In "The Big Bang Theory", Xie Er traveled to the magnetic north pole with his three friends to find magnetic monopoles for his string theory research. Xie Er also thought that he had proved the existence of magnetic monopoles, believing that he would win the Nobel Prize for it.
Image source: Stills from The Big Bang Theory
Although Shea's magnetic monopole turned out to be oolong, in the scientific community, there are indeed countless scientists who are pursuing this magical hypothetical particle.
Many physical theories predict the existence of this unique particle, but it still does not appear.
Magnetic monopole
Magnets can be found everywhere in our lives. Whether it's in your refrigerator, or your smartphone, or your credit card, all magnets have one thing in common, and they must have an antarctic plus an arctic.
Even if you cut a magnet in two, no matter how many times you repeat it, no matter how small it is cut, 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 there are no isolated magnetic charges.
In the last century, the development of quantum mechanics has brought a new chapter to the story. In 1931, Dirac predicted the existence of such a hypothetical elementary particle with only a single magnetic pole in the universe, which is what we now call a magnetic monopole.
All the magnets we've ever seen have an antarctic and an arctic. But a magnetic monopole has only one pole. | Image credit: Dominguez, Daniel/CERN
Many,and probably even most, physicists believe in the existence of magnetic monopoles. Physicists already know that electricity and magnetism are essentially forces, and if positive and negative charges can exist on their own, like electrons with only a negative charge, then a particle with only one magnetic pole should exist.
Theoretically, magnetic monopoles should have arisen in the early universe, and they were stable, so there should be a remnant flux of magnetic monopoles left over from the Big Bang that still permeates all space.
But the crux of the matter is that until today, they have never been detected.
Look in the snow and ice
In the search for magnetic monopoles, a huge neutrino detector located in the South Pole may be able to help us.
The Ice Cube Neutrino Observatory is a "strange" telescope. It consists of more than 5,000 light sensors buried in a cubic kilometer of ice at the south pole, and its original goal was to study a lightweight elementary particle, known as neutrino, that pervades the universe.
Antarctic Ice Cube Observatory. | Image credit: IceCube Neutrino
Neutrinos are elusive, and the almost only way to study them is by analyzing the products of neutrinos' rare interactions with matter, as if studying them based on the footprints animals leave on the snow.
If a neutrino collides with an atom in the ice around the ice cube, it can produce a charged particle called a μ. When a μ passes through the ice fast enough, it produces a cone-shaped blue radiation along its path, known as Chelenkov light. This light energy travels through the ice and triggers the Ice Cube's sensors on the way, telling scientists about the particle's energy and direction.
Interestingly, not only neutrinos, but also magnetic monopoles, if they do exist, they will also emit Chelenkov light when they pass through the Ice Cube Detector. But one thing that sets them apart is that they are unusually bright. Magnetic monopoles emit about 8,000 times more Chelenkov light than μ, and the emission of these lights is evenly distributed along their trajectories. This will result in a unique and distinct pattern of features in the ice cube.
Magnetic monopoles should leave a very noticeable mark in the Antarctic ice cube. Blue represents the Ice Cube detector, and the dotted line indicates the trajectory of the particles. The colored area around the trajectory represents the Chelenkov light pattern emitted by different types of particles. The color from red to green indicates the time when light is generated from first to last. | Image credit: Alexander Burgman, IceCube Collaboration
Because of this, Ice Cube scientists decided to look for signs of magnetic monopoles in the Ice Cube data over an 8-year period. They first screened out exceptionally bright events and looked for uniform emissions along the path. Since magnetic monopoles can fully penetrate the detector, they want to exclude those non-penetrating trajectories. Next, the researchers trained a machine learning event description tool (boosted decision tree) to distinguish between magnetic monopole events and μ events in the sample.
Recently, the team released the results of this search. Unfortunately, scientists have not been able to find any characteristics of the magnetic monopoles of the universe.
Look in the collider
In addition to capturing magnetic monopoles in the universe, scientists are also working to "create" such particles in the lab.
A new class of experiments in the LHC (Large Hadron Collider), which was credited with the discovery of the Higgs boson, created conditions that were more likely to produce magnetic monopoles, allowing researchers to refine the possible properties of magnetic monopoles.
LHC is most commonly pulverized by protons at extremely high energies. But in 2018, the LHC crushed a different particle, the heavy ion, specifically the lead nucleus. These particles contain hundreds of protons and neutrons, and this heavy nature means that they can only collide at lower energies than single-proton collisions.
However, if these lead nuclei accidentally collide or rub shoulders very close together, their interaction can produce something spectacular that would be one of the strongest known magnetic fields in the universe, a million times stronger than those found in neutron stars. These magnetic fields can only last for a very short time, but their presence provides a different mechanism for generating magnetic monopoles.
By colliding heavy ions, an attempt is made to create a magnetic monopole. | Image credit: James Pinfold, MoEDAL Collaboration
This theory follows a mechanism that produces an "electrical version" of a unipolar. The Schwenger mechanism proposed in the 1930s suggested that a strong electric field would interact with quantum fluctuations in a vacuum, producing positive and negative "electric monopoles" (electrons and positrons). Similarly, scientists speculate that a strong magnetic field should bring about magnetic monopoles from north and south.
In proton collisions, particles arise in a fierce collision, unlike in the Schwenger mechanism, particles are produced in a large number of small interactions. Researchers can theoretically describe these effects.
Importantly, this allows researchers to predict how many unipolars this mechanism will produce. This prediction depends on how multiple magnetic monopoles there are. Simply put, if they are too heavy, then the lead nuclei of the LHC do not have enough energy to make them.
In a 2018 experiment, the researchers found this situation, and they failed to produce magnetic monopoles.
For science, even negative outcomes have positive and positive implications. They still allow us to explore the nature of nature. These experiments also proved that if magnetic monopoles do exist, their weight must exceed a certain limit (75 GeV/c). The new results were published in a recent issue in the journal Nature.
The Mystery of The Disappearance
The mystery of the disappearance of the magnetic monopole remains unsolved, but many physicists still believe that this does not mean that the magnetic monopole does not exist. In our current "favorite" early model of the universe, a magnetic monopole is a kind of particle that "packs the ticket."
If they are ultimately proven to not really exist, we will have to rethink the most fundamental assumptions of our current model of the world.
#创作团队:
Written by: Takeko
Typography: Wenwen
#参考来源:
https://icecube.wisc.edu/news/research/2022/01/icecube-and-the-mystery-of-the-missing-magnetic-monopoles/
https://www.imperial.ac.uk/news/233494/strongest-magnetic-fields-universe-search-magnetic/
https://www.symmetrymagazine.org/article/the-hunt-for-the-truest-north
https://physicsworld.com/a/magnetic-monopoles-seen-in-the-lab/
#图片来源:
Cover image: Dominguez, Daniel/CERN
Top image: Dominguez, Daniel/CERN