes by a phase, much like in the Aharonov–Bohm effect. This phase is proportional to the electric charge qe of the probe, as well as to the magnetic charge qm of the source. Dirac was originally considering an electron whose wave function is described by the Dirac equation.
Because the electron returns to the same point after the full trip around the equator, the phase φ of its wave function eiφ must be unchanged, which implies that the phase φ added to the wave function must be a multiple of 2π:
|SI units (weber convention)|
|SI units (ampere·meter convention)|
This is known as the Dirac quantization condition. The hypothetical existence of a magnetic monopole would imply that the electric charge must be quantized in certain units; also, the existence of the electric charges implies that the magnetic charges of the hypothetical magnetic monopoles, if they exist, must be quantized in units inversely proportional to the elementary electric charge.
At the time it was not clear if such a thing existed, or even had to. After all, another theory could come along that would explain charge quantization without need for the monopole. The concept remained something of a curiosity. However, in the time since the publication of this seminal work, no other widely accepted explanation of charge quantization has appeared. (The concept of local gauge invariance—see Gauge theory—provides a natural explanation of charge quantization, without invoking the need for magnetic monopoles; but only if the U(1) gauge group is compact, in which case we have magnetic monopoles anyway.)
If we maximally extend the definition of the vector potential for the southern hemisphere, it is defined everywhere except for a semi-infinite line stretched from the origin in the direction towards the northern pole. This semi-infinite line is called the Dirac string and its effect on the wave function is analogous to the effect of the solenoid in the Aharonov–Bohm effect. The quantization condition comes from the requirement that the phases around the Dirac string are trivial, which means that the Dirac string must be unphysical. The Dirac string is merely an artifact of the coordinate chart used and should not be taken seriously.
The Dirac monopole is a singular solution of Maxwell's equation (because it requires removing the worldline from spacetime); in more complicated theories, it is superseded by a smooth solution such as the 't Hooft–Polyakov monopole.
A gauge theory like electromagnetism is defined by a gauge field, which associates a group element to each path in space time. For infinitesimal paths, the group element is close to the identity, while for longer paths the group element is the successive product of the infinitesimal group elements along the way.
In electrodynamics, the group is U(1), unit complex numbers under multiplication. For infinitesimal paths, the group element is 1 + iAμdxμ which implies that for finite paths parametrized by s, the group element is:
The map from paths to group elements is called the Wilson loop or the holonomy, and for a U(1) gauge group it is the phase factor which the wavefunction of a charged particle acquires as it traverses the path. For a loop:
So that the phase a charged particle gets when going in a loop is the magnetic flux through the loop. When a small solenoid has a magnetic flux, there are interference fringes for charged particles which go around the solenoid, or around different sides of the solenoid, which reveal its presence.
But if all particle charges are integer multiples of e, solenoids with a flux of 2π/e have no interference fringes, because the phase factor for any charged particle is e2πi = 1. Such a solenoid, if thin enough, is quantum-mechanically invisible. If such a solenoid were to carry a flux of 2π/e, when the flux leaked out from one of its ends it would be indistinguishable from a monopole.
Dirac's monopole solution in fact describes an infinitesimal line solenoid ending at a point, and the location of the solenoid is the singular part of the solution, the Dirac string. Dirac strings link monopoles and antimonopoles of opposite magnetic charge, although in Dirac's version, the string just goes off to infinity. The string is unobservable, so you can put it anywhere, and by using two coordinate patches, the field in each patch can be made nonsingular by sliding the string to where it cannot be seen.
In a U(1) gauge group with quantized charge, the group is a circle of radius 2π/e. Such a U(1) gauge group is called compact. Any U(1) that comes from a grand unified theory is compact – because only compact higher gauge groups make sense. The size of the gauge group is a measure of the inverse coupling constant, so that in the limit of a large-volume gauge group, the interaction of any fixed representation goes to zero.
The case of the U(1) gauge group is a special case because all its irreducible representations are of the same size – the charge is bigger by an integer amount, but the field is still just a complex number – so that in U(1) gauge field theory it is possible to take the decompactified limit with no contradiction. The quantum of charge becomes small, but each charged particle has a huge number of charge quanta so its charge stays finite. In a non-compact U(1) gauge group theory, the charges of particles are generically not integer multiples of a single unit. Since charge quantization is an experimental certainty, it is clear that the U(1) gauge group of electromagnetism is compact.
GUTs lead to compact U(1) gauge groups, so they explain charge quantization in a way that seems logically independent from magnetic monopoles. However, the explanation is essentially the same, because in any GUT that breaks down into a U(1) gauge group at long distances, there are magnetic monopoles.
