A lot of times, when you’re pushing science to the absolute limits, you want an experiment to have one of two results. First, you want to see a positive result-something that confirms your models and suspicions and says: “Yes, that thing we thought we understood? We do, in fact, understand it.” Or second, you want to see a result so confusing that it confirms we don’t really know anything at all.
In 2021, a team from Fermilab ran an experiment called Muon g-2 (pronounced “muon g minus two”). Just recently, they announced that they had managed to replicate their results with twice the precision. So, what was the result? Oh, nothing much … just an implication that we may be about to uncover a fifth, previously unknown fundamental force that flies in the face of the entire Standard Model of particle physics.
If that sounds big, you’re right-it is. If it sounds confusing, that’s fair. Let’s back up a bit.
We’ll start at the beginning with the Standard Model, the blueprint for pretty much everything. As far as we know, everything in existence is comprised of fundamental particles and controlled by fundamental forces. There are four known fundamental forces of nature-electromagnetism, the strong nuclear force, the weak nuclear force, and gravity. The Standard Model is the description of the way that all of those fundamental particles and forces (except for gravity, which is in its own bucket) interact. It’s been able to predict and explain a huge amount of scientific phenomena.
But the Standard Model is also incomplete. Especially as we’ve started to explore more and more complex physics-such as dark matter, what goes on inside black holes, and how exactly quantum mechanics works-researchers have been poking holes in the model. That doesn’t mean it bad, wrong, or useless. It just can’t describe everything. And as we’ve been able to perform more and more complex experiments, we’ve been able to see more and more phenomena first-hand that can’t be explained by the Standard Model.
Which brings us to the Muon g-2 experiment. Many years in the making, the experiment aimed to achieve very, very good measurements of the behavior of something called the magnetic moment of a muon. A magnetic moment is like a very small magnet inside a particle that controls how it moves through a magnetic field. And a muon is basically like a really big electron.
Researchers shot a bunch of muons through a superconducting magnetic storage ring, where they spun around and around about 1,000 times at nearly the speed of light. The ring was filled with detectors that allowed researchers to make very precise measurements of the muons’ magnetic moments, and once they had them, they were able to compare those measurements to the predictions of the Standard Model.
And when they compared them, they found that the predictions didn’t match their experimental results. While this experiment has yet to tell researchers why the two numbers are different, the fact that prediction and reality produce two separate results means one thing-we’re missing something. And if we’re missing something in an experiment based on predictions made using the Standard Model, that means the Standard Model is missing something.
“If the measurements don’t line up with the prediction, that could be a sign that there is some unknown particle appearing in the loops-which could, for example, be the carrier of a fifth force,” particle physicist Jon Butterworth told The Guardian.
More data on this experiment is still to come, as it still has three more years to run. And the team expects the next round of analysis to provide a reading even twice as precise as this one, much like this new result is twice as precise as the first one.
But from what they have already, hypotheses are starting to arise. The team believes that this may be a never-before-detected fundamental force-one that would bring our tally from four to five and open up an entirely new understanding of the fundamental physics of our universe.
If they’re right, that wouldn’t just be a new discovery. It would be entirely new science.