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Argos
2010-Jul-08, 05:11 PM
ON Nature (http://www.nature.com/news/2010/100707/full/news.2010.337.html)


The proton seems to be 0.00000000000003 millimetres smaller than researchers previously thought, according to work published in today's issue of Nature (...) The difference is so infinitesimal that it might defy belief that anyone, even physicists, would care. But the new measurements could mean that there is a gap in existing theories of quantum mechanics.

grapes
2010-Jul-08, 06:17 PM
According to the article linked, that's a 4% difference--so large that their window of observation didn't include it at first, and they missed the results.

Argos
2010-Jul-08, 06:24 PM
Seems a big difference, no?

grapes
2010-Jul-08, 06:27 PM
I'm still looking into what it is that is the difference. If it's the difference in the measurements, and they're outside each others error bars, maybe the theory is miscalculating the error bars. Or did the original measurements confirm the theory, and now the latest measurements are disputing the theory?

Ivan Viehoff
2010-Jul-09, 01:13 PM
The point is that these are not very direct measurements. They observe the properties of electrons orbiting protons, and derive the size of the proton based on the quantum theory of electrons. Now they've tried to do it with muons orbiting protons, and that gives a different number. Seems likely that the theory for deducing the size of the proton based upon its effect on orbiting leptons has a hole in it.

tusenfem
2010-Jul-09, 05:58 PM
I think I can download it at work, I will try tomorrow, pm me if you would like it and cannot get it.

mr obvious
2010-Jul-10, 02:29 PM
I haven't read this article, but I'd be somewhat surprised if indirect measurements had errors of less than 4%. I'm not saying it's impossible, but I also recall reading how different groups measured G, the universal gravitational constant, but due to uncertainties, if you aggregate all the measurements and their error bars, some measurements are mutually exclusive. So, I'm taking a wait-and-see approach on this.

Jerry
2010-Jul-12, 05:06 PM
http://www.bobpark.org



PROTON SIZE: IS THAT A CRACK IN THE FOUNDATION?
The only problem we could solve exactly was the hydrogen atom. No matter, we just built the universe out of hydrogen atoms, using quantum electrodynamics (QED), and few approximations to take care of the other stuff, it all worked great -- until now. A group led by R. Pohl at the Paul Scherrer Institute in Switzerland has measured the Lamb shift in muonic hydrogen, in which the electron has been replaced by a negative muon.

That should give a far more accurate measure of the proton width. The problem is it doesn't agree with other methods of determining the proton width. It's too early to speculate about what the problem might be, but I find it reassuring that there are still foundational problems.

ToSeek
2010-Jul-12, 08:25 PM
Thought this blog entry provided a good overview and explanation of the finding:

Protons: Even Smaller Than We Thought (http://scienceblogs.com/principles/2010/07/protons_even_smaller_than_we_t.php)


What's your guess as to the reason?

I'm inclined to think it's in the theory somewhere, but that's mostly because I'm an experimentalist by inclination and training. There's an awful lot of theoretical stuff going into the conversion, and it wouldn't surprise me if six months from now, somebody discovers a small tweak that brings this measurement into line with the others, or brings the other ones in line with this measurement. It's a whopping huge error as such things go-- 64 times the uncertainty they think is associated with the theory-- but I wouldn't be too surprised if that turns out to be unduly optimistic. They mention some other determination that gives results more in line with their result, which may point to something.

Jerry
2010-Jul-12, 10:30 PM
Assuming there is not a gross error in this experiment and the results are reproducible; what should go away is the myth that QCD is 'the most successful theory' about stuff ever. QCD is a system that has needed major modifications in the past (viz asymtonic freedom) and is now in need of an proton engine rebuild.

Since the theory is phenomenologically derived in the first place; we are kind of stuck with WYSIWYG. New shims will be splined in, and everyone wil be happy. Could we ask for anything more?

korjik
2010-Jul-12, 11:32 PM
Assuming there is not a gross error in this experiment and the results are reproducible; what should go away is the myth that QCD is 'the most successful theory' about stuff ever. QCD is a system that has needed major modifications in the past (viz asymtonic freedom) and is now in need of an proton engine rebuild.

Since the theory is phenomenologically derived in the first place; we are kind of stuck with WYSIWYG. New shims will be splined in, and everyone wil be happy. Could we ask for anything more?

Cause everyone knows that a theory must be perfect, and if it isnt theory has to go back to 'Og pick up rock'

Jerry
2010-Jul-13, 04:50 AM
We could have hung onto phlogiston with a couple of dozen of parametric nudges.

Hungry4info
2010-Jul-13, 06:08 AM
Forgive the possibly silly question, but I was under the impression that subatomic particles didn't have actual sizes, but were better approximated as a wave form. If so, what does a reduction in radius actually mean?