12 ms·
Proton’s mass radius is apparently shorter than its charge radius
- JPLeRouzic 4y agoIt looks like the questions about electron's mass at the end of the XIX century. https://en.wikipedia.org/wiki/Electromagnetic_mass https://en.wikipedia.org/wiki/Electromagnetic_mass
- b112 4y agoSome help for you, citizen. https://m.youtube.com/watch?v=fjFaKD9BuOc https://m.youtube.com/watch?v=fjFaKD9BuOc
- DoctorOetker 4y agoI don't see the relevance of your comment, would you mind explaining?
- peterfirefly 4y agoIt's "19th century" in English. Many (all?) Romance languages + a few of the other ones in Europe use Roman numerals for centuries.
- nine_k 4y agoI think it's styled "XIX" and not "19" to make it look archaic.
- int_19h 4y agoIn those languages it is used in all contexts and for all centuries. It's just a formatting convention that is occasionally helpful in e.g. history textbooks.
- simiones 4y agoIt's also common to write XXI century, it's not meant to look archaic. It is only a textual convention for centuries.
- JPLeRouzic 4y agoThanks, English is not my native language, today I learned something!
- peterfirefly 4y agoIt seems almost nobody caught that "Some help for you, citizen" is more or less a quote form the sketch you linked to -- and not an expression of condescension towards the, likely French, person who wrote "XIX".
- narag 4y agoThe proton is a collection of quarks and gluons moving at relativistic speeds around a central point. What? Moving at relativistic speeds? I had never heard that.
- daveguy 4y agoI think that just means "moving fast enough that relativity is significant and has to be accounted for". It doesn't mean they are right at the speed of light (although they could be, I don't know). You could call a rocket "moving a relativistic speeds" because relativity has to be included in the calculations for accurate missions within the solar system.
- frutiger 4y agoGluons are massless so they must move at the speed of light. However they are strongly interacting with quarks and themselves so their worldlines will be a crazy mess of intersections.
- bgirard 4y agoI wonder what the cut off speed is for relativistic speeds. If you think about it, it's not only a matter of speed but also how much precision you want in your calculations. Even if something was moving very slowly, you need to account for relatively if you want very accurate calculations.
- quchen 4y agoA rule of thumb in classical mechanics is that 0.1c is when you should start taking (special) relativity into account, I think it’s 0.5% contribution then. For general relativity I don’t know such a cutoff rule of thumb. Astronomy-wise, Mercury is the only planet that is obviously general-relativistic (its orbit is not an ellipse because it’s so close to the sun). On Earth, we don’t have strong/inhomogeneous enough gravity, so unless you’re synchronizing satellites or atomic clocks, GR is not something to worry about.
- 4y ago
- teovall 4y agoI really wish articles about physics or mathematics, especially ones aimed at non-experts like this one, would include the English name or pronunciation of symbols and terms. Not all of us know Greek or advanced mathematics and I need something to say in my head as I read along. How do you say J/ψ meson?
- archgoon 4y agoJay-sigh
- bgirard 4y agoIt would become redundant very quickly I think. Reading stories about quantum computing feels this way. The first 3 paragraph include the same 'qubits can exists as a 0, 1 or a superposition' over simplification followed by a single paragraph of explanation of the new discovery. IMO it's much easier for you to answer your specific question with an easy search than it is for article authors to anticipate every question and keep answering it in every article.
- BobbyJo 4y agoSomeone use GPT to automatically annotate articles with domain specific knowledge so that I can highlight words/sentences/symbols/formulae/etc. and have it explain it to me.
- bgirard 4y agoThere was a news site that did something like that, I don't recall which. It was incredibly distracting and I think they started annotating ads in their popovers as well.
- BobbyJo 4y agoThis is why it's have to be a separate lyer from the page itself. Otherwise it would become yet more add real estate.
