A fanciful illustration of a metal clamp holding a proton.
Enlarge / One option to measure the cost radius of a proton is to bounce one thing off it (proton-sized clamp is simply accessible by way of metaphor).

How large is a proton? This does not sound like a really difficult query, however it’s one which turned out to have the potential to wreck lots of trendy physics. That is as a result of completely different strategies of measuring the proton’s cost radius produced outcomes that disagreed—and never simply by a bit bit: the solutions had been 4 commonplace deviations aside. However now, a brand new and doubtlessly improved measurement brings them a lot nearer to settlement, though not fairly shut sufficient that we are able to contemplate the problem resolved.

We appear to have an issue

There are a few methods to measure a proton’s cost radius. One is to bounce different charged particles off the proton and measure its measurement primarily based on their deflections. One other is to discover how the proton’s cost influences the habits of an electron orbiting it in a hydrogen atom, which consists of solely a single proton and electron. The precise vitality distinction between completely different orbitals is the product of the proton’s cost radius. And, if an electron transitions from one orbital to a different, it’s going to emit (or take up) a photon with an vitality that corresponds to that distinction. Measure the photon, and you’ll work again to the vitality distinction and thus the proton’s cost radius.

(The precise wavelength relies on each the cost radius and a bodily fixed, so that you truly have to measure the wavelengths of two transitions with a view to produce values for each the cost radius and the bodily fixed. However for the needs of this text, we’ll simply concentrate on one measurement.)

A tough settlement between these two strategies appeared to go away physics in good condition. However then the physicists went and did one thing humorous: they changed the electron with its heavier and considerably unstable equal, the muon. In response to what we perceive of physics, the muon ought to behave precisely just like the electron apart from the mass distinction. So, when you can measure the muon orbiting a proton within the temporary flash of time earlier than it decays, you must be capable of produce the identical worth for the proton’s cost radius.

Naturally, it produced a special worth. And the distinction was massive sufficient {that a} easy experimental error was unlikely to account for it.

If the measurements actually had been completely different, then that signifies a major problem in our understanding of physics. If the muon and proton do not behave equivalently, then quantum chromodynamics, a significant concept in physics, is irretrievably damaged not directly. And having a damaged concept is one thing that makes physicists very excited.

Combing the frequencies

The brand new work is essentially an improved model of previous experiments in that it measures a particular orbital transition in commonplace hydrogen composed of an electron and a proton. To start with, the hydrogen itself was dropped at a really low temperature by passing it by a particularly chilly metallic nozzle on its means into the vacuum container the place the measurements had been made. This limits the influence of thermal noise on the measurements.

The second enchancment is that the researchers labored within the ultraviolet, the place shorter wavelengths helped enhance the precision. They measured the wavelength of the photons emitted by the hydrogen atoms utilizing what’s known as a frequency comb, which produces photons at an evenly spaced sequence of wavelengths that act a bit just like the marks on a ruler. All of this helped measure the orbital transition with a precision that was 20 instances extra correct than the group’s earlier try.

And the outcome the researchers get additionally disagrees with earlier measurements of regular hydrogen (although not a newer one). And it is a lot, a lot nearer to the measurements made utilizing muons orbiting protons. So, from the attitude of quantum mechanics being correct, that is excellent news.


However not nice information, for the reason that two outcomes are nonetheless outdoors of one another’s error bars. A part of the issue there may be that the added mass of the muon makes the error bars on these experiments extraordinarily small. That makes it very tough for any outcomes obtained with a standard electron to be in line with the muon outcomes with out utterly overlapping with them. And the authors acknowledge that the distinction is prone to simply be unaccounted for errors that broaden the uncertainty sufficient to permit overlap, citing the prospect of “systematic results in both (or each) of those measurements.”

So, the work is a vital landmark when it comes to discovering methods to up the precision of the outcomes, and the end result means that quantum chromodynamics might be advantageous. Nevertheless it would not truly utterly resolve the distinction, which means we’ll want some extra work earlier than we are able to actually breathe simply. Which is annoying sufficient to presumably clarify why Science selected to run the paper on Thanksgiving, when fewer folks could be paying consideration.

Science, 2020. DOI: 10.1126/science.abc7776 (About DOIs).


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