The Mystery Of The Missing Magnetic Monopoles
By Judith E Braffman-Miller
The night sky above Earth blazes with the distant fierce fires of countless stars, and when we stare up at this magnificent spectacle of stellar fireworks, we cannot help but wonder how this show came to be. What scientists know now, or at least what they think they now know, is that the Universe was born about 13,800,000,000 years ago in the Big Bang, when it began as an exquisitely small Patch, much smaller than an elementary particle, and then–in the tiniest fraction of a second–expanded exponentially to reach macroscopic size. Something–we do not know what–made that tiny Patch experience this bizarre runaway inflation. Mysteries are enticing, singing a haunting sirens’ song to those who care to listen to its captivating melody. One of the best-kept secrets of the Cosmos involves a weird hypothetical elementary particle called a magnetic monopole. According to theory, these exotic magnetic monopoles should exist somewhere in the Universe–and yet not one solitary magnetic monopole has ever been found lurking anywhere in Spacetime.
If a bar magnet is cut in half, the outcome is a duo of smaller bar magnets–and each magnet sports its own south pole and north pole. But hypothetical magnetic monopoles–if they really are out there somewhere–travel to the beat of a different drummer. These exotic elementary particles that clearly “do their own thing” can have either a south pole, or a north pole, but not both.
Alas, for the past 70 years, physicists have hunted for these exotic particles that should have been born in abundance in the Big Bang, only to come up empty-handed. A monopole is defined as a magnetic version of a charged particle, such as a negatively charged electron, or a positively charged proton. Because in particle physics a monopole is an isolated magnet with only one magnetic pole (a north without a south pole, or vice versa), a magnetic monopole would have a net magnetic charge.
Electric monopoles exist as particles that sport either a positive or negative electric charge. Magnetism, of course, seems somewhat analogous to electricity. This is because there exists in nature a magnetic field that possesses a direction that is defined as running from north to south. However, the analogy breaks down in scientific attempts to detect the magnetic counterpart of the electric charge. Even though we can find electric monopoles in the form of charged particles, scientists have never been able to observe a magnetic monopole.
The only magnets that we know of are all dipoles–with north and south ends. When a bar magnet is split into two pieces, you do not get either a north or south pole–both separated pieces still possess both poles. The two new dipole magnets are simply identical, smaller versions of the original dipole magnet. No matter how many times the magnets are split into individual particles, all that will emerge are increasingly more numerous, smaller dipole progeny.
When we study the way magnetism works in the world that we are familiar with, what we see is consistent with Maxwell’s equations. Maxwell’s equations describe the unification of electric and magnetic field theory in respect to one of the four known fundamental forces of nature: the electromagnetic force. The other three known forces of nature are the strong nuclear force, weak nuclear force, and gravity.
Maxwell’s equations were first published by the Scottish mathematical physicist James Clerk Maxwell (1831-1879) between 1861 and 1862, and they demonstrate that we could swap electric for magnetic fields and not observe any appreciable difference. This means that the two are symmetrical. Even today Maxwell’s equations are still used on a practical level in telecommunications, engineering, and medical applications–to list only a few. However, one of these equations–Gauss’s law for magnetism–indicates that there are no magnetic monopoles in the Universe. Nevertheless, many physicists think that there is good reason to suspect that these elusive elementary particles are really there. This is because their existence in nature would explain why the electric charge is quantized–that is, why it always appears to come in integer multiples of the charge of an electron, rather than in a continuous array of values. Indeed, the French physicist Pierre Curie (1859-1906), as far back as 1894, pointed out–in contrast to Maxwell’s Gauss’s law–that magnetic monopoles could really exist in nature, despite the fact that none had been detected.
The quantum theory of magnetic charge began with a paper by the English theoretical physicist Paul A.M. Dirac (1902-1984) in 1931. In this paper, Dirac demonstrated that if any magnetic monopoles exist in the Cosmos, then all electric charge in the Cosmos must be quantized. Since Dirac’s paper, several systematic hunts for the elusive magnetic monopoles have been conducted. Alas, not one has found a single magnetic monopole anywhere in the Universe.
Historically, many researchers attributed the magnetism of lodestones to two different “magnetic fluids” (“effluvia”). These early scientists proposed that there existed a north-pole “fluid” at one end and a south-pole fluid at the other, which attracted and repelled each other in a way similar to positive and negative electric charges.
