8 ms·

(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, i

(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
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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

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