The argument is topological:
Hence, the Dirac monopole is a topological defect in a compact U(1) gauge theory. When there is no GUT, the defect is a singularity – the core shrinks to a point. But when there is some sort of short-distance regulator on space time, the monopoles have a finite mass. Monopoles occur in lattice U(1), and there the core size is the lattice size. In general, they are expected to occur whenever there is a short-distance regulator.
In the universe, quantum gravity provides the regulator. When gravity is included, the monopole singularity can be a black hole, and for large magnetic charge and mass, the black hole mass is equal to the black hole charge, so that the mass of the magnetic black hole is not infinite. If the black hole can decay completely by Hawking radiation, the lightest charged particles cannot be too heavy. The lightest monopole should have a mass less than or comparable to its charge in natural units.
So in a consistent holographic theory, of which string theory is the only known example, there are always finite-mass monopoles. For ordinary electromagnetism, the upper mass bound is not very useful because it is about same size as the Planck mass.
A connection on a G-bundle tells you how to glue fibers together at nearby points of M. It starts with a continuous symmetry group G that acts on the fiber F, and then it associates a group element with each infinitesimal path. Group multiplication along any path tells you how to move from one point on the bundle to another, by having the G element associated to a path act on the fiber F.
In mathematics, the definition of bundle is designed to emphasize topology, so the notion of connection is added on as an afterthought. In physics, the connection is the fundamental physical object. One of the fundamental observations in the theory of characteristic classes in algebraic topology is that many homotopical structures of nontrivial principal bundles may be expressed as an integral of some polynomial over any connection over it. Note that a connection over a trivial bundle can never give us a nontrivial principal bundle.
If spacetime is ℝ4 the space of all possible connections of the G-bundle is connected. But consider what happens when we remove a timelike worldline from spacetime. The resulting spacetime is homotopically equivalent to the topological sphere S2.
A principal G-bundle over S2 is defined by covering S2 by two charts, each homeomorphic to the open 2-ball such that their intersection is homeomorphic to the strip S1×I. 2-balls are homotopically trivial and the strip is homotopically equivalent to the circle S1. So a topological classification of the possible connections is reduced to classifying the transition functions. The transition function maps the strip to G, and the different ways of mapping a strip into G are given by the first homotopy group of G.
So in the G-bundle formulation, a gauge theory admits Dirac monopoles provided G is not simply connected, whenever there are paths that go around the group that cannot be deformed to a constant path (a path whose image consists of a single point). U(1), which has quantized charges, is not simply connected and can have Dirac monopoles while ℝ, its universal covering group, is simply connected, doesn't have quantized charges and does not admit Dirac monopoles. The mathematical definition is equivalent to the physics definition provided that—following Dirac—gauge fields are allowed that are defined only patch-wise, and the gauge field on different patches are glued after a gauge transformation.
The total magnetic flux is none other than the first Chern number of the principal bundle, and depends only upon the choice of the principal bundle, and not the specific connection over it. In other words, it is a topological invariant.
This argument for monopoles is a restatement of the lasso argument for a pure U(1) theory. It generalizes to d + 1 dimensions with d ≥ 2 in several ways. One way is to extend everything into the extra dimensions, so that U(1) monopoles become sheets of dimension d − 3. Another way is to examine the type of topological singularity at a point with the homotopy group πd − 2(G).
In more recent years, a new class of theories has also suggested the existence of magnetic monopoles.
During the early 1970s, the successes of quantum field theory and gauge theory in the development of electroweak theory and the mathematics of the strong nuclear force led many theorists to move on to attempt to combine them in a single theory known as a Grand Unified Theory (GUT). Several GUTs were proposed, most of which implied the presence of a real magnetic monopole particle. More accurately, GUTs predicted a range of particles known as dyons, of which the most basic state was a monopole. The charge on magnetic monopoles predicted by GUTs is either 1 or 2 gD, depending on the theory.
The majority of particles appearing in any quantum field theory are unstable, and they decay into other particles in a variety of reactions that must satisfy various conservation laws. Stable particles are stable because there are no lighter particles into which they can decay and still satisfy the conservation laws. For instance, the electron has a lepton number of one and an electric charge of one, and there are no lighter particles that conserve these values. On the other hand, the muon, essentially a heavy electron, can decay into the electron plus two quanta of energy, and hence it is not stable.
The dyons in these GUTs are also stable, but for an entirely different reason. The dyons are expected to exist as a side effect of the "freezing out" of the conditions of the early universe, or a symmetry breaking. In this scenario, the dyons arise due to the configuration of the vacuum in a particular area of the universe, according to the original Dirac theory. They remain stable not because of a conservation condition, but because there is no simpler topological state into which they can decay.
