Does This Nucleus Make Me Look Fat?

As we may remember from high school science classes, the nucleus of an atom is made up mostly of protons and neutrons. Turns out there are other things in there as well, but throughout the re-thinkings brought on by new discoveries and new theories, the protons have remained constant.

Or maybe not. Physicists had used the movements of another group pf subatomic particles -- electrons -- to determine the size of a proton and found it to have a radius of 0.8768 of a femtometer. A femtometer is a millionth of a billionth of a meter. Recently, just for grins, similar experiments have been done using a different subatomic particle, called a muon. Muons are heavier than electrons and so they are closer to the protons in the nucleus, allowing for a more careful measurement. Using this method, the proton was found to have a radius of about 0.84087 of a femtometer.

Since it would take ten trillion femtometers to span a human hair, it may seem kind of silly to get all worked up over a difference of less than three one-hundredths of one. Kind of like cutting $85 billion out of a multi-trillion dollar budget.  Or measuring a person who's five-nine and finding out that they are actually almost six feet tall -- hmm, maybe this is a significant difference.

And for physicists, it is. The four percent difference in size could come from experiment error, of course, but it could also come from the presence of other particles in the nucleus that haven't been detected. More experiments will be needed to find out just how big the proton is. In the meantime, I would like my doctor to use muon-based measurements to calculate my weight. A four percent drop is just fine by me.