12 ms·

Protons are probably smaller than long thought

211 points by hhs 5y ago

brabel 5y ago
I wish they explained how exactly the fact that protons are 5% smaller than previously thought actually affects our current knolwedge. Or it just doesn't?
- davrosthedalek 5y ago
  (Disclaimer: I work in this field, having done of the original measurements. I do not believe this is a settled case at all.) If the proton is indeed smaller, it changes the Rydberg constant by several sigma. The Rydberg constant is one if not the best determined constant of nature. This has implications for precision tests of QED, for example.
  - H8crilA 5y ago
    What is even the size of a proton? I mean it's not like it's a nice spherical ball [made of something else]. From reading the article it seems like there's a hard distance boundary beyond which it will not "collide" with the electron?
    - dataflow 5y ago
      > What is even the size of a proton? Just a layman but I think they usually define size via the halfway distance between the centers of two identical bound particles. Not entirely sure what that would be in this case though, given helium-2 is unstable and helium-3 might give a different result. (?)
      - gowld 5y ago
        Protons don't bind without neutrons mixed in.
        dataflow 5y ago
        That's why I wrote He-2 is unstable, yes.
        vikingerik 5y ago
        They do for a very short duration (10^-9 s.) Nuclear fusion happens by protons binding via the strong force, then one beta-decays into a neutron before the electromagnetic repulsion pushes them beyond the range of the strong force. The beta decay happens fast enough in 0.01% of proton-proton collisions.
      - contravariant 5y ago
        As far as I know the size in these discussions is some kind of scattering cross-section. So roughly it's just how likely you are to hit it / how precise you need to aim. I don't know the exact details of the definition though.
        davrosthedalek 5y ago
        That's one way to determine the size. But the same thing pops up in spectroscopy.
      - whatshisface 5y ago
        You are right about the atomic radii often quoted in chemistry, but the sizes of subatomic particles are usually defined in ways related to their cross-sections.
        dataflow 5y ago
        Cool to know, thanks!
        dr_dshiv 5y ago
        But isn't the cross section dependent upon what is interacting with them? Like, my understanding is that the cross section of hafnium is massive with respect to neutron capture, but not with respect to photons.
        whatshisface 5y ago
        Although the size isn't simply the square root of the cross section, you've hit on a truth that you'd get different sizes if you were asking about the distributions of different things.
    - davrosthedalek 5y ago
      In this context, it's the root-mean-square of the electric charge distribution. (Or more precisely, it's related to the slope of the electric form factor at Q^2=0.) There is also at least a magnetic radius (similar to the electric), and a gravitational radius.
      - nobrains 5y ago
        Does the electrical charge ever reach 0 or does it keep getting less and less and for practical purposes, we have to set a cut-off limit and count the boundary of the proton from there?
        davrosthedalek 5y ago
        In some sense, the extend is infinite -- but the radius is not the "distance to cut-off". It's the root-mean-square average, which makes it finite.
        analog31 5y ago
        As we understand it, the extent is infinite. However, isolated charges have a hard time staying isolated. They tend to rearrange the charges around them, so that the net effect of their charge becomes effectively zero if you define a vicinity for other particles. I believe the term for this is "shielding."
  - MrLeap 5y ago
    Is there.. any reason why all protons must be the same size?
    - stronglikedan 5y ago
      layman, but I believe it's the "size" (defined by the wave function) that makes a particle a proton. at least, one of the factors that does, anyway
    - gowld 5y ago
      Protons are made of 3 quarks and a bunch of tiny gluons. if a big proton existed, it would be made of different or more quarks/stuff, and be called something else. Now, is there a reason why there isn't a something else that acts like a proton? That gets into Elementary particle physics and what combinations are stable and have matching charge. Quarks have charge +- n/3 and generally come in triples. Could the number of gluons vary? Maybe? But they wouldn't affect the size measurements much? And variations wouldn't be stable?
    - db48x 5y ago
      Yes. All protons are indistinguishable from each other. No matter what aspect of a proton you measure, it will be identical to any other proton you could measure instead. This has important implications. Consider a quantum mechanics experiment where you emit a proton at A and try to detect it at A′. You will find that that there is some quantifiable chance of detecting the particle at A′ some time T. Call that probability P. Now consider a second experiment where you emit a proton at both A and B, and try to detect them at A′ and B′. What is the probability of finding a proton at A′ at time T? You will find that you get a different number! This is because the proton at A could travel to A′ and be detected, but also the proton at B could travel to A′ and be detected too. Since you cannot distinguish the two protons, you won’t be able to distinguish between these two outcomes, and so the probability must be different from P.