The length scale over which this special vacuum configuration exists is called the correlation length of the system. A correlation length cannot be larger than causality would allow, therefore the correlation length for making magnetic monopoles must be at least as big as the horizon size determined by the metric of the expanding universe. According to that logic, there should be at least one magnetic monopole per horizon volume as it was when the symmetry breaking took place.
Cosmological models of the events following the big bang make predictions about what the horizon volume was, which lead to predictions about present-day monopole density. Early models predicted an enormous density of monopoles, in clear contradiction to the experimental evidence. This was called the "monopole problem". Its widely accepted resolution was not a change in the particle-physics prediction of monopoles, but rather in the cosmological models used to infer their present-day density. Specifically, more recent theories of cosmic inflation drastically reduce the predicted number of magnetic monopoles, to a density small enough to make it unsurprising that humans have never seen one. This resolution of the "monopole problem" was regarded as a success of cosmic inflation theory. (However, of course, it is only a noteworthy success if the particle-physics monopole prediction is correct.) For these reasons, monopoles became a major interest in the 1970s and 80s, along with the other "approachable" predictions of GUTs such as proton decay.
Many of the other particles predicted by these GUTs were beyond the abilities of current experiments to detect. For instance, a wide class of particles known as the X and Y bosons are predicted to mediate the coupling of the electroweak and strong forces, but these particles are extremely heavy and well beyond the capabilities of any reasonable particle accelerator to create.
Experimental searches for magnetic monopoles can be placed in one of two categories: those that try to detect preexisting magnetic monopoles and those that try to create and detect new magnetic monopoles.
Passing a magnetic monopole through a coil of wire induces a net current in the coil. This is not the case for a magnetic dipole or higher order magnetic pole, for which the net induced current is zero, and hence the effect can be used as an unambiguous test for the presence of magnetic monopoles. In a wire with finite resistance, the induced current quickly dissipates its energy as heat, but in a superconducting loop the induced current is long-lived. By using a highly sensitive "superconducting quantum interference device" (SQUID) one can, in principle, detect even a single magnetic monopole.
According to standard inflationary cosmology, magnetic monopoles produced before inflation would have been diluted to an extremely low density today. Magnetic monopoles may also have been produced thermally after inflation, during the period of reheating. However, the current bounds on the reheating temperature span 18 orders of magnitude and as a consequence the density of magnetic monopoles today is not well constrained by theory.
There have been many searches for preexisting magnetic monopoles. Although there have been tantalizing events recorded, in particular the event recorded by Blas Cabrera on the night of February 14, 1982 (thus, sometimes referred to as the "Valentine's Day Monopole"), there has never been reproducible evidence for the existence of magnetic monopoles. The lack of such events places an upper limit on the number of monopoles of about one monopole per 1029 nucleons.
Another experiment in 1975 resulted in the announcement of the detection of a moving magnetic monopole in cosmic rays by the team led by P. Buford Price. Price later retracted his claim, and a possible alternative explanation was offered by Alvarez. In his paper it was demonstrated that the path of the cosmic ray event that was claimed due to a magnetic monopole could be reproduced by the path followed by a platinum nucleus decaying first to osmium, and then to tantalum.
High energy particle colliders have been used to try to create magnetic monopoles. Due to the conservation of magnetic charge, magnetic monopoles must be created in pairs, one north and one south. Due to conservation of energy, only magnetic monopoles with masses less than the center of mass energy of the colliding particles can be produced. Beyond this, very little is known theoretically about the creation of magnetic monopoles in high energy particle collisions. This is due to their large magnetic charge, which invalidates all the usual calculational techniques. As a consequence, collider based searches for magnetic monopoles cannot, as yet, provide lower bounds on the mass of magnetic monopoles. They can however provide upper bounds on the probability (or cross section) of pair production, as a function of energy.
The MoEDAL experiment, installed at the Large Hadron Collider, is currently searching for magnetic monopoles and large supersymmetric particles using nuclear track detectors and aluminum bars around LHCb's VELO detector. The particles it is looking for damage the plastic sheets that comprise the nuclear track detectors along their path, with various identifying features. Further, the aluminum bars can trap sufficiently slowly moving magnetic monopoles. The bars can then be analyzed by passing them through a SQUID.
A true magnetic monopole would be a new elementary particle, and would violate Gauss's law for magnetism ∇⋅B = 0. A monopole of this kind, which would help to explain the law of charge quantization as formulated by Paul Dirac in 1931, has never been observed in experiments.
