[New post] Scientists’ search for the elusive magnetic monopole | Physics Conference 2022
Conference Alerts posted: " 3rd International Conference on Applied Physics and Materials Science | August 01-02, 2022 | Zurich, Switzerland While theories such as the grand unified theories (GUTs), which unify electromagnetism, nuclear forces and gravity in one framework, predi" European Conferences
ONE knows that matter is composed of atoms and that each atom is made up of a positively charged nucleus with negatively charged electrons moving around it in designated orbits. One was taught in school that electric current is due to the motion of electrons in a conductor. That is, one talks of positive and negative charges existing in the universe as separate entities that can exist in isolation. One was also taught that the flow of current in a wire results in a magnetic field around it (Ampere's law), and similarly, a changing magnetic field can cause an electric current to flow through a coil of wire (Faraday's law of induction). This unity and reciprocity between electricity and magnetism are described in classical physics by Maxwell's equations (1864), according to which the two are different manifestations of one unified electromagnetic field. Their properties can, therefore, be described by a single unified mathematical framework.
But despite this apparent symmetry between electricity and magnetism, one never talks of isolated magnetic charges. One only talks of magnets with one end as the north pole (N) and the other end as the south pole (S) but never of north or south pole as separate entities. One would recall the school experiment with a bar magnet and iron filings to map the magnetic field due to such a "dipole" magnet (Fig. 1a). The magnetic field lines are continuous loops that do not end at a source unlike in the case of a positive (or negative) electric charge where the electric field lines radiate from (or end at) the point of location of the isolated charge.
In fact, if one were to try and break a magnet into two, the two new ends would have opposite poles such that one ends up having two "dipole" magnets, both having north and south poles. This process can be carried on ad infinitum without the emergence of an isolated N or S pole, what would be a magnetic "monopole" (Fig. 1b). Even at the subatomic level, while an elementary particle like the electron carries an isolated electric charge, its intrinsic magnetism, which is due to its quantum mechanical property called spin, is that of a dipole. All known matter—every atom on the periodic table and every elementary particle—has zero magnetic monopole charge. So magnets, and the ordinary phenomenon of magnetism, are not due to magnetic monopoles; they arise from the interplay of electric charges, electric currents and the quantum mechanical intrinsic dipole magnetism of elementary particles.
Quantisation of electric charge
Because of the empirical fact that isolated magnetic charges, or magnetic monopoles, are not seen, James Clerk Maxwell did not introduce a term for magnetic charge (or source) analogous to the electric charge term in his equations of electromagnetism. The magnetic and electric parts of Maxwell's equations are thus not strictly symmetric. If magnetic monopoles existed and if one were to introduce a corresponding magnetic source term in Maxwell's equations, the two parts would be entirely symmetric and the duality between electricity and magnetism would be complete (Figs 2a and b).
In 1894, Pierre Curie was the first to discuss the possibility of the existence magnetic monopoles. But scientists only began to take the idea seriously after Paul Dirac, one of the architects of modern quantum theory, provided a physical basis to argue why magnetic monopoles can exist. One knows that charges can have only discrete values: they exist only as integral multiples of a unit e , which has a value of about 1.602 × 10−19 coulombs and is equal to the charge of a proton or an electron (a proton has the charge +1 e and an electron −1 e ). But there is no understanding of this quantisation of electric charge from basic principles.
Using fundamental quantum principles, Dirac posited that the existence of a magnetic monopole provided a natural explanation for this. He showed that quantum mechanics constrained the values of the smallest electric and magnetic charges in the universe by requiring that they satisfy the condition eg = h/2 , where g is the smallest magnetic charge and h is a constant known as Planck's constant. Later, in 1936, the Indian physicist Meghnad Saha provided a simpler and more elegant derivation of this result.
Quantisation of electric charge can then be understood as arising from the possible existence of a magnetic monopole of strength g somewhere in the universe. "Quantum mechanics does not preclude the existence of magnetic monopoles," Dirac concluded in his landmark 1931 paper and added that he "would be surprised if Nature had made no use of it". So, is the observed quantisation of electric charge evidence for the existence of magnetic monopoles? Not quite, the Dirac quantisation condition does not imply that monopoles must exist but that they can .
However, an important consequence of the quantisation condition is that, given the known value of e , the strength of the unit of magnetic charge turns out to be very high. The magnetic force between two monopoles with unit magnetic charge g would be 4,700 times the force between two electrons. Such a large value for g also makes it difficult to calculate reliably the production rate of magnetic monopoles in elementary particle interactions, particularly in high-energy particle accelerators, within the highly successful Standard Model of particle physics. Reliable estimates are necessary to even design a suitable monopole search experiment let alone to interpret the results.
However, theories beyond the Standard Model, such as grand unified theories (GUTs), which unify electromagnetism, nuclear forces (both weak and strong) and gravity in one framework, predict the existence of magnetic monopole–like entities. It is well recognised that despite its current enormous success, the Standard Model is an incomplete theory because it does not accommodate gravity in its framework and does not account for dark matter, cosmological inflation and the preponderance of matter over antimatter in the universe. Magnetic monopoles arise as an inevitable mathematical consequence of grand unification (of forces of nature) in emergent formalisms such as GUTs and superstring theory.
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