However, an improved understanding of electromagnetism in the 19th-century indicated that the magnetism of lodestones was better explained by Ampere’s circuital law, rather than “fluids”. Andre-Marie Ampere (1775-1836) was a French physicist and mathematician who was one of the founders of classical electromagnetism. Ampere’s circuital law relates the integrated magnetic field around a closed loop to the electric current flowing through the loop. However, it was actually James Clerk Maxwell (not Ampere) who derived it using hydrodynamics in his 1861 paper.
The magnetism that we see today can be attributed entirely to the movement of electric charges. Indeed, the equations describing electricity and magnetism are “mirror images” of one another. However, there is one important difference between the two. Protons and electrons carry electric charges, but there is no known particle that carries a magnetic charge. A magnetic monopole would be the first to carry a charge, and if one were ever detected, electricity and magnetism would finally be equal. If even one solitary magnetic monopole were found inhabiting the Universe, this important discovery would profoundly effect the foundations of physics.
Elusive Magnetic Monopoles And The Ancient Cosmos
In scientific cosmology, baryon acoustic oscillations (BAOs) are regular, periodic fluctuations in the density of the visible atomic matter of the Universe. Beginning from what started out as exquisitely tiny anisotropies caused by quantum fluctuations in the primeval Cosmos, the anisotropies ballooned in size–growing larger, and larger, and larger–as the Universe expanded with the passage of Time. The Arrow of Time points in the direction of the expansion of Space (Spacetime). In physics, a quantum is the minimum amount of any physical entity that is involved in an interaction.
The regions of greater density in the ancient Universe collapsed more quickly under the extremely powerful pull of their own gravity–eventually resulting in the foam-like, large scale structure of the Universe called the Cosmic Web. The primordial Cosmos itself was composed of a searing-hot, extremely dense plasma that was made up of electrons and baryons (protons and neutrons). Packets of light (photons) bounced around brightly in the very ancient Cosmos. This is because they were trapped–essentially unable to move freely for any great distance before interacting with the plasma that kept them imprisoned. During this era, the opaque Universe glared like the surface of a star similar to our Sun.
As the Universe expanded, the plasma cooled off considerably to reach a temperature lower than 3000 Kelvin. This cooler temperature was of a sufficiently low energy to allow the photons and electrons in the ancient plasma to mix themselves up together and form atoms of neutral hydrogen. This era of recombination occurred when the Universe was only 379,000 years old. The photons interacted to a lesser degree with the neutral hydrogen. Because of this, during the recombination, the Universe became transparent to photons. These packets of liberated light were finally free, and they have been shining their way through Spacetime ever since. The mean free path of the dancing photons essentially grew to become the size of the entire Universe. The cosmic microwave background (CMB) radiation is the lingering light that was sent forth following the era of recombination–it is the relic radiation of the Big Bang itself, that has been blown up to the immense size of the expanding Universe.
The physics of the Cosmos, during that very ancient era of exponential expansion (inflation), is described by particle theory. Many of these theories predict the formation of topographical defects. These defects resulted from phase transitions that occur in particle models. Because the temperature of the Universe cools as the expansion continues, these phase transitions are natural consequences of symmetry breakings that occur in particle models.
There are several types of defects:
Magnetic monopoles are considered to be point defects, where the field points radially away from the defect, which shows a characteristic mass. These defects also show a magnetic field configuration at infinity that makes them analogous to that of the magnetic monopoles first hypothesized by James Clerk Maxwell and others.
Out of all of the proposed defects, monopoles are the most prevalent in particle theories. Alas, this presents a disturbing problem for hot Big Bang models of the birth of the Universe. This is because calculations of the number of monopoles that would be churned out in the first seconds of the Universe’s existence indicate that they should be the dominant form of matter. This is, of course, contrary to the fact that not one single monopole has ever been found anywhere in the Cosmos–either directly or indirectly. These monopoles would effect the curvature of the Universe. Therefore, magnetic monopoles are the undiscovered (so far) relics that are an anomalous component of hot Big Bang theory.
Magnetic Monopoles Gone Missing
The empty-handed hunt for hypothetical magnetic monopoles has been a frustrating endeavor. More recent work, conducted at the Large Hadron Collider (LHC) at the particle physics lab CERN in Geneva, Switzerland, has inspired new efforts among members of the particle physics community. It is possible that magnetic monopoles are churned out when protons crash into one another at record-high energies of 13 trillion electron volts.
