Physics: Proton radius smaller than believed, European scientists say

Physicists might have to rethink what they know about, well, everything.

European researchers dropped a potential bombshell on their colleagues around the world Wednesday by reporting that sophisticated new measurements indicate the radius of the proton is 4% smaller than previously believed.

In a world where measurements out to a dozen or more decimal places are routine, a 4% difference in this subatomic particle — found in every atom’s nucleus — is phenomenally large, and the finding has left theoreticians scratching their heads in wonderment and confusion.

If the startling results are confirmed, a possibility that at least some physicists think is unlikely because the calculations involved are so difficult, they could have major ramifications for the so-called standard model on which most modern physics is based.


In an editorial accompanying the report in the journal Nature, physicist Jeff Flowers of the National Physical Laboratory in Teddington, England, said there were three possibilities: Either the experimenters have made a mistake, the calculations used in determining the size of the proton are wrong or, potentially most exciting and disturbing, the standard model has some kind of problem.

If the theory turns out to be wrong, “it would be quite revolutionary. It would mean that we know a lot less than we thought we knew,” said physicist Peter J. Mohr of the National Institute of Standards and Technology in Gaithersburg, Md., who was not involved in the research. “If it is a fundamental problem, we don’t know what the consequences are yet.”

Whatever the explanation, however, it will have far more import for physicists than for anyone else, he added. The standard model “works pretty well in most cases,” explaining lasers, magnetic resonance scanning and a host of other modern-day miracles.

The standard model, which defines the structure and behavior of matter, radioactivity, electricity — pretty much everything other than gravity — is based upon the hydrogen atom. That atom, composed of a single proton orbited by a single electron, is the most thoroughly studied atom in physics, primarily because of its simplicity.


“To understand hydrogen is to understand all of physics,” said physicist Aldo Antognini of the Paul Scherrer Institute in Villigen, Switzerland, a coauthor of the report, quoting the late MIT physicist Victor Weisskopf.

The newfound lack, or potential lack, of understanding of hydrogen is disconcerting, to say the least.

First some background: Electrons circling the nucleus of an atom can occupy many discrete energy levels, separated by characteristic frequencies that can be observed by spectroscopy when the atom is excited by light or other radiation. The foundation for the current work was laid in the late 1940s by Willis Lamb and R.C. Retherford, who discovered that two energy levels of the electron in a hydrogen atom, previously thought to be identical, were actually different.

That difference, known as the Lamb shift, forced a rethinking of previous physics theories and led to the development of quantum electrodynamics, which explains all interactions between light and electromagnetism. Most electronic devices, for example, are defined by QED, as it is commonly called.

But the mathematical foundations of the QED theory are still incomplete, and researchers are constantly trying to improve them.

The Lamb shift has been used to calculate the radius of the proton to an accuracy of about 1%, yielding a value of 0.8768 femtometers (1 femtometer equals 0.000 000 000 000 001 or 10-15 meter). That is roughly equal to the value obtained by other experiments, such as shooting electrons at the nucleus and measuring their scatter.

Researchers have long known that accuracy of the calculation could be improved by a factor of 10 by replacing the electron in a hydrogen atom with a muon, a particle that is also negatively charged but is 200 times as heavy as an electron. It thus orbits closer to the proton, giving a larger and more readily measurable Lamb shift.

But muons exist for only about 2 millionths of a second, so performing the experiment is exceptionally difficult. The international team at the Scherrer Institute has been working on it for 12 years. In essence, researchers aim a beam of muons at hydrogen atoms. Some atoms capture the muons. Before the newly formed muon-hydrogens can decompose, the team flashes a laser at them to measure the Lamb shift.


They report that the value they calculated in this manner is 0.84184 femtometers.

“We are confident in the experimental results,” Antognini said. The precision of the measurement “is equivalent to measuring the distance from here to the moon with one micrometer precision,” he said. “It seems everything [experimentally] is correct, but something is wrong. We cannot say what is wrong.”

The team is busy rechecking all its calculations, as will be physicists around the world. Meanwhile, the Scherrer team plans to repeat the experiment using helium atoms, which have two protons and two electrons, instead of hydrogen. That should either confirm or refute their findings.

For now, it’s unclear whether the result will be confirmed as an experimental mistake or a revolution in physics. “I wouldn’t bet on anything now,” Mohr said. “It’s not at all clear.”