Protons Aren't round

drox writes "USA Today reports that protons are ovoid rather than spherical, as most of us learned in school." In related news, thousands of high school science labs have thrown out a bunch of little plastic balls.

Comments

Can't say I'm surprised...

[jargonCCNA]

Protons are made up of only three quarks. Of course they'd be a little misshapen. Find me a subnuclear particle with four or six quarks, then I'll surprised to find out it's not a perfect sphere.

Junk Post

[rjh]

In the article it said quarks travel at around ninety percent of the speed of light. So, tell you what, let's compute just how much more massive those quarks are. Fire up your LISP interpreter. We're taking a trip into Mathemagicland.

In LISP notation...

(defun relativistic-mass (m v) (/ m (sqrt (- 1 (/ (* v v) 1)))))

(relativistic-mass 1 .9)
2.294157


... So as you can tell, relativity tells us that at ninety percent of c, an object is only going to have two and a quarter times its normal mass. I don't see how you come up with the notion that "if the quarks are traveling relativistically, then the proton's mass must be near-infinite, thus quarks aren't traveling relativistically". If anything, your science is just as much junk as the USA Today article you're blasting.

Potentially relevant links

[Scott Carnahan]

This appears to be the abstract for the announced results. Note the lack of words like "round" in the abstract and article. You may need a subscription to Physical Review Letters to reach it and download the paper.

This appears to be the abstract of the paper of Miller and Frank attempting to explain the phenomena. You will have to accept cookies to get any sort of information out of the APS site.

This seems to be the experimental project page. It doesn't appear to be an informative resource for the uninitiated.

I'd never read a nuclear physics paper before, so I wasn't sure what to expect. It looks like straight pQFT calculation with the Feynman diagrams, etc. would be computationally intractible for these problems, so people are always looking for reasonable approximation schemes. I guess the ones that had been used in the past didn't factor in relativistic effects as much as they should have, and the recent models corrected this.

"Shape" in nuclear physics

[radtea]

A number of comments posted here are mistaken about the nature of shape in the quantum world. It has nothing to do with the probability distribution of the proton's location; it has to do with the probability distributions of the quarks within the proton.

Only entities that are extended in space have shape. Electrons, for example, are to the best of our knowledge pointlike, and therefore we say they do not have shape. Atoms, on the other hand, are extended in space and therefore do have shape. Their shape is given to them by the probability distributions for the locations of the electrons that surround them, not by the probability distribution of their own location.

Shape is described in nuclear physics using the mathematical device of the "structure function", which is just a function of the three spatial co-ordinates that describes the size and (a)symmetries of the nucleus relative to some interaction. Structure functions are ususally expressed as linearly weighted sums of spherical harmonics or Legendre polynomials, which capture physically interesting processes in different terms.

Protons are believed to have a spherically symmetric structure function. On a number of both theoretical and experimental grounds I'd be extremely doubtful that the dynamical structure function of the proton is asymmetric, and the article in fact suggests that the result is a purely kinematic one due to relativistic effects, which is suprising but in the final analysis not the kind of earth-shattering news dynamical asymmetry would be.

In particular, a dynamical asymmetry of the proton would imply naively a comparable asymmetry of the neutron, and this is known not to exist experimentally at very high precision (and whose existence at a very small level is predicted by electro-weak theory.)

--Tom