The mysterious muon starts a revolution
Physics advances because scientists come to know what they don’t know
Muons are very common cousins to electrons. Recent tests by the Muon g-2 Collaboration have produced data on muons that differs from the Standard Model of Particle Physics, prompting a call for a new way to explain particle physics. Above, an image of a muon.
By STEPHEN SEKULA

One of my favorite quotes about science comes not from a practicing scientist but from a comedian, Dara O’Briain. He brilliantly summarized the whole point of science and how it makes progress: “Science knows it doesn’t know everything, otherwise it would just stop.”

Physicists, those scientists who study energy, matter, space and time, have just discovered a vast gap in their knowledge thanks to the unpredictable behavior of the tiniest-of-tiny particles, the muon. And if history is repeated, the new generation of physicists will discover pathbreaking ideas that could lead to revolutionary technology.

Like all science, physics advances because scientists come to know what they don’t know. A great example of this happened at the end of the 19th century. At that time, the physical science community was riding high on the many successes of Isaac Newton’s laws of motion, forces and gravity, as well as the newly discovered laws of heat and electricity and magnetism. Many practicing physical scientists were convinced that the end of physics was in sight, that all questions would soon be answered, all of nature completely understood even down to the atoms that were assumed to make up everything in the cosmos.

There were just a few loose ends. Light and atoms didn’t quite behave as expected. Dedicated experiments revealed that light didn’t follow some of the precepts of Newton’s laws of motion. Other experiments showed that atoms didn’t emit all possible forms of light as was expected from the laws of motion, electricity and magnetism.

It was these small problems, seemingly esoteric misunderstandings about tiny things, that turned out to be explicable only by big breakthroughs, discoveries that changed our entire view of the universe. Those breakthroughs have famous names, like Albert Einstein’s relativity (a better description of motion and gravity than Isaac Newton could have conceived) and quantum physics (the recognition that at its smallest scales the universe prefers to be described as wavelike and probabilistic).

Only by inventing and applying whole new branches of mathematics to complement an arsenal of novel investigative methods could the observed behavior of light and atoms be reconciled with our understanding of physical law. These discoveries, in turn, initiated a century-long technological revolution that birthed modern electronics, telecommunications and non-invasive medicine. Societies are now largely built on the answers to questions like: Why do light and atoms behave as observed?

When, as a public school kid, I became interested in physics in the late 1980s and early 1990s, there was a sense again that physics was about to wrap everything up, just as it seemed it would in the 1880s. Upon being hired into the same professorship once held by Sir Isaac Newton at Cambridge University, the brilliant physicist Stephen Hawking even provocatively titled his inaugural lecture, “Is the End in Sight for Theoretical Physics?” I grew up in the shadow of that lecture and the century that preceded it.

The revolutions launched by light and atoms in the 20th century had culminated in the most precise and, seemingly, accurate description of nature ever discovered: the Standard Model of Particle Physics, or “Standard Model” for short. It predicted the behavior of the most fundamental building blocks of nature and described the forces that bind those building blocks together.

The particles contained in the Standard Model, discovered throughout the 20th century, are more fundamental even than atoms. The Standard Model even explains how some of them come together to forge every known atom in the universe. Atoms exist because the Standard Model is true. Every new experiment seemed to yield to the Standard Model’s predictive genius. There came again the sense that everything could once again be known, that physics would just stop.

But a true thing is not necessarily a complete thing. Since the 1990s, physicists have been disabused of their hubris by several discoveries about nature. The most recent example of one came on April 7 when the latest results from an incredibly difficult and detail-oriented scientific investigation were revealed: the Fermilab Muon g-2 Experiment.

What is a muon (it’s pronounced myoo-on )? Quite simply, it’s the less stable and much heavier cousin of the more familiar electron. Electrons are found in every atom in the cosmos, and most people are introduced to them during high school chemistry. Muons are a result of natural processes, making them also very common.

For example, at Southern Methodist University we house a small instrument, about the size of a gallon milk jug, that observes almost 1,500 muons every hour. They rain down on us all the time, a by-product of cosmic radiation smacking into our planet’s atmosphere.

The muon was first discovered somewhat accidentally during the investigations of light and atoms in the early 20th century. They are one of those more fundamental building blocks of nature I mentioned earlier, of which we now know 12 that have been cataloged.

Muons are interesting because their established properties make them potentially sensitive to previously undetected building blocks of nature, perhaps more than just those 12 we already know and that are described by the Standard Model.

Muons, like their cousin electrons, behave a lot like extremely tiny refrigerator magnets. Just like a fridge magnet, if you expose such particles to another, exterior magnetic field, they will experience a force. Their response to that force, summarized in a single number called g-2 (G minus two), can be elegantly and precisely predicted by the Standard Model.

