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Is a new “muon” finding evidence for a fifth force of nature? Rob Sheldon weighs in

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The conventional four forces of nature are gravity, electromagnetism, the strong force, and the weak force. The current buzz revolves around the muon, similar to the electron but 200 times heavier:

Now, physicists say they have found possible signs of a fifth fundamental force of nature.…

The findings come from research carried out at a laboratory near Chicago…

The Muon g-2 experiment involves sending the particles around a 14-metre ring and then applying a magnetic field. Under the current laws of physics, encoded in the Standard Model, this should make the muons wobble at a certain rate.

Instead, the scientists found that muons wobbled at a faster rate than expected. This might be caused by a force of nature that’s completely new to science.

No one yet knows what this potential new force does, other than influence muon particles.

Pallab Ghosh, “Muons: ‘Strong’ evidence found for a new force of nature” at BBC News

There are people in the world who would stop short of speculating about a fifth force of nature in a situation that could easily be explained by a less spectacular facts. So, of course, we asked our obliging physics color commentator Rob Sheldon, what do you think?, and he wrote back:


I hate to disappoint you, but most of my gut reaction is negative. Full Disclosure: I am not a particle physicist. There were two tracks in graduate school, simply because there was too much material to cover. So budding particle physicists took the “relativistic QM track” and budding laser jocks took the “solid state” or “many bodied QM” track. I had spent the previous summer between college and grad school at IBM’s Thomas J Watson research center testing sub-micron linewidth bipolar transistors, and was sure that the future was in computers and lasers. As it turned out, that summer the “Standard Model” of particle physics was completed and has not been modified since, whereas Moore’s law has progressed for 40 years and transistor line widths are now down to 0.005 micron and getting close to the fundamental limit. So I don’t think I was wrong in choosing a career in solid state physics.

In fact, this 40-year stasis in particle physics has meant that two generations of graduate students have never had a successful breakthrough experiment, or confirmed a new theory. The field, as Sabine Hossenfelder reminds everyone, is littered with wrong papers. For example, the theory of “supersymmetry” known as SUSY which was taught as fact to two generations of students has now been abandoned by experimentalists. Combined with the cancellation of the Texas supercollider, the end-of-life of the Tevatron, the last upgrade of LHC, and the fact there haven’t been any new hires in a decade or so, and the future of a particle physicist’s career is grim indeed.

With that as a background, every experiment that deviates from the Standard Model is seized upon like a lifesaver beside the Titanic. The amount of expectation concentrated in a 4-sigma deviation of the muon magnetic moment is breath-taking. Even the detail of the sealed envelope reveals the level of deferred hope within the community. Every ounce of my scientific empathy is drawn to their difficult plight.

But still there are grounds for skepticism. Again, Sabine points out that 40 years of success of the Standard Model makes it unlikely to be overturned easily. Further, the 4-sigma result is not really 1:40,000 (which it would be if it were gaussian distributed), but more like a 1:10 once “fat tails” of non-gaussian distributions are taken into account. That was last week’s blog from an astronomer explaining the 5-sigma rule. But the deviation is not between two experiments, but between experiment and theory. And calculating the muon magnetic moment in the theory is not trivial, but an exercise in super-computer bragging rights because it involves “lattice-QCD” calculations, and in fact, this experimental result has spurred one of the theory groups to publish:

“The new calculations required hundreds of millions of CPU hours at multiple supercomputer centers in Europe and bring theory back in line with measurement. However, the story is not over yet. New, more precise experimental measurements of the muon’s magnetic moment are expected soon.
`If our calculations are correct and the new measurements do not change the story, it appears that we don’t need any new physics to explain the muon’s magnetic moment—it follows the rules of the standard model,’ said Fodor.”

So there you have it, the US experimentalists claim to overturn the Standard Model, the European theorists say it is all under control.

The Long Ascent: Genesis 1–11 in Science & Myth, Volume 1 by [Robert Sheldon, David Mackie]

But as I pointed out yesterday, this Standard Model thing is like bioethics, only important when we need to justify some action on our part. For example, the lifetime of the neutron is uncertain in the 3rd decimal place, an error far larger than the muon magnetic moment problem. Not only are the Standard Model theories not agreeing with experiment, but the two sets of neutron lifetime experiments don’t agree with each other. Neutrons are a part of every piece of matter in your body, room and world, whereas muons only flicker into existence in the upper atmosphere as cosmic rays collide with the atmosphere, and then vanish by the time they reach the ground. The discrepancy with neutrons is far more materially significant than muons, yet the Standard Model stands like a granite promontory in the raging sea of neutron data. What made anyone think that the splash of a muon magnetic moment was going to topple it?


Rob Sheldon is the author of Genesis: The Long Ascent and The Long Ascent, Volume II.

3 Replies to “Is a new “muon” finding evidence for a fifth force of nature? Rob Sheldon weighs in

  1. 1
    AaronS1978 says:

    This input is very much appreciated thank you

  2. 2
    PaV says:

    Thanks, Rob, for the overview. You’ve captured things quite nicely.

    I do want add this: I believe it will only be a matter of time before the 4.2 Sigma event becomes a 5.0+ event. When they process the three other ‘runs’ they’ve already performed, I’m rather sure that will get them over the hump.

    But, now, that new calculation–“There’s the rub.” I’m not exactly sure that they’ve used a new method of calculation or simply new levels of calculations, or a little bit of both. That should prove interesting and I’m sure there are lots of people looking over those calculations right now. We will have to wait on both fronts.

    But it sure would be fun to have a deviation from the Standard Model.

    As to your decision of “solid state” physics, that was a good one. And my suspicion is that any move forward in particle physics will come from the “solid state” community. I forget the exact connection, but the whole idea of Cooper pairs fed into advances towards the Standard Model. I suspect that will be repeated. As I look at summaries of what’s going on, to me it seems like the “solid state” guys are doing all the interesting work.

  3. 3
    Fasteddious says:

    Surely the fifth force is mental: mind, spirit, consciousness, etc.? There has to be something that allows the non-physical (spirit) to influence the physical (e.g. human body movements). This may be the same force that causes the Schrodinger equation to collapse probabilities into actualities? That force would not be seen in physics experiments which are purposely planned to keep the humans out of the setup. It may also be a short-range force given the fact that my mind cannot cause someone else’s body to act. But presumably mental or spiritual force does not affect muons.

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