      - RobotCaleb 5y ago
        Does this mean that there's possibly only one proton?
  - saurik 5y ago
    (Disclaimer: I don't do anything even slightly related to this field, and I somehow managed to skip taking any physics in college except quantum, which I literally slept through and dropped because it was too early in the morning three times before simply giving up. So, my question is probably super super dumb ;P.) Isn't a sigma a lot? Like, I think that's a standard deviation? If there is even a longshot chance that we are might be off on that constant by multiple standard deviations, isn't that certainly a less-determined constant than, say, the acceleration of gravity? I feel like there is no chance in hell we could one day discover that our calculations are that far off for gravity.
    - jacksnipe 5y ago
      The acceleration due to gravity isn’t a constant of nature —- it’s dependent on the mass and mutual distance of both objects. EDIT: distance, not size
      - moron4hire 5y ago
        Perhaps they meant the gravitational constant, which is not known to a very high degree of precision.
        saurik 5y ago
        But we've got to have it down within two standard deviations, right? ...No? Do I just not know what a sigma is? :(
        penteract 5y ago
        I think you have a misunderstanding about sigma. When describing the measurement of a particular physical constant, the "standard deviation" is something that changes as we get better at making measurements. It basically means "If all our assumptions (e.g. assumptions about how good our equipment is, uncertainties about other physical constants) are correct, then it is unlikely that we would have made the measurements we did if the true value is not within 2 standard deviations of the result we got". When a more accurate measurement is made, then "1 standard deviation" gets smaller, so we know the value better, but it's always true to say "we know the value to within a few standard deviations (given some assumptions made by experimenters)" . If it turns out the measurement was wrong by several standard deviations, then it's very likely that some assumptions were wrong.
  - deleted 5y ago
    [deleted]
- brian-armstrong 5y ago
  Most likely it indicates protons have devalued their currency slightly. Probably not a big deal for now but we should keep a close eye on it in case it happens again.
  - AnimalMuppet 5y ago
    Cosmic inflation?
- bonzini 5y ago
  We had two methods to measure the proton and they gave different results. People used to trust the old value more, but a study in 2019 implied that it was the incorrect one and the correct value is the new one. See https://en.wikipedia.org/wiki/Proton_radius_puzzle https://en.wikipedia.org/wiki/Proton_radius_puzzle, https://www.quantamagazine.org/physicists-finally-nail-the-protons-size-and-hope-dies-20190911/ https://www.quantamagazine.org/physicists-finally-nail-the-p.... The latter says "The new result implies that earlier attempts to measure the proton’s radius in electronic hydrogen tended to overshoot the true value. It’s unclear why this would be so" and it seems that these researchers have now shown why.
- fsh 5y ago
  The value of the proton charge radius in itself is pretty much irrelevant, otherwise it wouldn't be so hard to measure. As scientists we want to know if our understanding of nature is correct. To test this, we measure the same quantity, for example the proton charge radius, in different ways. If the underlying theory is correct, the results should agree within the experimental uncertainties. Since 2010 there was a big disagreement between the proton charge radii measured by hydrogen spectroscopy and electron/proton scattering (which roughly agreed at the time), and a much more accurate measurement using muonic hydrogen spectroscopy. This has lead to a lot of excitement, since discrepancies could be a hint for new physics. Since then, more accurate hydrogen spectroscopy experiments have been performed and most agree with the muonic hydrogen value. This probably indicates that the discrepancy is due to underestimated error bars in the old measurements. In contrast to laser spectroscopy which gives relatively direct results, getting the charge radius out of electron scattering data is notoriously hard. Different groups have found different charge radii from the same data for a long time.
justinator 5y ago
The difference is .04 femtometers and a femtometer is one quadrillionth of a meter. So, I'll be sitting down for a little while.
- munk-a 5y ago
  Did you happen to know that the difference between 1 lightyear and 1.000001 light years is 9.4605284 × 10^24 femtometers? That means they're not at all similar distances on an astronomic scale!
  - justinator 5y ago
    Not sure why my og comment was downvoted. In absolute terms, the difference in size of the proton is quite large. In relative, the difference in size is quite small. And that blows my little sapien mind. And that was all.
  - YaBomm 5y ago
moron4hire 5y ago