The monopoles studied by condensed-matter groups have none of these properties. They are not a new elementary particle, but rather are an emergent phenomenon in systems of everyday particles (protons, neutrons, electrons, photons); in other words, they are quasi-particles. They are not sources for the B-field (i.e., they do not violate ∇⋅B = 0); instead, they are sources for other fields, for example the H-field, the "B*-field" (related to superfluid vorticity), or various other quantum fields. They are not directly relevant to grand unified theories or other aspects of particle physics, and do not help explain charge quantization—except insofar as studies of analogous situations can help confirm that the mathematical analyses involved are sound.
There are a number of examples in condensed-matter physics where collective behavior leads to emergent phenomena that resemble magnetic monopoles in certain respects, including most prominently the spin ice materials. While these should not be confused with hypothetical elementary monopoles existing in the vacuum, they nonetheless have similar properties and can be probed using similar techniques.
One example of the work on magnetic monopole quasiparticles is a paper published in the journal Science in September 2009, in which researchers described the observation of quasiparticles resembling magnetic monopoles. A single crystal of the spin ice material dysprosium titanate was cooled to a temperature between 0.6 kelvin and 2.0 kelvin. Using observations of neutron scattering, the magnetic moments were shown to align into interwoven tubelike bundles resembling Dirac strings. At the defect formed by the end of each tube, the magnetic field looks like that of a monopole. Using an applied magnetic field to break the symmetry of the system, the researchers were able to control the density and orientation of these strings. A contribution to the heat capacity of the system from an effective gas of these quasiparticles was also described. This research went on to win the 2012 Europhysics Prize for condensed matter physics.
In another example, a paper in the February 11, 2011 issue of Nature Physics describes creation and measurement of long-lived magnetic monopole quasiparticle currents in spin ice. By applying a magnetic-field pulse to crystal of dysprosium titanate at 0.36 K, the authors created a relaxing magnetic current that lasted for several minutes. They measured the current by means of the electromotive force it induced in a solenoid coupled to a sensitive amplifier, and quantitatively described it using a chemical kinetic model of point-like charges obeying the Onsager–Wien mechanism of carrier dissociation and recombination. They thus derived the microscopic parameters of monopole motion in spin ice and identified the distinct roles of free and bound magnetic charges.
In superfluids, there is a field B*, related to superfluid vorticity, which is mathematically analogous to the magnetic B-field. Because of the similarity, the field B* is called a "synthetic magnetic field". In January 2014, it was reported that monopole quasiparticles for the B* field were created and studied in a spinor Bose–Einstein condensate. This constitutes the first example of a quasi-magnetic monopole observed within a system governed by quantum field theory.
In mathematics, the phrase customarily refers to a static solution to these equations in the Bogomolny–Parasad–Sommerfeld limit λ → ϕ, which realizes, within topological class, the absolutes minimum of the functional
This means that it in a connection A on a principal G-bundle over ℝ3 (cf. also Connections on a manifold; principal G-object) and a section ϕ of the associated adjoint bundle of Lie algebras such that the curvature FA and covariant derivative DA ϕ satisfy the Bogomolny equations
and the boundary conditions.
Pure mathematical advances in the theory of monopoles from the 1980s onwards have often proceeded on the basis of physically motived questions.
The equations themselves are invariant under gauge transformation and orientation-preserving symmetries. When γ is large, ϕ/||ϕ|| defines a mapping from a 2-sphere of radius γ in ℝ3 to an adjoint orbit G/k and the homotopy class of this mapping is called the magnetic charge. Most work has been done in the case G = SU(2), where the charge is a positive integer k. The absolute minimum value of the functional is then 8πk and the coefficient m in the asymptotic expansion of ϕ/||ϕ|| is k/2.
The first SU(2) solution was found by E. B. Bogomolny, J. K. Parasad and C. M. Sommerfield in 1975. It is spherically symmetric of charge 1 and has the form
In 1980, C.H.Taubes showed by a gluing construction that there exist solutions for all large k and soon after explicit axially symmetric solutions were found. The first exact solution in the general case was given in 1981 by R.S.Ward for k = 2 in terms of elliptic functions.
There are two ways of solving the Bogomolny equations. The first is by twistor methods. In the formulation of N.J. Hitchin, an arbitrary solution corresponds to a holomorphic vector bundle over the complex surface TP1, the tangent bundle of the projective line. This is naturally isomorphic to the space of oriented straight lines in ℝ3.
The boundary condition show that the holomorphic bundle is an extension of line bundles determined by a compact algebraic curve of genus (k − 1)2 (the spectral curve) in TP1, satisfying certain constraints.
The second method, due to W.Nahm, involves solving an eigen value problem for the coupled Dirac operator and transforming the equations with their boundary conditions into a system of ordinary differential equations, the Nahm equations.
where Ti(s) is a (k × k)-matrix valued function on (0,2).