Every prediction made by the Standard Model can, in principle, be turned into a test. That aspect is what makes it a great scientific theory. The Standard Model has been very good at generating predictions that pass almost every test. Note that I said almost.

Failing an experimental test is the moment when we concretely realize Dara O’Briain’s succinct point about scientists knowing that they don’t know everything. Sometimes the revealed knowledge gaps are big, and sometimes they are subtle or small.

Even small knowledge gaps can yield profound scientific moments. The gaps in the 1800s between then-established physical law and the actual behavior of light and atoms seemed small at the time, which was why physicists were so confident they would just be explained by what was already known. They could not.

As we have learned repeatedly in the history of science, sometimes a small gap is a profound marker. It can mean you have just discovered a signpost that will lead to a more reliable description of nature, far better than the one you already have. You need only the wisdom and the courage to follow the sign wherever it points, if first you can figure out where it is pointing. This is what preceded the scientific revolutions of the 20th century.

The Muon g-2 Experiment was first performed at the CERN Laboratory in Geneva in the late 1950s and early 1960s. The U.S. got into the game in the 1980s and the first unexpected results came from an incarnation of it 20 years ago at the Brookhaven National Laboratory in Upton, N.Y.

The experiment was recently relocated to the Fermi National Accelerator Laboratory (Fermilab, for short). A key component of the project, a complex and large magnet system, had to be moved intact from Upton to Batavia, Ill. This alone was a major feat of engineering and planning, and is one example of just how remarkable this entire experiment has been.

Since the relocation, an international scientific collaboration has been collecting new data, re-evaluating the experimental methods, and preparing to update our collective knowledge of those minuscule refrigerator magnet-like properties of the muon. What has been most tantalizing about this new phase of the experiment is its past: Previous measurements over the past 20 years seemed to reveal a slight mystery.

As one measured the muon’s magnetic properties more precisely, and in concert improved the mathematical predictions from the Standard Model to match the experiment, a gap opened, wider and wider, between what is observed and what is predicted. That tension has grown with time, but it has never been sufficient to convince the scientific community that it’s more than a statistical fluke. Scientists generally set high standards for evidence to avoid fooling themselves or others.

April 7, 2021, changed everything. The Muon g-2 Collaboration unveiled its latest analysis, combining fresh data from the newest incarnation of the experiment at Fermilab with the experiment’s previous findings. The past and current results agree even at the new and enhanced level of precision. As in the past and in parallel, the mathematical physics community has independently sharpened its predictions of the muon. The dust has settled and the results are definitive: These two ways of understanding the muon, the Standard Model and the Muon g-2 Collaboration, strongly disagree.

To give you a sense of just how tiny a disagreement has been discovered, it would be like someone asking my age, and me answering with a number that is off from the truth by just three seconds. Three seconds of incorrectness out of a human life seems trivial, but when you think about it, we humans are so good at keeping time (we can measure time with precision better than billionths of a second), three seconds is actually a large and easily measurable difference from the truth.

This is the case for the Fermilab Muon g-2 Experiment and the magnetic properties of the muon; the difference is the level of the ninth decimal place in the comparison, but that difference is easily measured with this kind of cutting-edge experiment. We can even quantify the chance that this is just a random statistical accident. As of now, there appears to be only a 1-in-40,000 chance the Standard Model description of the muon explains the experiment.

A signpost is now coming into focus. The muon seems to be trying to tell us something deep and fundamental about the universe.

First and foremost, it’s telling us: You don’t know me. Which screams for a new direction that can explain it.

What direction? The most compelling possibility is that there is a whole pantheon of new forces and building blocks of nature that are beginning to reveal themselves through their influence on the muon. Just as the magnetic properties of the muon are a consequence of the particles and forces described in the Standard Model, new particles and new forces would alter that behavior.

It is incumbent upon us now to explain this observation. We need independent ways to try to directly observe those particles and forces, either using ongoing experiments (like the Large Hadron Collider in Geneva) or by conceiving new methods and new ideas.

In the early 20th century, our misunderstanding of tiny things initiated new intellectual and technological revolutions. The mobile phone and the MRI medical scanner are two benefits facilitated by those revolutions, after a long chain of human ingenuity.

Now, the muon may have just yielded a huge clue to undiscovered forces and building blocks of nature. Perhaps it will be a signpost pointing a new generation of young physicists to revolutionary ideas and technology.

Stephen Sekula is chair of physics and an associate professor of experimental particle physics at Southern Methodist University in Dallas. He is co-author of the book, “Reality in the Shadows (or) What the Heck’s the Higgs?” He wrote this column for The Dallas Morning News.

Seiichi Yamamoto