Is this going to mean going back and looking at a bunch of old experiments and slapping your forehead and saying, "god dammit, THAT'S why it didn't work" like I do with my code when I find a wrong sign or off by one error? Or even worse, saying "how the hell did that ever work?" That one... ooooh, that one.
- kspacewalk2 5y ago
  > Or even worse, saying "how the hell did that ever work?" The most unsettling feeling. I often have that feeling when doing the devops and infrastructure part of my job, e.g. encountering paradoxes in the dark dreary bowels of systemd. The idealist and perfectionist in you wants to keep digging deeper to arrive at a proper understanding. The realist and lazy SOB in you wants to slowly back away and pretend this never happened. This dialectic hopefully guides you toward a happy compromise, somewhere down the middle.
  - Karunamon 5y ago
    Worse, you dig at it for too long and collapse the wave function, and it never works again despite you having made no changes.
    - mensetmanusman 5y ago
      Because the code you are looking at isn’t what is compiled and deployed, doh!
  - anyfoo 5y ago
    The more critical the infrastructure you work on, the less you will be willing to take the "slowly back away" route. Because as long as you don't understand every single detail of the failure, that feeling of "what horrible thing do I not understand here" won't go away, and you dread having a major "oh... crap, so that's what I did not understand" moment when it's inevitable. This week, we had such a failure when testing a change. The root cause was found eventually, but what two extremely good engineers plus me still could not figure out was why that problem was only surfacing now, after years of having that buggy code in there. Very important code. I did not want to risk some obviously unknown property of it be our demise later on. Fortunately, in this case it just turned out that the seemingly unrelated changes did, after all, hold the now failing thing differently than before. As it had been used, the problem was masked entirely, and the code did work well for all those years.
asdfman123 5y ago
I misread this as "pronouns." I need to spend less time on Twitter.
Victerius 5y ago
What are quarks made of?
- labster 5y ago
  Snips, snails, and gold-pressed latinum. Or possibly strings. If that’s not true, the best answer is the tautology: quarks are made of quarks.
- 323 5y ago
  Quarks are excitations in the quark fields. Thinking about them as particles is a useful simplification/model. Analogy: there are no "red pixels", there are just red excitations in the display RGB field.
- davrosthedalek 5y ago
  As far as we know, quarks are not composite particles. They are fundamental (and point-like). Same as electrons, photons, etc.
sundarurfriend 5y ago
Ignorant outsider to the field here: I've often heard the claim that quantum mechanics has been verified to amazing accuracy, that its predictions match with reality to the maximum degree that our instrumental precision allows, etc. A 5% difference seems big enough that at least some of our experiments should have error limits less than that. So how is it that this is only now being found, and there's still uncertainty surrounding it? I feel like I'm missing something fundamental here, and I'd like to know what it is.
- Yajirobe 5y ago
  Quantum theory has the largest error between theory and prediction (120 orders of magnitude). See https://en.wikipedia.org/wiki/Cosmological_constant_problem https://en.wikipedia.org/wiki/Cosmological_constant_problem
  - evanb 5y ago
    That’s just a back-of-the-envelope calculation. Nobody knows how to do a bone-fide calculation because we don’t have an accepted quantum theory of gravity.
    - mcswell 5y ago
      I guess that's why they invented slide rules. I could get within 3 significant digits on a slide rule (and compute the exponent in my head), whereas I guess the margin of this envelope is too small to contain the calculation.
- gowld 5y ago
  Well this 5% is the instrument precision.
- db48x 5y ago
  There are lots of different numbers that you can use quantum mechanics to predict. It turns out that the size of the proton is both harder to predict and harder to measure than many of those other numbers. Meanwhile we can measure the fine structure constant to 12 decimal places (https://en.wikipedia.org/wiki/Electromagnetic_coupling_constant#History_of_measurements https://en.wikipedia.org/wiki/Electromagnetic_coupling_const...) and that measurement is in very close accord with the predictions of quantum mechanics.
miika 5y ago
https://hiup.org/wp-content/uploads/2017/05/Addendum_QGHM_2.pdf https://hiup.org/wp-content/uploads/2017/05/Addendum_QGHM_2....
- fsh 5y ago
  This is not even wrong [1]. [1] https://en.wikipedia.org/wiki/Not_even_wrong https://en.wikipedia.org/wiki/Not_even_wrong
  - miika 5y ago
    It’s easy to drop that link. Much harder to deny the math that works perfectly. But probably it’s even harder to convince you to take serious look at it :)
ur-whale 5y ago
What does it even mean? We're at quantum scales and the very notion of "size" is rather ill-defined. The position of a proton, is - as a matter of fact - a probabilistic affair, so when you talk about size ... if you don't even know where the darn thing is, how are you going to measure where it starts and where it ends ...
- fsh 5y ago
  It is a misconception that things are ill-defined just because they are at a quantum scale. The proton has an electric charge distribution. The definition of the proton size used here is the root mean square charge radius of that charge distribution.
