Physics textbooks gave the proton a radius. Independent labs around the world kept arriving at the same number, so physicists stopped worrying about it.
The proton’s size was settled.
Then in 2010, a lab in Germany swapped the electron in a hydrogen atom for a heavier particle and came back with a proton nearly four percent smaller.
That result sparked 15 years of disagreement nobody could fully resolve.
Where the puzzle began
The team conducting the 2010 experiment built a special hydrogen atom by replacing its usual electron with a much heavier particle called a muon.
The measurement gave a proton radius about 4 percent smaller than the established value – a gap large enough to shake confidence in some of the most precise work in physics.
It opened what became known as the proton radius puzzle, sending researchers hunting for either a hidden experimental error or signs of unknown forces or particles.
Dr. Lothar Maisenbacher, who is now at the University of California, Berkeley (UC Berkeley), led one of the latest efforts to solve the puzzle.
He and colleagues at the Max Planck Institute of Quantum Optics (MPQ) in Garching, Germany, built a setup precise enough to distinguish between the two competing values.
Reading a tiny atom
Hydrogen is the simplest atom in existence – one proton, one electron, locked together by their opposite charges.
The two particles pull on each other electrically, and the proton’s size appears to shift the energies available to the electron.
Precisely how large that shift is depends on the proton’s actual dimensions.
That connection is the lever physicists pull on. By measuring how electrons jump between energy levels inside hydrogen, they can work backward to the proton’s size.
Each new measurement has to be cleaner than the last.
Dr. Maisenbacher’s group used finely tuned lasers to nudge electrons between two energy states never measured at this precision before.
The transitions had been observed in earlier experiments, but never sharply enough to settle the puzzle.
The textbook number was wrong
After three measurement runs and months of cataloguing every possible source of error, the team arrived at a proton radius of 0.8406 femtometers – less than one millionth of a billionth of a meter.
The result is 2.5 times more precise than any previous atomic hydrogen measurement.
It matches the smaller value from muonic hydrogen almost exactly and confirms what a 2019 paper had already suggested – the old textbook number was wrong.
Dylan Yost, a physicist at Colorado State University (CSU), led a second team that ran a complementary experiment and landed on the same answer.
The identical results from two independent experiments make an instrument-specific error highly unlikely.
The challenge of studying hydrogen
These experiments look modest from the outside – a vacuum chamber, lasers, a small cloud of hydrogen atoms chilled to near absolute zero. Behind that simplicity is brutal difficulty.
Hydrogen atoms need a near-perfect vacuum, and the lasers require calibration so exacting that minor environmental shifts can corrupt the data.
Collecting raw numbers might take a few weeks. Cataloguing every possible source of error can take years.
Each lab’s setup is unique enough that tracing divergences between results to a specific cause is difficult.
That difficulty becomes a strength when the results agree. Any quirk specific to one instrument would not survive two independent setups.
Testing the rulebook
With the proton’s size locked in, the team pushed further.
The experts compared their measurement against the Standard Model – physics’ master set of equations for how particles and forces behave. Agreement to 0.7 parts per trillion.
One match in roughly a trillion. That kind of precision is usually reserved for atomic clocks – one of the sharpest checks ever run on how light and matter interact at the fundamental level.
Everything matched. There was no hint of unknown forces, no exotic particles betraying themselves through the electron’s behavior.
The current rulebook held up under unusually heavy scrutiny.
A new search begins
The result may sound like a closed door, but Yost sees it differently.
Knowing the proton’s size with confidence lets researchers use the same hydrogen experiments to set tighter limits on what new physics might still be hiding.
Giant accelerators like the Large Hadron Collider in Switzerland are built to smash particles together and create heavy new ones.
Tabletop hydrogen experiments are sensitive to a different breed – extremely light particles that giant colliders cannot easily see.
With the proton question settled, those tabletop setups become a viable tool for particle physics searches.
The hardware was already there. What was missing was certainty about the background.
Broader implications of the study
For the first time, physicists have a proton radius they can stake their reputations on.
The atomic hydrogen number and the muonic hydrogen number finally agree, precisely enough that the 15-year disagreement is no longer scientifically alive.
That settles a long-standing question. Researchers can now use the same techniques to search for particles or forces beyond the Standard Model, with hydrogen serving as a sensitive detector instead of a source of uncertainty.
Other groups working on deuterium and similar transitions stand to gain from the methods this team developed.
Within a few years, the field could move from one resolved puzzle to fresh questions about physics beyond the known.
The study is published in the journal Nature.
—–
Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.
Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
—–