- 4y ago
- drdeca 4y agoThe article refers to a gravitational something tensor, and, I’m wondering whether that is because this is actually dealing with gravitational effects, or whether it is just called that because it is the same tensor (or basically the same, or analogous in some way) to the stress energy tensor? It also isn’t obvious to me what “mass radius” ought to mean... I guess like, the average distance from the center of the mass, but like... Idk, I guess that makes sense..? But I guess I’m not sure what the “meaning” of that quantity would be? Like, what makes that a relevant quantity for describing the system? Is it in case you are hoping to describe some gravitational effects? Or...? Hmm... well, I guess one could ask the same question about the “charge radius”... but that one to me sounds like it would have clearer uses? Like, if you are describing the EM forces on/from a charge, then the charge being distributed over a region would have different results than if it were at a single point I’d think. Though, “the center of the proton” also doesn’t have one single position either.. but I imagine one could kind of separate those two things, comparing “what if we had a point particle with the charge and mass of a proton (which of course would be in a superposition over a range of positions)” to “what if we had a proton, with charge distributed about the center (and the center distributed over a range of positions in the same way as the hypothetical point particle)”
- evanb 4y agoA good cartoon is that a form factor is the function that describes how an object deforms when exposed to an outside influence with a particular momentum. The form factor is a function of momentum. There are many different kinds of outside influence. They can be scalar (think: just increasing the pressure uniformly), vector (put in an electric field), tensor (zap with a gravitational wave), pseudovector (magnetic field), pseudoscalar (zap with a pion). Of course, you can apply a scalar outside influence and a vector at once. But the scalar, vector, tensor, pseudovector, and pseudoscalar labels denote different representations of the Lorentz group [lorentz]. What's more: the Wigner-Eckhart theorem [wigner] basically says [cheat] that the response can be factored into three pieces: the strength of the external influence, a factor that depends only on the representation of the external influence, and a factor that depends only on the property of the thing you're talking about (a proton, in this instance). So people call it the gravitational form factor because if you exposed the proton to a gravitational wave, it's the thing you need to know about the proton to know how it deforms. Note that because of the factorization you don't actually have to zap the proton with a gravitational wave! You can measure it by zapping the proton with other stuff, as long as you can get that stuff to have the right rotational properties or measure the response to many different perturbations and sum the responses the right way to mock up a tensor operator. The experiment at JLab doesn't use gravitational waves, it uses these latter approaches. Roughly speaking at zero momentum the form factor is the charge of the object you measure if it's just sitting there. So the electric form factor evaluated at zero momentum is the electric charge, the gravitational form factor evaluated at zero momentum is the mass. What are radii? Express the form factor as a function of momentum^2 [possible]. In units that physicists like to work in (where c=1, hbar=1), the units of momentum are 1/length. Expand the form factor as a Taylor series in momentum^2 and you will get form factor(p) = charge + # radius^2 p^2 + ... where # is a known dimensionless number. The above story is a cartoon but can be made more-or-less precise depending on how much quantum field theory you learn. lorentz: https://en.wikipedia.org/wiki/Representation_theory_of_the_Lorentz_group https://en.wikipedia.org/wiki/Representation_theory_of_the_L... wigner: https://en.wikipedia.org/wiki/Wigner%E2%80%93Eckart_theorem https://en.wikipedia.org/wiki/Wigner%E2%80%93Eckart_theorem cheat: this is a little bit of a cheat, it's only true to leading order in a taylor series in the strength of the external influence. possible: it's always possible to arrange this, or at least to separate the momentum dependence into a factor dictated by the rotational symmetry properties and another factor dictated by the object, just like in the Wigner-Eckhart theorem.
- raattgift 4y agoSee also <https://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/ https://profmattstrassler.com/articles-and-posts/largehadron...> (2011) Preprint of Nature paper: https://arxiv.org/abs/2207.05212 https://arxiv.org/abs/2207.05212 HTML5 version: https://ar5iv.org/abs/2207.05212 https://ar5iv.org/abs/2207.05212 ("x"->"5" opens up ar5iv.labs.arxiv.org).
