A potential fifth fundamental force

For over 50 years, physicists have generally agreed that the interactions between elementary particles are governed by four fundamental forces: the strong force, weak force, electromagnetic force and gravity. The former three forces are related through the Standard Model of particle physics, while gravity derives its explanation from Einstein’s general relativity.

However, similar to the fashion industry, sometimes models can give inaccurate conclusions of what our perceived reality should look like. And like any Victoria’s Secret catalog, we often don’t get the full picture. Experimentalists have pointed out several phenomena that cannot be explained by our current understanding of physics, such as the nature of dark matter and dark energy, as well as the subatomic darties that may be going on inside black holes.

As a result, since as early as the 1980s, some scientists have postulated the existence of a fifth fundamental force to describe such “anomalous” behavior. This idea has been the subject of significant debate among the physics community ever since, with disparate studies proving and disproving the concept in an ever-perplexing will-they-won’t-they battle featuring theory, experiment and a lot of lasers. Nevertheless, recent findings from Fermilab have shed light (or as they say in the particle physics community, shed photons) on the potential for a fifth force after all.

The evidence of a fifth force comes from the fact that the standard model can make precise predictions about particle behavior, and if we run an experiment such that the particles behave in a way that doesn’t align with our predictions, there must be some unknown additional force at play. This notion underlies the workflow of the Muon g-2 experiment (pronounced “gee minus two") — a collaboration of nearly 200 scientists from 33 institutions, based at Fermilab in Batavia, Illinois. The experiment analyzes the behavior of muons, the heavier cousin of the electron by a factor of over 200. Muons are a temporary byproduct from collisions between positively-charged protons colliding with the nuclei of air molecules, part of the ever fluctuating “quantum foam” of appearing and disappearing particles in the boundless Diet Coke that is our universe. 

Researchers at Fermilab shot a bunch of muons through a 50-foot diameter electromagnetic ring, where the muons sped around the ring about 1,000 times near the speed of light, like the coolest NASCAR race whose ionizing radiation would probably mutate your DNA into a sheep. The ring was filled with detectors that can make precise measurements of the muon’s behavior. 

When elementary particles are exposed to a magnetic field, they act like mini-magnets themselves and exhibit an intrinsic property called spin, where they act as if they were a spinning object but aren’t actually spinning. The muon's internal magnet wants to rotate itself to align along the magnetic field axis like a compass aligning with the Earth's magnetic field. However, the muon’s spin prevents this from happening. Instead, the muon starts to act like a spinning top, where it spins around a wide axis, a type of rotation called precession which is notoriously one of the most difficult things to calculate in any mechanics course. Physicists can precisely measure this “wobbling” and eventually calculate the gyromagnetic ratio of a muon, denoted by g. All of this math theoretically predicts g for a muon to be around two, differing by a factor of 0.1, hence the name of the experiment.

But this is not what happened. Fermilab reported as of last month that they found the g for a muon to be about 0.002, five standard deviations smaller than expected. This means that there’s only a 1-in-3.5 million chance of this being a statistical fluke! Such a discrepancy has provided staggering evidence that there might be some additional force at play. However, the researchers did note a high uncertainty in the theoretical calculation of g due to contributions from the four known fundamental forces at play in the experimental environment, slightly dampening the novelty of the result. Nevertheless, the staggering nature of these findings are enough to get theorists sharpening their pencils to figure out what’s going on, and experimentalists to sharpen…whatever tool you need to build a particle detector.

When we say that something is “defying the laws of physics,” what we’re really saying is that something is defying our current understanding of how physics should work (or that we just know someone who’s really good at calisthenics).  Although particle physics seeks to describe the most fundamental aspects of the universe, the fundamentals of physics themselves are always subject to change in response to new information. This makes studying physics exciting and full of the feeling that you’re always on some cutting edge, but also frustrating in that you’re constantly bombarded with ideas that fundamentally conflict with one other, like with relativity and quantum mechanics. Relativity produces conclusions that are definite, whereas quantum mechanics produces conclusions that are probabilistic. Relativity sees time as malleable, whereas quantum mechanics sees time as fixed. Relativity is continuous or “smooth,” whereas quantum mechanics is discrete or “chunky.”