The muons do not rotate in the way they predict the best physical model. Why not? This may be due to completely unknown subatomic particles popping up and disappearing into the quantum foam.

This is not some science fiction technobook. This is from very real experimental results, and it is very likely that the universe is telling us that we still do not understand everything about it.

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

Muons are similar to electrons ̵

Using everything we know about subatomic particles (called the standard model), physicists can predict a lot about muon behavior. For example, a rotating charged particle has an associated magnetic property called a *moment*, which is a measure of the strength of its magnetic field and its orientation. If you place a muon in a magnetic field, it will oscillate *precession*; it’s physically similar to rocking a toy from above as it rotates on the table top.

The models predict this precession extremely accurately. *Exceptional*. Physicists assign a value to what is called *g-factor*and is very close, but not exactly 2.

Here we have fun: On our macroscopic scale, we like to think that space is smooth and continuous. But in quantum scale, an incredibly small scale (like 10^{-35} meters!) quantum mechanics suggests that space is *no* continuously and smoothly and instead may have separate units, such as chart bookmarks. This means that on this scale the space may not be empty, but instead boil and foam with energy.

Sometimes this energy will spontaneously create a pair of subatomic particles (since mass and energy are two sides of the same coin, E is equal to mc^{2} and all that). These particles can appear, but these same laws of quantum reality require the particles to immediately interact and become energy again, returning to the vacuum energy. It’s called (and I love it) *quantum foam*.

The muon, which rotates in a magnetic field, is affected by the quantum foam. If there were no foam, the value of the g-factor would be very close to 2. But the particles that pop up and disappear affect the muon oscillation. This is called *anomalous magnetic moment,* the deviation from the normal value.

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

If not … fine. That would be interesting, wouldn’t it?

The standard model predicts the anomalous value of the magnetic moment of the muon to be **0,00116591810** (± 0.00000000043; as I said, very accurate).

The new experiment gets a value of **0.00116592061** (± 0.00000000041).

These are different. The difference is small, of course, only 0.0002%. But still they must be equal. And they are not.

This small difference means a lot. 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 on the wrench: The results are not *quite* to a statistical snuffbox. It is very unlikely that they are due to a chance. It’s like tossing a coin: If it appears three times in a row, you might think the coin is counterfeit, but there is a chance of every eight that will happen at random. The more times you turn it over and it appears, the less likely it is to be accidental.

Scientists use a term called *sigma* to measure this chance. The gold standard in particle physics experiments is when the experiment is in the five sigma range, which means that there is a chance of it happening around one in three million, or, if you prefer, a 99.99997% chance of being real (one sigma is about 68%, two is 95%, three is 97%, etc., crawling closer and closer to 100%). The results of the Muon g-factor experiment are only 4.2 sigma, which means that they still have about 1 in 38,000 chances of being due to random noise.

Still, this is a 99.997% chance that it’s not due to a random chance, and that’s pretty good^{*}. It is simply not enough for physicists to declare victory. The good news is that they are not over yet. So far, the experiment has been conducted three times, a fourth is being done and a fifth is planned. The researchers studied the data from these first experiments, but that’s only about 6% of the total amount of data they expect from the experiment. To use the analogy above, it is as if they have turned the coin over several times and obtained strange results, but will continue to turn it many more times to be sure.

If the rest of the data is aligned with what they have seen so far, they will transmit 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 that *already* he told us that the universe was hellishly strange.

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

So this is potentially very exciting. The standard model is quite successful (for example, it predicts the existence of the Higgs boson, which was first discovered a few years ago), but we know that there are cracks in it, things that it also does not anticipate. In this case, muons floating and rotating and swaying in a magnetic field draw us further down this path, waving us to more physics that we still don’t understand or even know.

And this is the dream of every particle physicist. When experiments test the theory, it’s good because it’s like showing that the road behind us is paved smoothly.

But what’s next?

*[[[[ Correction (16:00 UTC on April 8, 2021): I initially calculated the percentages incorrectly on these odds, adding an additional two 9 in the decimal point (in other words, I had written them as correct odds, not as percentages, with a 0.01 chance being 1%). Arg! The numbers are already fixed. Also, I changed the phrase a bit; the statistics cover only a chance chance. There may also be systematic errors, that is, something that is not reported in the equipment, or in mathematics, or whatever. They are not random and are difficult to account for. I just want to make sure I’m covering the basics here.]*