The proton just got smaller

11 July 2010
Posted by Chris Smith.

Scientists might have to re-write the quantum rule book thanks to a new series of experiments which have shown that the proton is significantly smaller than we originally thought.

Writing in Nature this week, researcher Randolf Pohl, from the Max Planck Institute of Quantum Optics in Garching, Germany, explains how, together with colleagues, he used a particle accelerator to replace the electrons in atoms of hydrogen with related particles called muons. Although negatively-charged like electrons, muons are about 200 times heavier, which means that they orbit much closer to the proton in the nucleus than would an electron and are therefore more sensitive to the proton's size.

The team zapped these muon-containing hydrogen atoms with a powerful laser. When energy is added to atoms like this it normally makes the electrons temporarily move up to a higher energy level. When they then fall back to their normal energy level, the extra energy they absorbed is re-emitted as light waves of a certain frequency, which can be measured.

The same is true with muons orbiting atoms except that, owing to the closer proximity of the proton, the energy levels that the muons can occupy, and hence the frequency of the light they emit - in this case x-rays - when they fall back to their starting positions, are different. But based on the accepted size of the proton - about 0.8768 femtometres (just under one quadrillionth of a metre) - when the team looked for the x-rays of a certain frequency that should be being given out, they couldn't find it.

Instead, they detected x-rays at a very different frequency, which could only be explained if the current measurement of the proton is made 4% smaller!

This may not sound like much, but since this value underpins the workings of quantum electrodynamic theory, without an accurate measurement predications made using the existing models will be wrong...

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