Saturday, 26 January 2013

The proton size puzzle

The proton is one of the so-called ‘building blocks’ of the nucleus of an atom, along with neutrons, although these basic constituents themselves consist of three  much smaller entities called quarks.

a proton comprises
3 different kinds of quark bound together
Over the decades the mass and size of the proton has been determined with increasing accuracy and the received view of physicists has been that they are both constant. For proton mass, this still holds; but for proton size the situation changed with a measurement made in 2010 by Pohl et al (Max Planck Institute, Germany) and published in Nature.

It had been determined that the diameter of a proton was 0.87 femtometres (1 fm = 1 metre divided by a quadrillion, i.e. a 1 followed by 24 zeros). It was always assumed that this was a constant property of a proton throughout space and time, independent of the measurement method, which was normally done using a hydrogen atom, which comprises a proton and an orbital electron.

 The method used in 2010 was a new one and involved focusing a fine laser beam onto a muonic hydrogen atom. Like ordinary hydrogen this comprises a central proton of positive charge attended by a negatively charged particle. In the case of normal hydrogen the negative particle is an electron whereas with muonic hydrogen it is a similar but heavier particle called a muon.  The larger mass of the muon, 200x that of an electron, would indirectly result in a more accurate measurement of proton size than the method employing the usual electron. That was the reason for using it; but the result was entirely unexpected: a 4% smaller radius.

 Another experiment in 2012 employed a slightly different method, though it still used muonic hydrogen, and the same result was obtained –i.e. a diameter of 0.84 instead of 0.87 fm.

No one has been able to find any fault in the methodology of these new measurements and existing physics does not explain why the size of the proton depends on whether an electron or a muon is accompanying the proton. Does this mean that the photons in the laser beam interact with muons in a fundamentally different way than with electrons? Maybe the Large Hadron Collider will give a clue. The chase is on, one of many in modern physics that will hopefully converge to reveal more of the splendour of God’s creation.

see also

Radioactivity half life is not contant


Author of 2077 AD