Proton Smaller Than Thought—May Rewrite Laws of Physics

Protons, among the building blocks of atoms, are even smaller than we thought—and the unexpected discovery may alter some of the most trusted laws of physics.

All atoms are made up of nuclei orbited by electrons. The nuclei, in turn, are made of neutrons and protons, which are themselves made of particles called quarks.

For years the accepted value for the radius of a proton has been 0.8768 femtometers, where a femtometer equals one quadrillionth of a meter.

The size of a proton is an essential value in equations that make up the 60-year-old theory of quantum electrodynamics, a cornerstone of the Standard Model of particle physics. The Standard Model describes how all forces, except gravity, affect subatomic particles. (See "Einstein's Gravity Confirmed on a Cosmic Scale.")

But the proton's current value is accurate only by plus or minus one percent—which isn't accurate enough for quantum electrodynamics, or QED, theory to work perfectly. So physicists have been searching for ways to refine the number.

Smaller Proton Size Revealed by Lasers

In a ten-year experiment, a team led by Randolf Pohl of the Max-Planck Institute of Quantum Optics in Garching, Germany, used a specialized particle accelerator to alter hydrogen atoms, which are each made of a single proton orbited by an electron.
For each hydrogen atom, the team replaced the atom's electron with a particle called a muon, which is 200 times more massive than an electron.

"Because the muon is so much heavier, it orbits very close to the proton, so it is sensitive to the proton's size," said team member Aldo Antognini, of the Paul-Scherrer Institute in Switzerland.

Muons are unstable, and they decay into other particles in just 2.2 microseconds. The team knew that firing a laser at the atom before the muon decays should excite the muon, causing it to move to a higher energy level—a higher orbit around the proton. The muon should then release the extra energy as x-rays and move to a lower energy level.

The distance between these energy levels is determined by the size of the proton, which in turn dictates the frequency of the emitted x-rays.

But based on the accepted proton radius, the experiment failed to produce x-rays at the anticipated frequency.

In the summer of 2009 the team decided to widen their search to include other possible proton sizes. To their astonishment, the scientists detected x-rays at an assumed proton radius of 0.8418 femtometers—4 percent smaller than expected.

"We were totally surprised and don't have any explanation for it currently," Antognini said.

Smaller Proton a "Significant Shake-up"

The proton finding won't impact most people's daily lives. But if it proves correct, it means something fundamental is wrong in particle physics.

It's possible the smaller proton means the Rydberg constant hasn't been correctly measured. This value describes the way light gets emitted from various elements—a key component of spectroscopy, which is used, for instance, to tell which kinds of elements exist in galaxies and the vast interstellar gas-and-dust clouds called nebulae.

Or, if the Rydberg constant is correct, the smaller size of a proton could mean the equations in QED theory will fail to work.

"It is a significant shakeup and could mean a complete rethink of QED, potentially opening the door to a new theory," said Jeff Flowers, a scientist with the National Physical Laboratory in the U.K., who wasn’t involved with the experiment.

Over the coming weeks physicists all over the world will be scrutinizing the experimental setup and complex calculations, making sure that there are no mistakes.

Assuming no errors are found, the scientists may have to get to work rebuilding the Standard Model.
Findings appear in this week's issue of the journal Nature.