- yonixw 4y agoWow! What a rabbit hole finding a proton is not just 3 quarks!. Suggesting these 2 articles: 1] Experimental results of proton collision as of 2013 (3 quarks is an observation, but for low energies) : https://physics.stackexchange.com/questions/81190/whats-inside-a-proton https://physics.stackexchange.com/questions/81190/whats-insi... 2] Another article from the same author (Matt Strassler). Suggesting to read his answers for the comments of: Harry Bostock, Bob Anderson, and the 2 top comments of "aa. sh." (for more history and Neutron decay) https://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/proton-collisions-vs-quarkgluonantiquark-mini-collisions/ https://profmattstrassler.com/articles-and-posts/largehadron...
- hibbelig 4y agoIt would have been nice to get some idea about the relative sizes.
- iamerroragent 4y agoIt's really really tiny and rather incredibly small. https://arxiv.org/abs/2102.00110 https://arxiv.org/abs/2102.00110 " Collaboration data to extract the r.m.s. mass radius of the proton Rm=0.55±0.03 fm. The extracted mass radius is significantly smaller than the charge radius of the proton RC=0.8409±0.0004 fm. "
- gus_massa 4y agoSo 0.55/0.84 = 0.65, i.e. 35% smaller.
- dr_dshiv 4y agoCharge radius is 53% bigger, got it.
- pacaro 4y agoIt's almost as if the charge radius is in miles and the mass radius is in kilometers. It's imperial vs metric all the way down
- plank 4y agoIf charge is surface* effect, and mass a 'volume' effect, you might expect a ratio of 1.0/(0.5^[1/3]). This is 1.26 (or 0.79). Does not seem to fit experiment, even when fiddling with error bars. OK, so no volume vs surface effect then. *Suppose that both gluon and quarks are really in the exact same region, but that the 'effective' behaviour is "on the surface" for one of them, while "in the whole volume" for the other. In three dimensions, the "effective" radio would differ, in one it would be a factor (0.5)^(1/3) smaller.
- hibbelig 4y agoSo the charge radius is about 150% of the mass radius. Thank you.
- AnimalMuppet 4y agoTL;DR: Most of the mass comes from the energy of the strong force interactions of the gluons, not from the quarks. The gluons are not charged; the quarks are. The quarks sometimes go beyond the region of most of the gluons, which gives the proton a larger charge radius than mass radius.
- pavel_lishin 4y ago> The quarks sometimes go beyond the region of most of the gluons, which gives the proton a larger charge radius than mass radius. This is one hypothesis, but not a stated fact in the article.
- AnimalMuppet 4y agoFair. On the other hand, the charge can't be coming from the gluons, nor from the strong force itself, so it kind of has to be coming from the quarks. (Um, unless it's inducing virtual pair production out there, and the destruction mechanism involves the pair getting destroyed by interaction with the quarks, and it takes one of the pair longer than the other... something like that could lead to charge out there farther than the quarks actually go.)
- phkahler 4y agoWhy are gluons needed to hold quarks together. If the 3 are in a line, equally spaced, with the oddball charge in the middle, they will attract quite strongly. +2. -1. +2 If these charges are equally spaced, the attraction of the middle one in stronger than the repulsion of the outer two.
- dguest 4y agoGood point! But it's also an unstable system: the -1 will get attracted to one of the +2 charges and (from a distance) look like a +1, and the other charge will fly off to infinity. I guess we could design a bunch of picometer-scale scaffolds that hold everything in place and this might work, but that doesn't seem to be the way nature put things.
- sebzim4500 4y agoThis is not surprising given my own (presumably horribly wrong) mental image of what a proton is. Is it surprising given current theoretical models?
- mannykannot 4y agoIn your your mental image, was the mass radius necessarily smaller than the charge radius? If so, then your model has survived an empirical challenge. If not, then you have learned something new that was not fully determined or explained by your model.
- etrautmann 4y agoI feel that it’s important to point out that a scientific result doesn’t need to be surprising in order to be important and grounding for future work. I agree this aligns with my intuitions (to the extent that I have any for subatomic particles), but it seems important to document and measure these properties.