The most recent chase, conducted by particle physicist Dr. James Pinfold of the University of Alberta in Edmonton, Canada, and his team, using the Monopole and Exotics Detector (MoEDAL) at the LHC failed to find its elusive quarry. The good news is that this most recent hunt has set some of the tightest constraints so far on how readily the hypothetical, troublesome particles may dance with matter. The team’s findings were reported on December 28, 2017 at arXiv.org.
Magnetic monopoles may also haunt strange regions of the Universe where temperatures are extraordinarily high and magnetic fields are particularly powerful. Under such extreme conditions, duos of monopoles may be born spontaneously. Such extreme environments exist around a special class of stellar relic known as a magnetar, as well as in the aftermath of collisions of heavy atomic nuclei in particle accelerators.
If magnetic monopoles sport small masses, the elusive particles would suck the strength out of a magnetar’s magnetic fields. This indicates that the possible particles must be more massive than approximately 0.3 billion electron volts–which amounts to about a third of the mass of a proton, a second team of particle physicists from University College London (UCL) reported in the December 15, 2017 issue of Physical Review Letters.
Part of the problem that the UCL team identified was that if magnetic monopoles were churned out within particle colliders, there was a very good chance they would be strongly stuck to one another. Therefore, what was needed was yet another method to narrow down the secretive nature of the properties these potential particles might possess–and then compare those with MoEDAL’s findings.
In order to accomplish this, the UCL physicists took a slightly different approach from the scientists at the LHC. The UCL team pondered how magnetic monopoles would appear within searing-hot, intense magnetic fields similar to those within a magnetar. Magnetars are a special class of neutron star. Neutron stars are the stellar remnants of massive progenitor stars that went supernova after they had managed to burn their necessary supply of nuclear-fusing fuel–and, as a result, had collapsed, blasting themselves to smithereens, leaving only a dense neutron star behind to tell the tragic tale of how once there was a star that is a star no more. Neutron stars are extremely dense city-sized stellar corpses. A teaspoon full of neutron star stuff weighs as much as a fleet of school buses.
If the mass of the magnetar was small enough, their magnetic charge would influence the star’s magnetic field. But, of course, even the strength of the monopole’s charge at this point is hypothetical. However, based on a handful of reasonable assumptions the scientists were able to calculate what they would expect if the hypothetical particle’s mass is more than approximately one-third that of a proton.
No matter how physicists look at this puzzle, they will need to consider two possibilities; either the magnetic monopole does not exist, and the fractured symmetry between electricity and magnetism is a fundamental part of the way nature operates; or the magnetic monopole is really, really heavy.
It is possible that particle physicists must wait for bigger colliders to be developed. It’s also possible that magnetic monopoles are so big that only something as profoundly monumental as the Big Bang beginning of the Universe could churn them out–leaving bewitched, bothered, and bewildered physicists hunting for these relics that were produced at the birth of Spacetime.
Even though this most recent hunt for the still-hypothetical magnetic monopole has come up empty-handed–just like previous hunts–that still doesn’t rule out the possibility that these hypothetical particles do exist somewhere in the Universe.
Neverless, not everyone thinks these elusive particles exist in nature. In 2017, physicists argued that the symmetry between electricity and magnetism is broken at a deep and fundamental level. Nevertheless, for those physicists who see a cup that is half full, rather than half empty, the search goes on.
“A lot of people think they should exist,” Dr. Pinfold told the press on January 12, 2018.
Dr. Pinfold and his colleagues went through a large pile of data obtained from the LHC’s MoEDAL–and they came up with nothing, nothing, nothing at all. However, the scientists had six times the necessary information available in earlier efforts, that also involved MoEDAL. Furthermore, the team took into account magnetic monopoles with a different kind of spin than those hypothesized in earlier analyses. This shows just how much ground has been covered in this baffling search.
Even though the LHC team has failed to find any trace of a magnetic monopole, this may not be such a bad thing. This is because their study narrows down the places where physicists must look in order to find these elusive particles. Blasting protons into one another at immense speeds is one method physicists can use in order to create magnetic monopoles.
Many uncertainties still confront particle physicists in their quest to find the holy grail of even one lone magnetic monopole hiding somewhere in the Universe. But, even with all of these uncertainties, one certainty remains–the quest continues.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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