The Shrinking Proton

11 July 2010


New research published in the journal Nature suggests that the proton might be as much as 4% smaller than we previously thought, and this discovery might prompt a revaluation of some trusted laws of physics...

Protons are one of the basic subatomic particles - atoms are made of protons, neutrons and electrons (except hydrogen, which lacks a neutron).

The size of the proton is a value used in Quantum Electrodynamics (QED) and in spectroscopy, but so far has only been known to an accuracy of about 1%.  Physicists would obviously like a more accurate figure, but only recently has this been experimentally possible.

To get a more accurate measurement, Randolf Pohl at the Max Planck institute for Quantenoptik in Germany used a specialised particle accelerator to alter hydrogen atoms and replace the electron with a particle called a muon - creating muonic hydrogen.  Muons have the same charge as electrons, but roughly 200 times more mass.  This means it will orbit much closer to the proton and as such interacts more closely with it, allowing us to more accurately probe the proton's properties.

Muonic hydrogen only survives for around one microsecond, but this is long enough to blast the atoms with a pulse of laser light, which causes the muon to jump up to a higher energy level.  When it falls back down, it releases some energy in the form of x-rays.  Detecting and analysing the energy of the x-rays released tells us the energy difference between the 2 states.  This energy gap, known as the Lamb shift, is determined by the size of the proton.

This gives results that are more accurate than other methods of proton measurement, but they suggest the proton is actually 4% smaller than we thought.  This could have implications for the theories of quantum electrodynamics, or could imply that the Rydberg constant, a value used in spectroscopy - for identifying what elements we can see in interstellar dust, for example - may not be correct.

Physicists are likely to be queuing up to check their finding, putting every element of the experiment and calculations under scrutiny, so we'll have to wait and see what this measurement really means for modern physics - but this could be a very significant shakeup.


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