Proton-size puzzle deepens

http://www.nature.com/news/proton-size-puzzle-deepens-1.22760

by digital55 • 9 years ago

108 57 9 years ago

mabbo• 9 years ago

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"?

moomin• 9 years ago

No idea, but I find it amazingly cool that even today with our modern technology, we're finding there's more to be learned about the stuff of high-school physics.

davrosthedalek4• 9 years ago

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.

QAPereo2• 9 years ago

Just to add for GP’s benefit, QCD is monstrously complex, like electric charge x 3, which explains why the achievement would be so enormous.

killjoywashere• 9 years ago

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

(2) https://indico.mitp.uni-mainz.de/event/14/contribution/11/ma...

eesmith2• 9 years ago

As far as I can tell, on your axis it's a curiosity with no Mr. Fusion coming to a DeLorean near you.

If the proton and muon sizes were different, then a bunch of physicists would be chomping away at being able to replace the Standard Model and get the Nobel Prize.

As it stands, it looks like there's a question of why the old measurements were off, and that's about it.

danbruc• 9 years ago

If the proton and muon sizes were different [...]

They are different, in fact the muon is believed to be a point particle and not to have any internal structure like, for example, the proton. Nonetheless one can assign a non-zero radius to the muon in specific contexts due to its interaction with the vacuum.

mattheww• 9 years ago

So there's no obvious practical application (right now).

Also, the proton is not 4% smaller. Protons are obviously whatever size they are.

The discrepancy comes from the fact there are two techniques to measure the proton size. Both experiments do their thing and then there's a way to interpret the results that would tell you the size of the proton (look up proton form factors).

However, when you do the interpretations, which depend on some theoretical calculations, you get different results. The general thinking around this result, because nobody has found any issue with the experimental results, is that there are some additional interactions that are stronger than expected that need to be accounted for (there are some unknown quantities that allow this).

One of the interactions would only affect the muonic hydrogen measurement - basically there are some different interactions between muons and protons than between electrons and protons because of the muon's mass and those might be different than originally thought.

The other is a type of interaction that could affect both normal and muonic hydrogen. This new measurement shows that the interactions that affect both has to play an important role in understanding this discrepancy. There are other measurements trying to measure this effect independently (not using hydrogen at all).

andrepd• 9 years ago

No, the radius of the proton being smaller than previously thought does not directly lead to any technological developments. However...

It does raise curious questions, namely what is it that we missed that lead us to believe the previous results. If the proton is indeed so much smaller, what is it that was skewing the results of the previous experiments? Perhaps this signals new physics and strange new effects.

And that may eventually lead to cold fusion and jetpacs :)

tim3332• 9 years ago

The "Oh my god" is more that the laws of physics as we have them could be wrong. Might not mean much but experiments like Michelson–Morley not agreeing with the understandings of the time have had big effects.

andy_ppp• 9 years ago

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

Koshkin8• 9 years ago

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.

SAI_Peregrinus• 9 years ago

I feel it necessary to point out that the proton is only an aggregate of three other particles on average. It's three quarks, plus several gluons mediating interactions between the quarks, and lots of virtual quark-antiquark pairs.

Koshkin8• 9 years ago

... and the muon that "spends more time inside the nucleus", according to the article.

(You direct your sarcasm at an easy target; it is also too easy to turn a blind eye to all the glaring omissions and gross misrepresentations - i.e., effectively, lies - purposefully made in popular texts for the sake of making the material more "accessible" for the general reader.)

beezle• 9 years ago

It might be more correct to say three valence quarks?

kobeya4• 9 years ago

Technically nothing has "size" in the sense that you are intuitively defining it to be. All matter in the universe is collections of point-like (but also wave-like) particles without defined shape. It'd be totally fair to throw the critique back at you.

Koshkin8• 9 years ago

I don't think it would - for the obvious reason that when we talk about the size of, say, a desk, there is never a concern about its meaning, and we rarely, if ever, think about the object itself in terms of waves or particles. Protons etc., on the other hand, is a totally different matter, and since everyday notions cannot be trivially applied to such objects it becomes important to be clear concerning the meaning we choose to give to the word.

oldandtired• 9 years ago

Technically, the point-like properties come out of the mathematics used, not the experimental measured. The problem with point-like is that this brings singularities to the fore when we do not measure singularities.

Singularities makes the mathematics simpler (an approximation to the reality being worked with). Keep in mind that QM and all of its associated trappings are approximations (useful but still approximations). This is no different to the use of mathematics in any other field.

om2• 9 years ago

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

Koshkin8• 9 years ago

the root mean square of the electron scattering cross-section

Thank you, and I wish the article had said that.

davrosthedalek4• 9 years ago

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)).

dmitriid• 9 years ago

It's a pop-sci article, and it does a wonderful job explaining what's going on

BugsJustFindMe• 9 years ago

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

The "normal sense" for _any_ measured value, in order to be considered a reliable measurement, whether it be length, weight, or number of electrons contained within, must necessarily be established as an average of independent samples sufficient to narrow the measurement error to within the desired precision boundaries. You might as well say that you don't have a height in the normal sense because some days you stand slightly more erect than others, except that would be a misunderstanding of "normal sense" and also "height". If you can only reliably measure protons along a single particular dimensional axis, and then only on average through sampling, then you have no way of saying that it isn't actually, factually, literally a sphere, and using spherical measurement terms is reasonable. If you want, feel free to mentally prepend the word "approximately" any time you read any measurement of anything. It will always be slightly more accurate, even when just counting things.

Also https://en.wikipedia.org/wiki/Spherical_cow

ggm3• 9 years ago

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.

idlewords• 9 years ago

I don't understand this question. Why would the muon's mass change the error rate?

ggm3• 9 years ago

because the imputed 'radius' of the proton in normal matter is a function of its interactions with electrons in orbit and its own properties. If you replace an electron with a Muon which orbits but is 400x more massive, then some element of distortion has to come into play. Its radii is now a function of its innate properties and an orbiting body 400x more massive than normal.

Therefore, I posit (plausibly wrongly btw, I am not a physicist)_ that there is some measurement inaccuracy which stems from this, which is a function of this 400x size difference.

monochromatic• 9 years ago

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.

Retra• 9 years ago

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

SubiculumCode• 9 years ago

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

comnetxr• 9 years ago

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.

Proton-size puzzle deepens

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