Why Muonic Hydrogen Matters


WC wrote about challenges facing experimental physics back in November 2011; there were some indications that physics might be moving into the a paradigm shift, a kind of scientific revolution that periodically rattles accepted theories. WC will briefly revisit those issues then a note a new, possibly issue that has arisen.

There were four observational results not easily addressed by current theories of physics back in November 2011.

First, there was the discovery over the last decade that dark matter and dark energy constitute something like 96% of the apparent mass of the universe, but we can’t see or interact with the dark stuff. Physics hasn’t found a way out of that conundrum yet, although there are some aspects of string theory that are interesting. So mark that one as unresolved.

Second, there is the observation that the universe is not only still expanding, but the rate of expansion is accelerating. Some form of dark energy is making the universe get larger faster. The word “strange” seems so inadequate in this context. No progress there, either.

Third, the Higgs Boson was found, lurking in the statistical weeds, so to speak, as WC reported earlier. The discovery was a triumph for high energy physics and hight speed computing. Too bad it happened in Europe at the CERN instead of at the abandoned superconducting supercollider in the United States. But regardless of patriotism, it was a massive triumph for the predictive power of science and the theories of quantum chromodynamics.

And lastly, those neutrinos that were thought to have exceed the speed of light, the universal speed limit? Turns out to be observational error. Probably.

Which takes us to the newest conundrum. Protons, of course, are part of what makes up the stuff a the center of every atom. The simplest atom is a hydrogen atom: just one proton and one electron. The structure of a hydrogen atom was thought to be pretty well understood. Among the things known was the apparent diameter of a proton, about 0.88 femtometers. A femtometer, for those who are still following along, is 10 to the -15 power meters – 0.0000000000000001 meters. Very, very tiny indeed.

A team led by Switzerland’s Paul Scherrer Institute created a different kind of hydrogen atom. They substituted a muon for the electron in a single hydrogen atom. A muon is in the same elementary particle family as an electron, and has the same electrical charge, but it weighs 207 times as much as an electron. This was a tricky business, because a muon decays to a smorgasbord of elementary particles and energy in just two-millionths of a second. But before that happened, the Scherrer Institute folks measured the diameter of  the proton in their cobbled together muonic hydrogen atom.

And it was different. Not a lot different, 0.84 femtometers, but different.

There is no theory of physics to account for the difference. It doesn’t seem to be experimental error, and the difference is statistically significant.

Now quantum physics is famously weird, and the Pauli Exclusionary Principle teaches us to be careful about what we think we observe in the quantum realm. But even for quantum physics, this is seriously strange.

Most serious theoretical physicists will tell you that the most thrilling thing they can do is prove a theory is wrong. The next few years of physics could be thrilling.

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