The Mystery of the Missing Magnetic Monopole
In The Big Bang Theory, for his string theory research, Sheldon and his three friends went to the magnetic north pole in search of magnetic monopoles. Sheldon once thought he had proven the existence of magnetic monopoles and believed he would win a Nobel Prize for it.
Image credit: Stills from The Big Bang Theory.
Although Sheldon's magnetic monopole turned out to be an oolong, there are indeed countless scientists in the scientific community who are pursuing this magical hypothetical particle. Several physical theories have predicted the existence of this unique particle, but it has yet to emerge.
Magnets are everywhere in our lives. Whether it's on your refrigerator, or in your smartphone, or in your credit card, all magnets have one thing in common, they must have a south pole and a north pole.
Even if you cut a magnet into two, no matter how many times you repeat it, and how small you cut it, all you get is a new magnet with its own north and south poles. This phenomenon is reflected in Maxwell's equations, which show that there are isolated positive and negative charges in the universe, but no isolated magnetic charges.
The development of quantum mechanics over the last century brought a new chapter to the story. In 1931, Dirac predicted that there should be such a hypothetical elementary particle with only a single magnetic pole in the universe, which is what we often call a magnetic monopole. All magnets we've seen have a south pole and a north pole. But magnetic monopoles have only one pole.
Many (probably most) physicists believe in the existence of magnetic monopoles. Physicists already know that electricity and magnetism are essentially one force, and if positive and negative charges can exist alone, like electrons with only negative charges, then particles with only one magnetic pole should exist.
In theory, magnetic monopoles should have arisen in the early universe, and they are stable, so there should be a relic flux of magnetic monopoles left over from the Big Bang, still permeating all space. But the crux of the matter is that, to this day, they have never been detected.
Find in the ice and snow
In the hunt for magnetic monopoles, a giant neutrino detector at the South Pole may be able to help us.
The IceCube Neutrino Observatory is a "weird" telescope. Consisting of more than 5,000 light sensors buried in a cubic kilometer of ice in Antarctica, it was originally aimed at studying neutrinos, a lightweight fundamental particle that pervades the universe.
Image: IceCube Observatory, Antarctica. | Image credit: IceCube Neutrino
Neutrinos are elusive, and almost the only way to study them is by analyzing the products of their rare interactions with matter, which is like studying animals based on their footprints in the snow.
If a neutrino collided with an atom in the ice layer surrounding IceCube, it could produce a charged particle called a muon. When a muon travels fast enough through the ice, it produces a cone of blue radiation, known as Cherenkov light, along its path. This light energy travels through the ice and on the way triggers IceCube's sensors, which tell scientists the energy and direction of the particle.
Interestingly, not only neutrinos, but magnetic monopoles, if they do exist, would also emit Cherenkov light as they pass through the IceCube detector at nearly the speed of light. But there's one thing that sets them apart, they're incredibly bright. Magnetic monopoles emit about 8,000 times more Cherenkov light than muons, and the emission of these light is distributed uniformly along their trajectory. This results in a unique and distinct pattern of features in IceCube.
Magnetic monopoles, penetrating muons, shower particles, dark muons
Magnetic monopoles should leave a very visible trace in the South Pole IceCube. Blue represents the IceCube detector, and the dotted line represents the particle's trajectory. The colored areas around the trajectories represent the Cherenkov light patterns emitted by different types of particles. Colors from red to green indicate when the light is generated from first to last. | Image credit: Alexander Burgman, IceCube Collaboration
That's why IceCube scientists decided to look for signs of magnetic monopoles in eight years of IceCube data. They first filtered out exceptionally bright events and looked for uniform emission along the path. Since magnetic monopoles can completely penetrate the detector, they have to exclude those non-penetrating trajectories. Next, the researchers trained a machine-learning event description tool (boosted decision tree) to distinguish magnetic monopole events from muon events in the samples.
Recently, the team released the results of this search. Unfortunately, scientists have not been able to find any signature of cosmic magnetic monopoles.
Look in the collider
In addition to capturing magnetic monopoles in the universe, scientists are also working to "create" such particles in the laboratory.
A new class of experiments at the LHC (Large Hadron Collider), credited with the discovery of the Higgs boson, has created conditions that are more likely to produce magnetic monopoles, allowing researchers to refine them possible properties of the child.
The LHC most commonly smashes protons at extremely high energies. But in 2018, the LHC shattered a different particle, a heavy ion, specifically a lead nucleus. These particles contain hundreds of protons and neutrons, and this heavy nature means they can only collide at lower energies than single-proton collisions.
However, if these lead nuclei accidentally slanted or passed very close, their interaction could produce something spectacular, one of the strongest known magnetic fields in the universe, stronger than those found in neutron stars The magnetic field is a million times stronger. These magnetic fields only last for a very short time, but their presence provides a different mechanism for creating magnetic monopoles.
Attempts to create magnetic monopoles by colliding heavy ions. | Image credit: James Pinfold, MoEDAL Collaboration
The theory follows a mechanism that produces an "electrical version" of a monopole. The Schwinger mechanism, proposed in the 1930s, believed that a strong electric field would interact with quantum fluctuations in a vacuum, creating positive and negative "electric monopoles" (electrons and positrons). Similarly, scientists speculate that a strong magnetic field should bring north and south magnetic monopoles.
Unlike in proton collisions, where particles arise from one violent collision, in the Schwinger mechanism, particles arise from a large number of small interactions. Researchers can theoretically describe these effects.
Importantly, this allowed the researchers to predict how many monopoles the mechanism would produce. This prediction depends on how heavy the magnetic monopole is. Simply put, if they're too heavy, the lead core of the LHC doesn't have enough energy to make them.
The researchers found this to be the case in a 2018 experiment, when they failed to create a magnetic monopole.
For science, even negative results have positive and positive implications. They still allow us to explore the essence of nature. These experiments also demonstrate that if magnetic monopoles do exist, their weight must exceed a certain limit (75 GeV/c²). The new results were published recently in the journal Nature.
The mystery of the monopole's disappearance remains unsolved, but many physicists still believe that doesn't mean it doesn't exist. In our current "favorite" model of the early universe, a magnetic monopole is a "bonus" particle.
If it turns out that they really don't exist, we'll have to rethink the most basic assumptions of our current model of the world.