Scientists measure protons again, mystery still unsolved

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Researchers at the York University in Toronto once again took a look at one of the most fundamental information about our universe: the actual size of a single proton. This is all in an attempt to finally solve a decade-long mystery that has baffled physicists.

The traditional and long known value of the proton’s charge radius (its effective “size”) is 0.877 femtometers. This was derived from two independent measurement methods, which yielded more or less the same result. However, last 2010, a team of physicists attempted to once again manually measure the charge radius of a proton using a new method that uses exotic (muonic) hydrogen atoms.

The resulting measurements caused worldwide controversy and intrigue in the scientific community. The value of the newer measurement, which has since then been confirmed by other researchers using the same method, was 0.842 femtometers, or about 4% smaller than what was previously known. For the next nine whole years afterward, an official explanation was never concluded for the apparent difference.

The main reason why this discrepancy was very important to physicists is due to the apparent “error” that fundamentally changes what has been known about subatomic particles within the last century. To quote an exact statement from the official press release of the research:

“The quest to resolve the proton-radius puzzle has far-reaching consequences for the understanding of the laws of physics, such as the theory of quantum electrodynamics, which describes how light and matter interact.”

This is considerably different from, let’s say, a miscalculation of the height of a mountain or building. Because it is at the macro scale, we can easily attribute the error to a number of external factors, be it geological, atmospheric, chemical, or even mathematical. At the subatomic level, there is only very, very little room for variations when it comes to absolute measurements.

The study made a few days ago is another attempt to officially confirm the real charge radius of a proton. The new measurement, which was also confirmed, peer-reviewed, and was published in the journal Science, turned out to be 0.833 femtometers — a measurement with a 5% difference from the original, traditional charge radius measurement.

The official press release did not directly address why the newly measured difference came to be. However, it did explain that the measurement accounted for both the old atomic hydrogen method, and the newer exotic (muonic) hydrogen method.

Most importantly, this new attempt pretty much confirms that the 2010 measurement was more precise. Protons are indeed smaller than expected. It yielded results that are “analogous to the muon-based measurement”, albeit still slightly different from the previous.

So what now then? The proton-radius puzzle still lingers. One specific research paper made by astrophysicist Laurent Nottale last April this year did propose another explanation, which suggested that scale relativity may be the key to answering this long-standing mystery.

Image Credit by jimothyjohn via Reddit, and the FOM Institute for Atomic and Molecular Physics

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