Surprise To Physicists -- Protons Aren't Always Shaped Like A Basketball!

Science Daily ^ | 2003-04-08 | Editorial Source

[vannrox]

This is surprising!

[Flightdeck]

I thought that in a high speed accelerator, protons flattened out like pancakes? Also, Doesn't Gell-Mann's liquid droplet model of the nucleus allow for some "floppiness"?

[The_Victor]

Kewl!

[demosthenes the elder]

and wierd. very wierd. one must wonder whether "protonic" shapes might influence nuclear structure in any way, whether certain shapes of "protons" are more closely associated with certain elements than others, and whether the classification of all positively charged nuclear particles as "protons" might need to be revised into a family of diverse particles.
This is quite possibly a revolutionary discovery.
Thanks for posting it.

[petuniasevan]

Ping!

[CollegeRepublican]

I thought that the point of quantum mechanics is that atomic particles only have certian probabilities of existance at any given point and that the Heisenberg uncertianty principle precludes us from actual true measurement of shape?

[Cicero]

Good to know that protons don't just play one kind of ball game.

[Doctor Stochastic]

I seem to remember scattering experiments from about 30 years ago which showed structure within protons. I always had the impression that protons looked like pawn-shop signs, but my sub-atomic intuition is rather miniscule.

[MrLeRoy]

I thought that in a high speed accelerator, protons flattened out like pancakes?

To an observer with respect to whom the proton is moving, yes. The shapes discussed in the article are presumably in the proton's rest frame.

Also, Doesn't Gell-Mann's liquid droplet model of the nucleus allow for some "floppiness"?

Yes. I'm not sure why these results are so surprising; as a bound state of three quarks, it stands to reason that the proton would have excited states as well as a spherical ground state. Maybe the excited states are at lower energies than previously assumed ...

[MrLeRoy]

one must wonder whether "protonic" shapes might influence nuclear structure in any way, whether certain shapes of "protons" are more closely associated with certain elements than others,

Possibly to both.

and whether the classification of all positively charged nuclear particles as "protons" might need to be revised into a family of diverse particles.

Don't bet on it; these nonspherical shapes are probably just excited states of the three-quark bound state that is the proton.

[MrLeRoy]

I thought that the point of quantum mechanics is that atomic particles only have certian probabilities of existance at any given point and that the Heisenberg uncertianty principle precludes us from actual true measurement of shape?

You have to expect a certain amount of oversimplification in a science article written for a general audience. The shapes discussed are presumably the shapes of the surfaces of equal probability density.

[razorbak]

Protons Aren't Always Shaped Like A Basketball!

No wonder they're so hard to dribble.

[lelio]

as well as a spherical ground state
The article seems to imply that a spherical shape isn't the ground state, did I read that right?
If that's the case, does that mean that experiments where you assume that you're bouncing particles off of one another and you're expecting billard ball reactions will be to be re-examined?

[coloradan]

The article seems to imply that a spherical shape isn't the ground state, did I read that right?

I don't think so. The article states a sphere is possible, and makes the analogy to prisoners walking around in a cell. The non-spherical shapes arise from the prisoners walking around and bumping into the walls. In the ground state there is no walking - and hence no bumping.

[MrLeRoy]

In the ground state there is no walking

Not "no" walking, but the slowest walking.

[VadeRetro]

In fact, by 1996 he and two colleagues were ready to publish a paper theorizing the angles at which protons would bounce off electrons after collisions in a nuclear accelerator.

This looks funny. A proton has about 1835 times the mass of an electron. Generally it would be the electron that really goes flying after a collion.

[MrLeRoy]

More oversimplification; I strongly suspect that electrons and protons were both accelerated.

[VadeRetro]

Yes, the momentum of both particles is different, post-collision. The article's wording was funny, however. (" ... The angles at which protons would bounce off electrons ...") When you say you're bouncing cannon balls off of BBs, you imply that the BB is the immovable object and the cannonball is a relatively resistible force.

[demosthenes the elder]

sub-nuclear particle physics is too deep for my little mind, so forgive me if these questions are utterly idiotic:
1. Can the "state" of quarks while bound within the structure of a proton be changed without outside influence? ie: can they go from "excited" to "ground" (? I am assuming these states are at some level comparable to the states of electrons) without some external trigger?
2. If they can make such an untriggered change in state, do they emit anything (some quarkish equivalent of a photon)?
3. If they cannot make such a change, des this not mean that a "lethargic quark" proton is a different beastie than a "lively quark" proton? Wouldn't their behaviors and characteristics be different? (again, the question of proton-shape/element correlation arises) Doesn't that mean that they are not really the same things?

[coloradan]

True. The zero point energy may be why the diameter is non-zero in the ground state. It is certainly why hydrogen atoms are as big as they are (how small they are notwithstanding).