  - davrosthedalek 5y ago
    Indeed, but it's actually a little bit hard to define what that charge distribution is (or in which frame...). The way it shows up, both in the cross section for elastic lepton scattering as well as in spectroscopy, is via the related quantity "form factor", as the slope at zero four-momentum transfer. While the form factor can be thought as the Fourier transform of the charge distribution for heavy objects (say, iron nuclei), for the proton, this becomes dicey.
ffhhj 5y ago
What if the size of the proton is variable?
- YaBomm 5y ago
mrlonglong 5y ago
What would it mean if they were actually 0.88 when first measured and at 0.84 now?
- swayvil 5y ago
  Either the protons shrank or the yardstick grew.
  - mrlonglong 5y ago
    But if protons shrank, wouldn't the yardstick also shrink seeing as it's also composed of protons? These measurements taken didn't use a physical yardstick.
ummonk 5y ago
So to summarize, standard model theory predicted that the radius of the proton would be 0.84 femtometers. High energy electron scattering experiments from the 90s and 2000s suggested it was more like 0.88 femtometers, which was a large discrepancy that causes some consideration that the standard model would have to be revised. These researchers performed a reanalysis of the data correcting for some confounding phenomena (might have been neutron formation) to suggest that the old experimental data was consistent with the 0.84 femtometers predicted by both theory and by newer lower energy scattering experiments. I’ll leave it as an exercise for the reader to decide whether the title of the article is clickbait.
- ISL 5y ago
  It's not clickbait. I've been to oodles of talks on the problem. It has had special sessions at APS meetings and I've watched acquaintances and colleagues expend meaningful fractions of their careers attempting to resolve the Proton Radius Puzzle. If an appeal to authority is required -- Nature gave it the cover. https://www.psi.ch/en/media/our-research/protons-smaller-than-we-thought https://www.psi.ch/en/media/our-research/protons-smaller-tha... Pohl's measurements were incontrovertible, yet they disagreed with decades of work. It has been a big problem in a quiet community for some time. If someone has a reliable angle to resolve the problem, it is big news. Enough really good physicists have tried and failed to resolve the conundrum that one should wait to see if this new approach holds up, too.
  - ummonk 5y ago
    The headline gives the impression that the researchers found a new discrepancy where protons are found to be smaller than theory would predict. In actuality, the researchers are proposing a resolution to a preexisting discrepancy. Additionally, I’m not sure why you’re saying we should wait to see whether the results hold up, if you don’t think writing “probably” in the headline is clickbait.
    - deleted 5y ago
      [deleted]
pomian 5y ago
I thought that someone had a measure for a thought and that a long thought was measurably shorter than a short(?) thought, by the length of a proton. This then went off into a consideration of neural message length, etc. Cosmic black hole of thought in a spilt second.
zw123456 5y ago
OK, another dumb question from a neophyte here, doesn't the Heisenberg uncertainty principle make it impossible to know the size of a proton for sure? It seems like if you cannot not know the position and momentum of a particle then measuring its size exactly would be impossible? Thanks in advance to anyone willing to take the time to give me an ELI5 on this :)
- ninkendo 5y ago
  I'm an ignorant layman here, but the uncertainty principal only says that if you're sure about the size/location, then you're very unsure about the momentum. It's possible that measuring its size does not need much certainty about its momentum, so they can get the accuracy they want.
- Bootvis 5y ago
  If I'm blindly shooting a lot of bullets at a lot of ducks, I should be hitting some ducks. If I then count the number of shots fired and the number of ducks dead on the ground I should be able to calculate the size of ducks with high precision.
  - J5892 5y ago
    A better analogy would be to subtract the amount of bullets embedded in the wall behind the ducks from the total bullets fired. Assuming ducks stop the bullets. And assuming it's not just one immortal duck flying around super fast.
- whatshisface 5y ago
  How big is the smell of a baking pie? The concentration of pie molecules in the air is not a binary yes/no function of position, but at the same time the volume is definitely smaller than the county, and larger than the kitchen. Fuzzy-edged objects have some sense of size but there is some freedom in where to define their edge. For the proton, the "size" is defined as a length-valued parameter in the function that expresses the charge distribution through space. The parameter is objective - but its association with the word "size" is imprecise. It is necessarily imprecise, not because of anything quantum or even unfamiliar, but because there's not another English word for the scale of objects without hard edges.
mcswell 5y ago
Looking at the picture in the article, I'd say the proton wasn't too small, rather the ruler was too big.

