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Humans humbled by a wobbling particle

We know less about the universe than we thought.

A particle storage ring at Fermilab in Batavia, Ill., where subatomic particles called muons appear to be violating the laws of physics.
A particle storage ring at Fermilab in Batavia, Ill., where subatomic particles called muons appear to be violating the laws of physics.FERMILAB/U.S. DEPT OF ENERGY/NYT

The world of physics was abuzz this week with the announcement that a subatomic particle called a muon, observed anew in an incredibly sophisticated experiment, did not behave as expected.

It was remarkable to get such strong evidence that physicists have not been accurately characterizing things that are happening in the physical world. But perhaps more stunning is that this also means there could be many more things in existence that we don’t know about at all. There might be dozens of undiscovered subatomic particles in the universe.

Muons — which are similar to electrons but heavier and shorter-lived — arise in nature when cosmic rays strike Earth’s atmosphere. Scientists also can produce them in particle accelerators such as Fermilab, in Illinois. Based on such experiments, an international team of 200 particle physicists spread among seven countries said Wednesday that muons do not behave as predicted in the presence of a magnetic field. They wobble, much like a spinning top or gyroscope, in a manner inconsistent with the so-called Standard Model, the theory of physics that has been used to describe the subatomic world for 50 years. Some unknown particles and forces could be giving the muons an extra push.

“Something is missing in the Standard Model,” says William Morse, an experimental particle physicist at Brookhaven National Laboratory who was part of the team that first saw muons misbehave 20 years ago — which called out for the confirmation that finally appeared to come this week. “We wouldn’t be here if the Standard Model were right and complete.”


He was hinting at the fact that, while we see observational evidence of the Big Bang that created the universe, the Standard Model cannot account for the presence of stars and galaxies. Another discrepancy is the presence of dark matter, which our telescopes cannot see but which we know is there from its gravitational footprint.


So if the Standard Model is incomplete, what could be going on?

One explanation for the wobbling of muons — and other unexplained aspects of the universe — could come from a theory called supersymmetry, which basically holds that every known subatomic particle has a hidden, heavier partner. But since none of these particles has yet been detected, supersymmetry remains debatable.

Finding them may be beyond the capabilities of existing particle smashers such as the Large Hadron Collider in Europe, so the answer might need to wait for the next generation of equipment. And it will be years before any such experiment is built and running.

In the meantime, many theorists are not yet willing to throw out the Standard Model. William Marciano, a theorist at Brookhaven, is waiting to see whether additional experiments on muons and electrons clear things up.

Zoltan Fodor at Penn State is another theorist who disagrees that the Standard Model is broken. He has done a theoretical simulation of how much the muon should wobble, using a different technique that called for hundreds of hours of supercomputer time.

“There is no new physics,” he says. “I have checked my results 1,000 times.”

Saswato R. Das is a science and technology writer.