The equivalence of the two methods for SU(2) and their general applicability was established in (see also:). Explicit formulas for A and ϕ are difficult to obtain by either method, despite some exact solutions of Nahm's equations in symmetric situations.
Maximally imbedded spherically symmetric magnetic monopole solutions in the Bogolomony–Parasad–Sommerfield limit for the gauge group SU(n) were exhibited by Bais. Gannoulis, Goddard and Olive, and Farwell and Minami showed that maximally imbedded spherically symmetric magnetic monopole solutions in the Bogolomony–Parasad–Sommerfield limit for an arbitrary simple gauge group G corresponding to a Lie algebra with Cartan matrix K and level vector R, are solutions to the Toda molecule equation:
Non-singular solutions have a magnetic field vanishes at the origin. Explicit finite energy solutions for the Lie algebras An, Bn and Cn have been obtained using this method.
The case of a more general Lie group G, where the stabilizer of ϕ at infinity is a maximal torus, was treated by M. K. Murray from the twistor point of view, where the single spectral curve of an SU(2)-monopole is replaced by a collection of curves indexed by the vertices of the Dynkin diagram of G. The corresponding Nahm construction was designed by J. Hustubise and Murray.
The moduli space (cf. also Moduli theory) of all SU(2) monopoles of charge k up to gauge equivalence was shown by Taubes to be a smooth non-compact manifold of dimension 4k − 1. Restricting to gauge transformations that preserve the connection at infinity gives a 4k-dimensional manifold Mk, which is a circle bundle over the true moduli space and carries a natural complete hyper-Kähler metric (cf. also Kähler–Einstein manifold). With suspected to any of the complex structures of the hyper-Kähler family, this manifold is holomorphically equivalent to the space of based rational mapping of degree k from P1 to itself.
The metric is known in twistor terms, and its Kähler potential can be written using the Riemann theta functions of the spectral curve, but only the case k = 2 is known in a more conventional and usable form (as of 2000). This Atiyah–Hitchin manifold, the Einstein Taub-NUT metric and ℝ4 are the only 4-dimensional complete hyper-Kähler manifolds with a non-triholomorphic SU(2) action. Its geodesics have been studied and a programme of Manton concerning monopole dynamics put into effect. Further dynamical features have been elucidated by numerical and analytical techniques.
A cyclic k-fold conering of Mk splits isometrically is a product M̃k × S1 × ℝ3, where M̃k is the space of strongly centred monopoles. This space features in an application of S-duality in theoretical physics, and in G. B. Segal and A. Selby studied its topology and the L2 harmonic forms defined on it, partially confirming the physical prediction.
Magnetic monopole on hyperbolic three-space were investigated from the twistor point of view by M. F. Atiyah (replacing the complex surface TP1 by the complement of the anti-diagonal in P1 × P1) and in terms of discrete Nahm equations by Murray and M. A. Singer.
Magnetic monopoles have also inspired condensed-matter physicists to discover analogous states and excitations in systems such as spin ices and Bose–Einstein condensates. However, despite the importance of those developments in their own fields, they do not resolve the question of the existence of real magnetic monopoles. Therefore, the search continues.
This article incorporates material from N. Hitchin (2001) , "Magnetic Monopole", in Hazewinkel, Michiel (ed.), Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4, which is licensed under the Creative Commons Attribution/Share-Alike License and GNU Free Documentation License.Bipolarity
The term bipolarity can refer to:
Polarity in international relations
Bipolar disorder in psychiatry
An object with an electromagnetic field which is not a magnetic monopole
A dipole antenna in radio broadcastingBlas Cabrera
Blas Cabrera (born September 21, 1946 in Paris, France) is a physicist at Stanford University best known for his experiment in search of magnetic monopoles. He is the son of Spanish physicist Nicolás Cabrera and the grandson of Blas Cabrera Felipe, also a Spanish physicist.
He received in 1968 his B.S. from the University of Virginia and in 1975 his Ph.D. from Stanford University this thesis The Use of Superconducting Shields for Generating Ultra Low Magnetic Field Regions and Several Related Experiments, with advisor William M. Fairbank and co-advisor William O. Hamilton.
On the night of February 14, 1982, his detector recorded an event which had the perfect signature hypothesized for a magnetic monopole. After he published his discovery,
a number of similar detectors were built by various research groups, and Cabrera's laboratory itself received a large grant to build an improved detector. However, no similar event has been recorded since, and his research group has since dropped the search. He is now a leader of the Cryogenic Dark Matter Search experiment.Dirac string
In physics, a Dirac string is a hypothetical one-dimensional curve in space, conceived of by the physicist Paul Dirac, stretching between two Dirac magnetic monopoles with opposite magnetic charges, or from one magnetic monopole out to infinity. The gauge potential cannot be defined on the Dirac string, but it is defined everywhere else. The Dirac string acts as the solenoid in the Aharonov–Bohm effect, and the requirement that the position of the Dirac string should not be observable implies the Dirac quantization rule: the product of a magnetic charge and an electric charge must always be an integer multiple of . Also, a change of position of a Dirac string corresponds to a gauge transformation. This shows that Dirac strings are not gauge invariant, which is consistent with the fact that they are not observable.
