The Proton Shrinks

A new finding indicates the proton’s size is 4 per cent smaller than estimated, raising questions about current theories.

Minute is not a that even begins to describe a difference of 0.00000000000003 millimetres, but the difference may still be large enough to overturn one of our most fundamental theories of matter. Researchers at the Paul-Scherrer Institute in Switzerland, using powerful lasers, have made the entirely unexpected finding that the proton is 4 per cent smaller than previous estimates. Those estimates had used calculations based on the theory of Quantum Electrodynamics (QED). 

While classical electrodynamics deals with the interplay of electricity and magnetism in the world around us, QED extends this to the quantum level, describing the interplay of light and matter. So precise has been the match between predictions based on QED and experiments, that Richard Feynman, one of the originators of QED, termed it the ‘jewel of physics’. 

Earlier experiments to measure the size of the Proton had used the hydrogen atom, where one electron orbits around a proton at the centre. The electron can only occupy discrete energy levels, and measuring the energy required to jump from one level to another provides a precise estimate of the size of the proton based on calculations using QED. The current piece of work instead knocked the electron out of the hydrogen and replaced it with a particle called a muon that is 200 times heavier. Measuring the energy levels of the muon provided a more precise estimate of the proton’s size. 

According to an editorial in Nature carried along with the publication of the research, ‘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.’ The experiment is very precise and an error is unlikely. Both the other cases imply a re-evaluation of some fundamental theories.

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Hartosh Singh Bal