211 points by hhs 5y ago

I wish they explained how exactly the fact that protons are 5% smaller than previously thought actually affects our current knolwedge. Or it just doesn't?

davrosthedalek 5y ago
(Disclaimer: I work in this field, having done of the original measurements. I do not believe this is a settled case at all.) If the proton is indeed smaller, it changes the Rydberg constant by several sigma. The Rydberg constant is one if not the best determined constant of nature. This has implications for precision tests of QED, for example.
- H8crilA 5y ago
  What is even the size of a proton? I mean it's not like it's a nice spherical ball [made of something else]. From reading the article it seems like there's a hard distance boundary beyond which it will not "collide" with the electron?
  - dataflow 5y ago
    > What is even the size of a proton? Just a layman but I think they usually define size via the halfway distance between the centers of two identical bound particles. Not entirely sure what that would be in this case though, given helium-2 is unstable and helium-3 might give a different result. (?)
    - gowld 5y ago
      Protons don't bind without neutrons mixed in.
      - dataflow 5y ago
        That's why I wrote He-2 is unstable, yes.
      - vikingerik 5y ago
        They do for a very short duration (10^-9 s.) Nuclear fusion happens by protons binding via the strong force, then one beta-decays into a neutron before the electromagnetic repulsion pushes them beyond the range of the strong force. The beta decay happens fast enough in 0.01% of proton-proton collisions.
    - contravariant 5y ago
      As far as I know the size in these discussions is some kind of scattering cross-section. So roughly it's just how likely you are to hit it / how precise you need to aim. I don't know the exact details of the definition though.
      - davrosthedalek 5y ago
        That's one way to determine the size. But the same thing pops up in spectroscopy.
    - whatshisface 5y ago
      You are right about the atomic radii often quoted in chemistry, but the sizes of subatomic particles are usually defined in ways related to their cross-sections.
      - dataflow 5y ago
        Cool to know, thanks!
      - dr_dshiv 5y ago
        But isn't the cross section dependent upon what is interacting with them? Like, my understanding is that the cross section of hafnium is massive with respect to neutron capture, but not with respect to photons.
        whatshisface 5y ago
        Although the size isn't simply the square root of the cross section, you've hit on a truth that you'd get different sizes if you were asking about the distributions of different things.
  - davrosthedalek 5y ago
    In this context, it's the root-mean-square of the electric charge distribution. (Or more precisely, it's related to the slope of the electric form factor at Q^2=0.) There is also at least a magnetic radius (similar to the electric), and a gravitational radius.
    - nobrains 5y ago
      Does the electrical charge ever reach 0 or does it keep getting less and less and for practical purposes, we have to set a cut-off limit and count the boundary of the proton from there?
      - davrosthedalek 5y ago
        In some sense, the extend is infinite -- but the radius is not the "distance to cut-off". It's the root-mean-square average, which makes it finite.
      - analog31 5y ago
        As we understand it, the extent is infinite. However, isolated charges have a hard time staying isolated. They tend to rearrange the charges around them, so that the net effect of their charge becomes effectively zero if you define a vicinity for other particles. I believe the term for this is "shielding."
- MrLeap 5y ago
  Is there.. any reason why all protons must be the same size?
  - stronglikedan 5y ago
    layman, but I believe it's the "size" (defined by the wave function) that makes a particle a proton. at least, one of the factors that does, anyway
  - gowld 5y ago
    Protons are made of 3 quarks and a bunch of tiny gluons. if a big proton existed, it would be made of different or more quarks/stuff, and be called something else. Now, is there a reason why there isn't a something else that acts like a proton? That gets into Elementary particle physics and what combinations are stable and have matching charge. Quarks have charge +- n/3 and generally come in triples. Could the number of gluons vary? Maybe? But they wouldn't affect the size measurements much? And variations wouldn't be stable?
  - db48x 5y ago