The Dirac string is the only way to incorporate magnetic monopoles into Maxwell's equations, since the magnetic flux running along the interior of the string maintains their validity. If Maxwell equations are modified to allow magnetic charges at the fundamental level then the magnetic monopoles are no longer Dirac monopoles, and do not require attached Dirac strings.Dual photon
In theoretical physics, the dual photon is a hypothetical elementary particle that is a dual of the photon under electric-magnetic duality which is predicted by some theoretical models and some results of M-theory in eleven dimensions.It has been shown that including magnetic monopole in Maxwell's equations introduces a singularity. The only way to avoid the singularity is including a second four-vector potential, called dual photon, in addition to the usual four-vector potential, photon. Additionally, it was found that the standard Lagrangian of electromagnetism is not dual symmetric that causes dual-asymmetric problems of the energy–momentum, spin and orbital angular-momentum tensors. To resolve this issue, a dual-symmetric Lagrangian of electromagnetism has been proposed, which has a self-consistent separation of the spin and orbital degrees of freedom. The Poincaré symmetries imply that the dual electromagnetism naturally makes self-consistent conservation laws.Duality (electricity and magnetism)
In physics, the electromagnetic dual concept is based on the idea that, in the static case, electromagnetism has two separate facets: electric fields and magnetic fields. Expressions in one of these will have a directly analogous, or dual, expression in the other. The reason for this can ultimately be traced to special relativity where applying the Lorentz transformation to the electric field will transform it into a magnetic field. These are special cases of duality in mathematics.
The electric field (E) is the dual of the magnetic field (H).
The electric displacement field (D) is the dual of the magnetic flux density (B).
Faraday's law of induction is the dual of Ampère's circuital law.
Gauss's law for electric field is the dual of Gauss's law for magnetism.
The electric potential is the dual of the magnetic potential.
Permittivity is the dual of permeability.
Electrostriction is the dual of magnetostriction.
Piezoelectricity is the dual of piezomagnetism.
Ferroelectricity is the dual of ferromagnetism.
An electrostatic motor is the dual of a magnetic motor;
Electrets are the dual of permanent magnets;
The Faraday effect is the dual of the Kerr effect;
The Aharonov–Casher effect is the dual to the Aharonov–Bohm effect;
The hypothetical magnetic monopole is the dual of electric charge.Dyon
In physics, a dyon is a hypothetical particle in 4-dimensional theories with both electric and magnetic charges. A dyon with a zero electric charge is usually referred to as a magnetic monopole. Many grand unified theories predict the existence of both magnetic monopoles and dyons.
Dyons were first proposed by Julian Schwinger in 1969 as a phenomenological alternative to
quarks. He extended the Dirac quantization condition to the dyon and used the model to predict the existence of a particle with the properties of the J/ψ meson prior to its discovery in 1974.
The allowed charges of dyons are restricted by the Dirac quantization condition. This states in particular that their magnetic charge must be integral, and that their electric charges must all be equal modulo 1. The Witten effect, demonstrated by Edward Witten in his 1979 paper, states that the electric charges of dyons must all be equal, modulo one, to the product of their magnetic charge and the theta angle of the theory. In particular, if a theory does not preserve CP symmetry then the electric charges of all dyons are integers.Inflation (cosmology)
In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10−36 seconds after the conjectured Big Bang singularity to some time between 10−33 and 10−32 seconds after the singularity. Following the inflationary period, the universe continues to expand, but at a less rapid rate.Inflation theory was first developed in 1979 by theoretical physicist Alan Guth at Cornell University. It was developed further in the early 1980s. It explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the Universe (see galaxy formation and evolution and structure formation). Many physicists also believe that inflation explains why the universe appears to be the same in all directions (isotropic), why the cosmic microwave background radiation is distributed evenly, why the universe is flat, and why no magnetic monopoles have been observed.
