Muons do not spin as the best physics model predicts. Why not? This may be due to completely unknown subatomic particles jumping in and out of the existence of the quantum foam.

This is not some kind of sci-fi technique. This is from quite real experimental results and may very well be that the universe tells us that we do not yet understand everything about it.

These extremely interesting and possibly game-changing results come from Fermilab, a high-energy particle accelerator lab in Illinois. They perform many different types of experiments there, and one is called Muon g-2 (literally “g minus 2”), which examines a subatomic particle called a *muon*.

Muons look like electrons ̵

Using everything we know about subatomic particles (called the standard model), physicists can predict a lot about a muon’s behavior. For example, a rotating charged particle has a magnetic property associated with it called a *moment*, which is a measure of the strength of its magnetic field and its orientation. If you put a muon in a magnetic field, it undergoes a wobble called *precession*; this physically resembles a toy stopper that tilts when rotating on a tabletop.

The models predict this precession extremely accurately. *Extremely*. Physicists assign a value to this call *g-factor*, and it is very close, but does not equal 2.

Here things get funny: On our macroscopic scale, we like to think that space is smooth and continuous. But on a quantum scale, an incredibly small scale (like 10^{-35} meters!) quantum mechanics implies that space is *does not* continuous and smooth and can instead come in discrete units, like crosses in a graph. This means that the space on that scale may not be empty, but instead boils and foams of energy.

Sometimes this energy will spontaneously create a few subatomic particles (because mass and energy are two sides of the same coin, where E is equal to mc^{2} and all that). These particles can occur, but the same laws of quantum reality require that the particles immediately interact and become energy again and go back into the vacuum energy. This is called (and I love it) *quantum foam*.

A muon rotating in a magnetic field is affected by the quantum foam. If there was no foam, the value of the g-factor would be very close to 2. But the particles that jump in and out of existence affect the muzzle’s wobbling. This is called *abnormal magnetic moment,* the deviation from the usual value.

The standard model predicts the value of this abnormal moment by looking at everything known about forces and particles. It must be very accurate. Still, it’s always nice to make sure, and that’s what the Muon g-2 experiment does. It injects muons into a very stable magnetic field and *measures* wobble, which can then be compared to the prediction. If they agree, then we understand how the quantum mechanical universe behaves.

If not … well. That would be interesting, right?

The standard model predicts that the muon’s anomalous magnetic moment value must be **0.00116591810** (± 0.00000000043; as I said, very accurate).

The new experiment gets a value of **0.00116592061** (± 0.000000000041).

They differ. The difference is small, for sure, only 0.0002%. But still, they must be equal. And they are not.

This small difference means a lot. It means that *there are forces and / or particles acting on the quantum scale that we do not know about!*

Well, maybe. Here is the monkey in the wrench: The results are not *quite* up to statistical snuff. It is very easily possible that they are due to random chance. It’s like flipping a coin: if it comes up heads three times in a row, you might think the coin is rigged, but there is a chance out of eight that will happen randomly. The more times you flip it and it comes up heads, the less likely it is random.

Researchers use a term called *sigma* to measure this chance. The gold standard in particle physics experiments is when an experiment is in the five sigma range, which means it has a random chance of occurring at around one in three million, or if you prefer, there is a 99.99997% chance of to be real is about 68%, two is 95%, three is 97%, and so on, creeps ever closer to 100%). The Muon g-factor experimental results are only 4.2 sigma, which means that they still have approx. 1 in 38,000 chances due to random noise.

Still, there’s a 99.997% chance it’s not due to chance, and that’s pretty good.^{*}. It’s just not quite enough for physicists to declare victory. The good news is that they are not finished yet. The experiment has been run three times so far, doing a fourth, and a fifth race is planned. The researchers have examined the data from the first runs, but it only amounts to approx. 6% of the total amount of data they expect from the experiment. To use the analogy above, it is as if they have flipped the coin a few times and got strange results, but will continue to flip it many more times to be sure.

If the rest of the data matches what they have seen so far, they surpass the five-sigma security. And if that happens, it certainly means that the universe is stranger and more mysterious than even the quantum mechanics we know tell us … and it is *already* has told us that the universe is damn weird.

If you want all this in comic form, then Jorge Cham has you covered:

So this is potentially very exciting. The standard model is quite successful (for example, it predicted the existence of the Higgs boson, which was first found a few years ago), but we know there are cracks in it, things it does not predict either. In this case, muons floating and spinning and staggering in a magnetic field wave at us further down that path and wave at us towards more physics that we do not yet understand or even know anything about.

And that is the dream of every particle physicist. When experiments confirm theory, it’s nice because it’s like showing that the road behind us is paved smooth.

But what lies ahead?

*[[[[ Correction (16:00 UTC on April 8, 2021): I originally calculated the percentages incorrectly on these chances and added two more 9s in the decimal point (in other words, I had written them as equal odds, not percentages, as a chance of 0.01 is 1%). Arg! The numbers are now fixed. I also changed the wording a bit; the statistics only cover random chance. There can also be systematic errors, that is, something that is not taken into account in the equipment or the math or whatever. These are not random and are difficult to account for. I just want to make sure I cover the bases here.]*