    Yes. All protons are indistinguishable from each other. No matter what aspect of a proton you measure, it will be identical to any other proton you could measure instead. This has important implications. Consider a quantum mechanics experiment where you emit a proton at A and try to detect it at A′. You will find that that there is some quantifiable chance of detecting the particle at A′ some time T. Call that probability P. Now consider a second experiment where you emit a proton at both A and B, and try to detect them at A′ and B′. What is the probability of finding a proton at A′ at time T? You will find that you get a different number! This is because the proton at A could travel to A′ and be detected, but also the proton at B could travel to A′ and be detected too. Since you cannot distinguish the two protons, you won’t be able to distinguish between these two outcomes, and so the probability must be different from P.
    - RobotCaleb 5y ago
      Does this mean that there's possibly only one proton?
- saurik 5y ago
  (Disclaimer: I don't do anything even slightly related to this field, and I somehow managed to skip taking any physics in college except quantum, which I literally slept through and dropped because it was too early in the morning three times before simply giving up. So, my question is probably super super dumb ;P.) Isn't a sigma a lot? Like, I think that's a standard deviation? If there is even a longshot chance that we are might be off on that constant by multiple standard deviations, isn't that certainly a less-determined constant than, say, the acceleration of gravity? I feel like there is no chance in hell we could one day discover that our calculations are that far off for gravity.
  - jacksnipe 5y ago
    The acceleration due to gravity isn’t a constant of nature —- it’s dependent on the mass and mutual distance of both objects. EDIT: distance, not size
    - moron4hire 5y ago
      Perhaps they meant the gravitational constant, which is not known to a very high degree of precision.
      - saurik 5y ago
        But we've got to have it down within two standard deviations, right? ...No? Do I just not know what a sigma is? :(
        penteract 5y ago
        I think you have a misunderstanding about sigma. When describing the measurement of a particular physical constant, the "standard deviation" is something that changes as we get better at making measurements. It basically means "If all our assumptions (e.g. assumptions about how good our equipment is, uncertainties about other physical constants) are correct, then it is unlikely that we would have made the measurements we did if the true value is not within 2 standard deviations of the result we got". When a more accurate measurement is made, then "1 standard deviation" gets smaller, so we know the value better, but it's always true to say "we know the value to within a few standard deviations (given some assumptions made by experimenters)" . If it turns out the measurement was wrong by several standard deviations, then it's very likely that some assumptions were wrong.
- deleted 5y ago
  [deleted]
brian-armstrong 5y ago
Most likely it indicates protons have devalued their currency slightly. Probably not a big deal for now but we should keep a close eye on it in case it happens again.
- AnimalMuppet 5y ago
  Cosmic inflation?
bonzini 5y ago
We had two methods to measure the proton and they gave different results. People used to trust the old value more, but a study in 2019 implied that it was the incorrect one and the correct value is the new one. See https://en.wikipedia.org/wiki/Proton_radius_puzzle https://en.wikipedia.org/wiki/Proton_radius_puzzle, https://www.quantamagazine.org/physicists-finally-nail-the-protons-size-and-hope-dies-20190911/ https://www.quantamagazine.org/physicists-finally-nail-the-p.... The latter says "The new result implies that earlier attempts to measure the proton’s radius in electronic hydrogen tended to overshoot the true value. It’s unclear why this would be so" and it seems that these researchers have now shown why.
fsh 5y ago
The value of the proton charge radius in itself is pretty much irrelevant, otherwise it wouldn't be so hard to measure. As scientists we want to know if our understanding of nature is correct. To test this, we measure the same quantity, for example the proton charge radius, in different ways. If the underlying theory is correct, the results should agree within the experimental uncertainties. Since 2010 there was a big disagreement between the proton charge radii measured by hydrogen spectroscopy and electron/proton scattering (which roughly agreed at the time), and a much more accurate measurement using muonic hydrogen spectroscopy. This has lead to a lot of excitement, since discrepancies could be a hint for new physics. Since then, more accurate hydrogen spectroscopy experiments have been performed and most agree with the muonic hydrogen value. This probably indicates that the discrepancy is due to underestimated error bars in the old measurements. In contrast to laser spectroscopy which gives relatively direct results, getting the charge radius out of electron scattering data is notoriously hard. Different groups have found different charge radii from the same data for a long time.