The detailed particle physics mechanism responsible for inflation is unknown. The basic inflationary paradigm is accepted by most physicists, as a number of inflation model predictions have been confirmed by observation; however, a substantial minority of scientists dissent from this position. The hypothetical field thought to be responsible for inflation is called the inflaton.In 2002, three of the original architects of the theory were recognized for their major contributions; physicists Alan Guth of M.I.T., Andrei Linde of Stanford, and Paul Steinhardt of Princeton shared the prestigious Dirac Prize "for development of the concept of inflation in cosmology". In 2012, Alan Guth and Andrei Linde were awarded the Breakthrough Prize in Fundamental Physics for their invention and development of inflationary cosmology.Loop algebra
In mathematics, loop algebras are certain types of Lie algebras, of particular interest in theoretical physics.Magnetic dipole
A magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the dimensions of the source are reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric dipole, but the analogy is not perfect. In particular, a magnetic monopole, the magnetic analogue of an electric charge, has never been observed. Moreover, one form of magnetic dipole moment is associated with a fundamental quantum property—the spin of elementary particles.
The magnetic field around any magnetic source looks increasingly like the field of a magnetic dipole as the distance from the source increases.Magnetic field
A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed in a wide range of size scales, from subatomic particles to galaxies. In everyday life, the effects of magnetic fields are often seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges (electric currents) such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location. As such, it is an example of a vector field.
The term 'magnetic field' is used for two distinct but closely related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla (in SI base units: kilogram per second2 per ampere), which is equivalent to newton per meter per ampere. H and B differ in how they account for magnetization. In a vacuum, B and H are the same aside from units; but in a magnetized material, B/ and H differ by the magnetization M of the material at that point in the material.
Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, and are both components of the electromagnetic force, one of the four fundamental forces of nature.
Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect. The Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass.Magnetic pole
Magnetic pole may refer to:
One of the two ends of a magnet
Magnetic monopole, a hypothetical elementary particle
The magnetic poles of astronomical bodies, a special case of magnets, especially:
The North Magnetic Pole of planet Earth, a point where the north end of a compass points downward
The South Magnetic Pole of planet Earth, a point where the south end of a compass points downwardMagneton
Magneton may refer to:
Bohr magneton, a physical constant of magnetic moment named after Niels Bohr
Nuclear magneton, a physical constant of magnetic moment
Parson magneton, a hypothetical object in atomic physics suggested by Alfred Lauck Parson in 1915
Weiss magneton, an experimentally derived unit of magnetic moment suggested in 1911 by Pierre-Ernest Weiss
Magneton, a term that some physicists use for magnetic monopole
Magneton (Pokémon), a Pokémon
Magneton, an album by The Octagon ManMonopole
Monopole may refer to:
Magnetic monopole, or Dirac monopole, a hypothetical particle that may be loosely described as a magnet with only one pole
Monopole (mathematics), a connection over a principal bundle G with a section (the Higgs field) of the associated adjoint bundle
Monopole, the first term in a multipole expansion
Monopole (wine), an appellation owned by only one winery
Monopole (album), a 2011 album by White Town
Monopole antenna, a radio antenna that replaces half of a dipole antenna with a ground plane at right-angles to the remaining half
Monopole, a tubular self-supporting telecommunications mast
The Monopole, a bar in Plattsburgh, NYMonopole, Astrophysics and Cosmic Ray Observatory
MACRO (Monopole, Astrophysics and Cosmic Ray Observatory) was a particle physics experiment located at the Laboratori Nazionali del Gran Sasso in Abruzzo, Italy. MACRO was proposed by 6 scientific institutions in the United States and 6 Italian institutions.
The primary goal of MACRO was to search for magnetic monopoles. The active elements of MACRO were liquid scintillator and streamer tubes, optimized for high resolution tracking and timing. This design also allowed MACRO to operate as a neutrino detector and as a cosmic ray observatory.
The experiment ceased operating in 2000. No monopole candidates were detected, meaning that the flux of monopoles is less than 1.4×10−16 per square centimetre per steradian per second (cm−2sr−1s−1) for velocities between 0.0001 c and 1 c (between 30000 m/s and 300000000 m/s).The magnetic monopole is a theorized particle that has not yet been observed. It is a possible solution to Maxwell's equations. One researcher claimed to have observed a monopole with a light-bulb-sized detector. The fact that a detector the size of multiple football pitches (MACRO) has not yet duplicated this feat leads most to disregard the earlier claim.
The MACRO project included a large underground cavern, approximately 800 metres underground, which was further hollowed out and housed hundreds of long chambers filled with scintillating fluid – a fluid that gives off photons when a charged or magnetic particle passes through it. At opposing ends of the chamber were a pair of photomultiplier tubes. Photomultiplier tubes contain a number of small charged "plates." They look like flood lights, but they are collectors that can take a handful of photons and "multiply" them. This multiplication begins by using the photo-electric effect to convert photons that hit the first "plate" into electrons. These electrons are then attracted to the next plate which gives off more electrons that it receives. The next plate does the same, thus amplifying the signal more at each plate. The photomultipliers used in the MACRO experiment were produced by Thorn-EMI, and were sensitive to a signal as small as five photons. After decommissioning, MACRO donated about 800 photomultiplier tubes to the Daya Bay Reactor Neutrino Experiment. The exact voltage put on each plate was determined by a custom circuit board designed by some of the scientists involved with the project.
