4 ms·

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 h

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."

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."