justinator 5y ago

The difference is .04 femtometers and a femtometer is one quadrillionth of a meter. So, I'll be sitting down for a little while.

munk-a 5y ago
Did you happen to know that the difference between 1 lightyear and 1.000001 light years is 9.4605284 × 10^24 femtometers? That means they're not at all similar distances on an astronomic scale!
- justinator 5y ago
  Not sure why my og comment was downvoted. In absolute terms, the difference in size of the proton is quite large. In relative, the difference in size is quite small. And that blows my little sapien mind. And that was all.
- YaBomm 5y ago

moron4hire 5y ago

Is this going to mean going back and looking at a bunch of old experiments and slapping your forehead and saying, "god dammit, THAT'S why it didn't work" like I do with my code when I find a wrong sign or off by one error? Or even worse, saying "how the hell did that ever work?" That one... ooooh, that one.

kspacewalk2 5y ago
> Or even worse, saying "how the hell did that ever work?" The most unsettling feeling. I often have that feeling when doing the devops and infrastructure part of my job, e.g. encountering paradoxes in the dark dreary bowels of systemd. The idealist and perfectionist in you wants to keep digging deeper to arrive at a proper understanding. The realist and lazy SOB in you wants to slowly back away and pretend this never happened. This dialectic hopefully guides you toward a happy compromise, somewhere down the middle.
- Karunamon 5y ago
  Worse, you dig at it for too long and collapse the wave function, and it never works again despite you having made no changes.
  - mensetmanusman 5y ago
    Because the code you are looking at isn’t what is compiled and deployed, doh!
- anyfoo 5y ago
  The more critical the infrastructure you work on, the less you will be willing to take the "slowly back away" route. Because as long as you don't understand every single detail of the failure, that feeling of "what horrible thing do I not understand here" won't go away, and you dread having a major "oh... crap, so that's what I did not understand" moment when it's inevitable. This week, we had such a failure when testing a change. The root cause was found eventually, but what two extremely good engineers plus me still could not figure out was why that problem was only surfacing now, after years of having that buggy code in there. Very important code. I did not want to risk some obviously unknown property of it be our demise later on. Fortunately, in this case it just turned out that the seemingly unrelated changes did, after all, hold the now failing thing differently than before. As it had been used, the problem was masked entirely, and the code did work well for all those years.

asdfman123 5y ago

I misread this as "pronouns." I need to spend less time on Twitter.

Victerius 5y ago

What are quarks made of?

labster 5y ago
Snips, snails, and gold-pressed latinum. Or possibly strings. If that’s not true, the best answer is the tautology: quarks are made of quarks.
323 5y ago
Quarks are excitations in the quark fields. Thinking about them as particles is a useful simplification/model. Analogy: there are no "red pixels", there are just red excitations in the display RGB field.
davrosthedalek 5y ago
As far as we know, quarks are not composite particles. They are fundamental (and point-like). Same as electrons, photons, etc.

sundarurfriend 5y ago

Ignorant outsider to the field here: I've often heard the claim that quantum mechanics has been verified to amazing accuracy, that its predictions match with reality to the maximum degree that our instrumental precision allows, etc. A 5% difference seems big enough that at least some of our experiments should have error limits less than that. So how is it that this is only now being found, and there's still uncertainty surrounding it? I feel like I'm missing something fundamental here, and I'd like to know what it is.

Yajirobe 5y ago
Quantum theory has the largest error between theory and prediction (120 orders of magnitude). See https://en.wikipedia.org/wiki/Cosmological_constant_problem https://en.wikipedia.org/wiki/Cosmological_constant_problem
- evanb 5y ago
  That’s just a back-of-the-envelope calculation. Nobody knows how to do a bone-fide calculation because we don’t have an accepted quantum theory of gravity.
  - mcswell 5y ago
    I guess that's why they invented slide rules. I could get within 3 significant digits on a slide rule (and compute the exponent in my head), whereas I guess the margin of this envelope is too small to contain the calculation.
gowld 5y ago
Well this 5% is the instrument precision.
db48x 5y ago
There are lots of different numbers that you can use quantum mechanics to predict. It turns out that the size of the proton is both harder to predict and harder to measure than many of those other numbers. Meanwhile we can measure the fine structure constant to 12 decimal places (https://en.wikipedia.org/wiki/Electromagnetic_coupling_constant#History_of_measurements https://en.wikipedia.org/wiki/Electromagnetic_coupling_const...) and that measurement is in very close accord with the predictions of quantum mechanics.

miika 5y ago

https://hiup.org/wp-content/uploads/2017/05/Addendum_QGHM_2.pdf https://hiup.org/wp-content/uploads/2017/05/Addendum_QGHM_2....

fsh 5y ago
This is not even wrong [1]. [1] https://en.wikipedia.org/wiki/Not_even_wrong https://en.wikipedia.org/wiki/Not_even_wrong
- miika 5y ago
  It’s easy to drop that link. Much harder to deny the math that works perfectly. But probably it’s even harder to convince you to take serious look at it :)

ur-whale 5y ago

What does it even mean? We're at quantum scales and the very notion of "size" is rather ill-defined. The position of a proton, is - as a matter of fact - a probabilistic affair, so when you talk about size ... if you don't even know where the darn thing is, how are you going to measure where it starts and where it ends ...

fsh 5y ago
It is a misconception that things are ill-defined just because they are at a quantum scale. The proton has an electric charge distribution. The definition of the proton size used here is the root mean square charge radius of that charge distribution.
- davrosthedalek 5y ago
  Indeed, but it's actually a little bit hard to define what that charge distribution is (or in which frame...). The way it shows up, both in the cross section for elastic lepton scattering as well as in spectroscopy, is via the related quantity "form factor", as the slope at zero four-momentum transfer. While the form factor can be thought as the Fourier transform of the charge distribution for heavy objects (say, iron nuclei), for the proton, this becomes dicey.

ffhhj 5y ago

What if the size of the proton is variable?

