• Rose Smith

How a Tiny Particle Could Change Physics Forever


Fermilab's Accelerator, where the Muon g-2 Experiments were performed

Photo Source: Wikimedia Commons


One of the most spectacular (and maybe infuriating, depending on how you think about it) things about science is that nothing is ever truly settled. The idea of the laws of physics is derived from the fact that we have shown enough times that the laws of physics hold upon experimentation, but science never commits to something as being unable to be disproven. With that said, there comes times in scientific communities where previously held notions of how the world works are challenged, and a debate over the nature of the observable world ensues. The world of physics has recently had this event all thanks to a little particle called the muon.


The muon, sometimes known as a “fat electron,” is a small, subatomic particle, a substance smaller than an atom (The New York Times). Discovered in 1936, American physicists Carl D. Anderson and Seth Neddermeyer discovered muons in cosmic ray particle “showers” (Britannica). Muons behave similarly to their more popular cousins, the electrons, but they are heavier, have magnetic properties, and decay extremely quickly into other, smaller particles. One thing to remember about muons, or any other particle, is that the theory of quantum mechanics dictates that there is no such thing as empty space, and every particle is surrounded by other particles. The particles that surround muons influence how muons express themselves. One of the muon’s properties is called its magnetic moment, illustrated in scientific equations as the letter ‘g.’


The set of equations that are meant to describe the relationships and properties of modern physics is referred to as the standard model. Around since the 1970s, the standard model serves to try to explain the phenomena that physicists find in their experiments, and for a long time, it has done an extremely good job doing that (CERN). There have been some times where the standard model has been poked at (Ars Technica). For instance, physicists have yet to find a way to factor in quantum gravity at the subatomic level, but for the most part, the standard model has been the go-to theory to describe the world around us.

The newest experiment done by a team of 190 researchers at Fermilab in Batavia, Illinois has potentially punched a hole in the standard model’s framework (Science). The experiment is actually building upon the work that was done at the Brookhaven National Laboratory in Upton, New York from 1997 to 2001. The experiments at Fermilab and Brookhaven have been trying to measure g factor of the muons. This is done with a giant contraption referred to as the Alternate Gradient Synchrotron. The machine sends a beam of muons through a 50-foot-wide ring with extremely powerful magnets (The New York Times). The scientists then study the muons’ magnetic moments. What turned the physics community on its head was that the measured magnetic moments were different than what the standard model’s equations predicted. This could imply then that there’s an issue with the standard model and that there is something else, perhaps a new kind of particle, that is having an effect on the magnetic moment of the muon.


Now, before we start burning the old standard model down and beginning anew, there are a couple caveats we need to consider. First of all, the experiments cannot be billed fully as a major discovery because its results are not deemed as fully statistically significant yet. Given the experiment results at Brookhaven and recently at Fermilab, the measurements are at four sigmas (Ars Tecnica). The amount of sigmas try to illustrate how much a result is significant and not due to chance. The more sigmas, the more likely the measurement is correct. The golden number of sigmas for an official major discovery is five; this would mean that there is only a 1 in 3.5 million chance that the measurement being recorded is happening by chance. Researchers are hoping that with more data, the results can be pushed up to five sigmas. There are also some detractors that believe that the results found in the experiments are already in line with the current physics framework (Nature). In a paper released in Nature, researchers in Pennsylvania State University argued that the standard model's physics equations actually explain the magnetic moments in muons perfectly well. The researchers recalculated one of the factors in the standard model equation with a new method, and they found the predicted muon g factor to more closely resemble the experimental results.

Science is a fascinating world. Plenty of people devote their lives trying to discover how the universe works and why it is what it is. Sometimes, we find a theory that describes our world, only to discover that our model was wrong the whole time. Right now, the experiments being done at Fermilab have the potential to upend physics as we know it. Because the muons’ magnetic moments were different from the expected value, some physicists are wondering whether there are perhaps more particles that we don’t know about yet. On the other side, the experiment is still collecting more data, and some scientists are claiming that the existing models are correctly guessing the muons’ magnetic moments. Either way, it’s exciting to see live scientific debate in action. This whole debate reminds us that we have so much to learn about the universe and inspires us to stay curious about the world around us.



Rose Smith is the blog editor of Twenty-two Twenty-eight. When she isn’t writing about the world around her, she is often found listening to music, watching movies, and going on walks with her dogs.


You can find her on Instagram here and on Twitter here.