Somewhere out in the cosmos, there's the physics equivalent of a unicorn. Catching even a glimpse of this oddity that looks like the isolated tip of a magnet would be like a beacon in the night, pointing the way to grand, unifying theories of absolutely everything.
Of course, physicists might simply be looking for them in all the wrong places. A new analysis by an international team of researchers has whittled down places to look by modeling magnetic monopole creation in the chaos of collisions high up in the atmosphere.
Their work uses the results of highly sensitive experiments already seeking signs of magnetic monopoles in the collisions of particles in powerful accelerators, presuming they also would have detected the same clues raining down from collisions above.
By modeling the production of magnetic monopoles in the debris of atoms blasted apart by high-speed cosmic rays, the team could confidently put some hard limits on the amount of energy it would take to make one.
It's not exactly the thrilling announcement we'd love to make on the particle's existence, but this is how science works. And frankly, its discovery would be well worth the wait.
If magnetic monopoles are unicorns, electric charges are horses. They're hard-working, easy to find, and nobody would argue they don't exist.
In deducing the equations for electromagnetism in the 19th century, Scottish mathematician James Clerk Maxwell modeled the movement of the electron's negative charge. From this, we get electric currents and the push and pull of a magnetic field.
Thing is, we can also swap out features of this equation and use the magnetic equivalent of a negative charge. A magnetic monopole. Intriguingly, these same equations now reveal how moving magnetic fields induce electric currents.
Physics is built on the back of symmetries like this, though on its own it could just be a shadow cast by the mathematics, doing little to prove a magnetic monopole actually exists.
It wasn't until the dawn of quantum physics that the theorist Paul Dirac reconsidered this symmetry in a new light, reasoning through more complex means that if a single magnetic monopole existed in the Universe, electric charges would have to come in discrete sizes.
The fact charges are indeed 'quantized' again isn't proof of anything. But little by little, as quantum field theories have grown, nothing has yet ruled out the existence of a magnetic monopole.
In fact, in the 1970s, as physicists began to realize quantum fields became indistinguishable at high enough energies, it became clear that a kind of wave would arise that for all purposes would behave just like a magnetic monopole.
Half a century later, the hunt for this unicorn of physics persists in the hope that maybe – if we catch one – we'll also have clues on the way physics might emerge from one unified, high-energy theory.
For the most part, in spite of a lot of looking, this search has come up empty-handed. A single blip in a Stanford experiment briefly stirred debate, but without much replication, it's since been seen as 'just one of those things' that happens in science.
Most searches focused on sifting out magnetic monopoles that would have been created in the furnaces of the early Universe. But models that explain their creation are frustratingly light on detail, meaning we can only hazard a guess on what they'd look like.
Particle accelerators could punch one out of the darkness, but only if magnetic monopoles can be created from relatively low energies. And even then, only when the accelerator is in operation.
Cosmic rays, on the other hand, are always sparking showers of fat, exotic particles down onto the surface, many at energies colliders can't yet reach.
Should one of these happen to spit out a suitably plump magnetic monopole in the future, we'll need to be on the lookout. According to this study's results, experiments like the IceCube Neutrino Observatory at the South Pole might be a sound bet for spotting them, so long as they have enough mass.
There are only so many corners of physics a massive unicorn can hide in, after all.
This research was published in Physical Review Letters.