
Proton-size puzzle deepens - digital55
http://www.nature.com/news/proton-size-puzzle-deepens-1.22760
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Koshkin
The article keeps talking about "the size of the proton", its "radius", etc.,
but I would like to point out that the proton is not a sphere and does not, in
fact, have a size in the normal sense, being an aggregate of three other
particles; so in this case the notion of "size" itself must be _defined_ in
some pragmatic fashion before any meaningful statements about it can be made.

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om2
It's true that there's a sense in which the size of the proton is undefined.
But there is also a sense in which it is very precisely defined, which is the
root mean square of the electron scattering cross-section. This can be
measured very accurately. And that's what the article is talking about.

[https://en.wikipedia.org/wiki/Charge_radius](https://en.wikipedia.org/wiki/Charge_radius)

~~~
davrosthedalek
While you can measure it via scattering, it's not the square root of the
scattering cross section. The radius is defined via the slope of the proton
electric form factor at Q^2=0. This translates to the root mean square radius,
that is the sqrt of the integral of r^2 weighted with the density, i.e. sqrt (
Integral r^2 rho(r)).

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dboreham
Hmm...perhaps it has been edited since but that's exactly what the parent
says.

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mabbo
Can someone who is more well-studied in physics tell me: what are the
implications of this? Is it a curiosity, but of no practical matter? Or is it
"Oh my god, a 4% smaller proton could mean cold fusion and jetpacks"?

~~~
davrosthedalek
The proton is very fundamental. Almost all of the visible matter around us is
either protons or neutrons, and their mass is almost completely generated
dynamically from QCD (forget Higgs). Its size and mass are cornerstones. Being
able to calculate them from first principles would be an enormous achievement.

For me, it's mainly interesting because two different fields of physics meet,
nuclear physics (electron scattering) experiments, and atomic physics
(spectroscopy).

Otherwise, it's the same as with all basic science. We don't know what it is
"good for" until somebody figures out how to cure cancer with it.

The paper this article refers to is in particular interesting, because it
finds a value not in agreement with earlier measurements of the /same/ type.
The indicated 3.3 sigma shift in the Rydberg constant, one of the most exact
measured quantities in existence, is a little bit worrying, but such shifts
happen more often than they should.

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killjoywashere
I'm a physics major, and still in science 20 years later, cancer research. I
originally came here to ask, in general, why would anyone care about this
result?

Since you asserted an answer to the question before I managed to ask, I'm
happy to subordinate my question. But I will also suggest your specific
answer, cure cancer, won't work. I need to kill cells, which requires a
cascade of large molecules interacting at energies on the order of a fraction
of an eV, or massive amounts of high energy radiation.

I can kill cells directly with high energy radiation, however, the energies
for this investigation, the hydrogen 2s-4p transition, (1,2) are trivial (486
nm is visible light). Also, radiotherapy isn't really good at interrogating
cell type, the current standard for new cancer therapies (immunotherapy).

(1)
[http://science.sciencemag.org/content/358/6359/79](http://science.sciencemag.org/content/358/6359/79)

(2) [https://indico.mitp.uni-
mainz.de/event/14/contribution/11/ma...](https://indico.mitp.uni-
mainz.de/event/14/contribution/11/material/slides/0.pdf)

~~~
TheSpiceIsLife
I don't think the parent comment meant that literally.

Sometimes we don't know what new knowledge will enable us to do until the
PhDs, engineers, and technicians get their hands on it.

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SubiculumCode
Are the old measurement method still being used today? What a wonderful oddity
it would be if those measurements changed as well.

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comnetxr
My interpretation of this is that the muon measured radius of the proton is
the correct one. This rules out the speculation that the muon measurement was
accessing some sort of new physics which caused it to be 4% off. The key quote
is here:

"Some researchers speculated that perhaps some previously unknown physics
could make muons act differently than electrons. This would have required a
revision of the standard model of particle physics, which predicts that muons
and electrons should be identical in every way except for their masses — and
might have pointed to the existence of yet-to-be-discovered elementary
particles."

This is the reason for this precision research: it could give you the hint for
where to look for new physics. This result, far from "deepening the proton-
size puzzle", says that muons do not have any new physics. In other words,
it's a null result.

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ggm
If the muon is 400x more massive, then is there not a f(400x) error rate,
which has to be imputed to the radius of the proton, influenced by the
significantly larger mass being made to act as an electron in orbit around it?

Radius being understood to be somewhat metaphorical here.

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monochromatic
Why would the error be linear in the mass of the electron or muon? I mean,
maybe it is... but it’s not at all obvious to me why that would be so.

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Retra
They didn't say the error was linear in the mass, only that it was
functionally dependent on the mass in some manner.

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andy_ppp
Slightly off topic but I strongly recommend subscribing to this amazing
YouTube channel: PBS Space Time. It'll give a bit of background and I've
enjoyed learning it.

[https://youtu.be/z3rgl-_a5C0](https://youtu.be/z3rgl-_a5C0)