YaBomm 5y ago

mrlonglong 5y ago

What would it mean if they were actually 0.88 when first measured and at 0.84 now?

swayvil 5y ago
Either the protons shrank or the yardstick grew.
- mrlonglong 5y ago
  But if protons shrank, wouldn't the yardstick also shrink seeing as it's also composed of protons? These measurements taken didn't use a physical yardstick.

ummonk 5y ago

So to summarize, standard model theory predicted that the radius of the proton would be 0.84 femtometers. High energy electron scattering experiments from the 90s and 2000s suggested it was more like 0.88 femtometers, which was a large discrepancy that causes some consideration that the standard model would have to be revised. These researchers performed a reanalysis of the data correcting for some confounding phenomena (might have been neutron formation) to suggest that the old experimental data was consistent with the 0.84 femtometers predicted by both theory and by newer lower energy scattering experiments. I’ll leave it as an exercise for the reader to decide whether the title of the article is clickbait.

ISL 5y ago
It's not clickbait. I've been to oodles of talks on the problem. It has had special sessions at APS meetings and I've watched acquaintances and colleagues expend meaningful fractions of their careers attempting to resolve the Proton Radius Puzzle. If an appeal to authority is required -- Nature gave it the cover. https://www.psi.ch/en/media/our-research/protons-smaller-than-we-thought https://www.psi.ch/en/media/our-research/protons-smaller-tha... Pohl's measurements were incontrovertible, yet they disagreed with decades of work. It has been a big problem in a quiet community for some time. If someone has a reliable angle to resolve the problem, it is big news. Enough really good physicists have tried and failed to resolve the conundrum that one should wait to see if this new approach holds up, too.
- ummonk 5y ago
  The headline gives the impression that the researchers found a new discrepancy where protons are found to be smaller than theory would predict. In actuality, the researchers are proposing a resolution to a preexisting discrepancy. Additionally, I’m not sure why you’re saying we should wait to see whether the results hold up, if you don’t think writing “probably” in the headline is clickbait.
  - deleted 5y ago
    [deleted]

pomian 5y ago

I thought that someone had a measure for a thought and that a long thought was measurably shorter than a short(?) thought, by the length of a proton. This then went off into a consideration of neural message length, etc. Cosmic black hole of thought in a spilt second.

zw123456 5y ago

OK, another dumb question from a neophyte here, doesn't the Heisenberg uncertainty principle make it impossible to know the size of a proton for sure? It seems like if you cannot not know the position and momentum of a particle then measuring its size exactly would be impossible? Thanks in advance to anyone willing to take the time to give me an ELI5 on this :)

ninkendo 5y ago
I'm an ignorant layman here, but the uncertainty principal only says that if you're sure about the size/location, then you're very unsure about the momentum. It's possible that measuring its size does not need much certainty about its momentum, so they can get the accuracy they want.
Bootvis 5y ago
If I'm blindly shooting a lot of bullets at a lot of ducks, I should be hitting some ducks. If I then count the number of shots fired and the number of ducks dead on the ground I should be able to calculate the size of ducks with high precision.
- J5892 5y ago
  A better analogy would be to subtract the amount of bullets embedded in the wall behind the ducks from the total bullets fired. Assuming ducks stop the bullets. And assuming it's not just one immortal duck flying around super fast.
whatshisface 5y ago
How big is the smell of a baking pie? The concentration of pie molecules in the air is not a binary yes/no function of position, but at the same time the volume is definitely smaller than the county, and larger than the kitchen. Fuzzy-edged objects have some sense of size but there is some freedom in where to define their edge. For the proton, the "size" is defined as a length-valued parameter in the function that expresses the charge distribution through space. The parameter is objective - but its association with the word "size" is imprecise. It is necessarily imprecise, not because of anything quantum or even unfamiliar, but because there's not another English word for the scale of objects without hard edges.

mcswell 5y ago

Looking at the picture in the article, I'd say the proton wasn't too small, rather the ruler was too big.