The scintillating chambers were assembled into high stacks and long rows. When a signal was detected, it was usually detected in multiple chambers. The timing of each signal from each photomultiplier told the approximate path and speed of the particle. The type of signal and the speed through the "pool" of chambers told researchers if they had observed a monopole or merely some common charged particle.
Very important results were obtained by MACRO in other sectors:
cosmic rays: flux, composition and shadow of the sun and the moon;
search for dark matter (WIMPS) from the center of the Sun and the Earth and dark matter with strange quarks;
search for low energy neutrinos from supernovae;
neutrino astronomy and neutrino oscillations.In particular, MACRO showed evidence of neutrino oscillations at the Takayama neutrino conference immediately before the announcement of the discovery of oscillations by the Super-Kamiokande experiment .Montonen–Olive duality
Montonen–Olive duality or electric-magnetic duality is the oldest known example of strong-weak duality or S-duality according to current terminology. It generalizes the electro-magnetic symmetry of Maxwell's equations by stating that magnetic monopoles, which are usually viewed as emergent quasiparticles that are "composite" (i.e. they are solitons or topological defects), can in fact be viewed as "elementary" quantized particles with electrons playing the reverse role of "composite" topological solitons; the viewpoints are equivalent and the situation dependant on the duality. It was later proven to hold true when dealing with a N = 4 supersymmetric Yang–Mills theory. It is named after Finnish physicist Claus Montonen and British physicist David Olive after they proposed the idea in their academic paper Magnetic monopoles as gauge particles? where they state:
There should be two "dual equivalent" field formulations of the same theory in which electric (Noether) and magnetic (topological) quantum numbers exchange roles.
S-duality is now a basic ingredient in topological quantum field theories and string theories, especially since the 1990s with the advent of the second superstring revolution. This duality is now one of several in string theory, the AdS/CFT correspondence which gives rise to the holographic principle, being viewed as amongst the most important. These dualities have played an important role in condensed matter physics, from predicting fractional charges of the electron, to the discovery of the magnetic monopole.P. Buford Price
Paul Buford Price, usually known as P. Buford Price, is a professor in the Graduate School at the University of California, Berkeley and a member of the National Academy of Sciences. His work has been wide-ranging over his career, but began with the study of physics and has included cosmic rays, astrophysics, nuclear physics, glaciology, climatology, biology in extreme environments, and origins of life.Praveen Chaudhari
Dr. Praveen Chaudhari (November 30, 1937 – January 13, 2010) was an Indian American physicist who has contributed to the field of material physics. His research focused on structure and properties of amorphous solids, defects in solids, mechanical properties of thin films, superconductivity, quantum transport in disordered systems, liquid crystal alignment on substrates, and the magnetic monopole experiment. He has published numerous papers and filed 22 patents, most notably one for the erasable read-write compact discs which are commonly used to burn music.He was at IBM for 36 years during which he was appointed director and later vice president of science in 1981 and 1982. In 2004, he became director of Brookhaven National Laboratory (BNL) and stepped down in 2006.
Praveen died on January 12, 2010, at the age of 72. He is survived by his wife, Karin; his son, Ashok; and his daughter, Pia.Steven T. Bramwell
Steven T. Bramwell (born 7 June 1961) is a British physicist and chemist who works at the London Centre for Nanotechnology and the Department of Physics and Astronomy, University College London. He is known for his experimental discovery of spin ice with M. J. Harris and his calculation of a critical exponent observed in two-dimensional magnets with P. C. W. Holdsworth. A probability distribution for global quantities in complex systems,
the "Bramwell-Holdsworth-Pinton (BHP) distribution", (to be implemented in Mathematica) is named after him.In 2009 Bramwell's group was one of several to report experimental evidence of magnetic monopole excitations in spin ice. He coined the term "magnetricity" to describe currents of these effective magnetic "monopoles" in condensed-matter systems.Bramwell studied chemistry at Oxford University, obtaining his PhD in 1989. He was a professor of physical chemistry at University College London from 2000-2009, before becoming a Professor in the Department of Physics and Astronomy.Wu–Yang monopole
The Wu–Yang monopole was the first solution (found in 1968 by Tai Tsun Wu and Chen Ning Yang) to the Yang-Mills field equations. It describes a magnetic monopole which is pointlike and has a potential which behaves like 1/r everywhere.