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Gravity is strictly accumulative. All other forces are paired with positive and negative carriers (electromagnetic, weak nuclear, strong nuclear). Protons can

Gravity is strictly accumulative. All other forces are paired with positive and negative carriers (electromagnetic, weak nuclear, strong nuclear). Protons can generate neutrons through beta decay. I suspect something like this happens in neutron stars, which don't spontaneously fly apart into a proton cloud, so far as we've observed. This SE comment explains why neutron stars don't contain (many) protons: <https://physics.stackexchange.com/a/149656 https://physics.stackexchange.com/a/149656> (The ratio seems to be ~100:1 neutron:proton.)

arcadi7 2y ago
protons can generate neutrons and neutrons can generate protons . why are there no nuclea with only neutrons ? — these could be as big as you want because there is no electric repulsion . the answer is that only neutron nucleus would have — by pauli exclusion principle — very large kinetic energy . so nucleus turns couple of its neutrons into protons to lower the kinetic energy of neutrons . same reason for small fraction of protons in neutron stars
- dredmorbius 2y ago
  There are nuclei with only neutrons. We call those "neutron stars". They're ... both big (by atomic scales) and massive (atomic and astronomical).
  - arcadi7 2y ago
    and they _always_ have few percent of protons and electrons — every single one of them .
  - arcadi7 2y ago
    and they _always_ have few percent of protons and electrons — every single neutron star .
    - adrianN 2y ago
      How do we know this? That seems difficult to observe from a couple of light years away.
      - dredmorbius 2y ago
        A good question, and mostly beyond my pay grade. Almost certainly based on models rather than direct observation, though informed by the latter. I'm struggling to find a strong source, but this page, by M. Coleman Miller at the University of Maryland, describes properties suggesting modelling over observation. Specifically it describes "the guts of a neutron star": Even further down, you mainly have free neutrons, with a 5%-10% sprinkling of protons and electrons. With an subsequent 'graph: Yes, you may say, that's all very well for keeping nuclear theorists employed, but how can we possibly tell if it works out in reality? Well, believe it or not, these things may actually have an effect on the cooling history of the star and their spin behavior! What follows is more description of theory with a few points based in what is directly observable, largely spin (via radio astronomy) and some temperature observations largely in X-ray observations, and the occasional gamma-ray burst. <https://www.astro.umd.edu/~miller/nstar.html https://www.astro.umd.edu/~miller/nstar.html> Miller's bio at UMD emphasizes his theoretical work: Cole Miller's research in the last few years has focused on theory and modeling of high-energy radiation from neutron stars and black holes. <https://www.astro.umd.edu/people/miller.html https://www.astro.umd.edu/people/miller.html>
    - abakker 2y ago
      Does that mean that in some ridiculous sense, a neutron star is a unique element?
      - dredmorbius 2y ago
        I've seen similar suggestions, but it's ... probably not an especially useful conceptualisation. The conceptualisation of elements is useful to us because physical characteristics of atoms are (mostly) stable over time, with atoms having distinctive masses, atomic numbers, and most importantly, electron shells. The latter account for most of what we consider to be "chemistry", along with some other effects, most notably the van der Waals force. Neutron stars ... lack most of this. They're in constant flux, they (probably) don't have stable masses (if only due to constant accretion) or atomic numbers, they probably don't have anything resembling an electron shell, and in interactions with other objects any electromagnetic forces would probably be overwhelmed by gravity or, say, spin-induced magnetism. If a measure of a concept or model's usefulness is how much it explains behaviours, "neutron-star-as-element" doesn't buy you much. Though there's possibly some truth or conceptual validity to it.

arcadi7 2y ago

protons can generate neutrons and neutrons can generate protons . why are there no nuclea with only neutrons ? — these could be as big as you want because there is no electric repulsion . the answer is that only neutron nucleus would have — by pauli exclusion principle — very large kinetic energy . so nucleus turns couple of its neutrons into protons to lower the kinetic energy of neutrons . same reason for small fraction of protons in neutron stars

dredmorbius 2y ago
There are nuclei with only neutrons. We call those "neutron stars". They're ... both big (by atomic scales) and massive (atomic and astronomical).
- arcadi7 2y ago
  and they _always_ have few percent of protons and electrons — every single one of them .
- arcadi7 2y ago
  and they _always_ have few percent of protons and electrons — every single neutron star .
  - adrianN 2y ago
    How do we know this? That seems difficult to observe from a couple of light years away.
    - dredmorbius 2y ago
      A good question, and mostly beyond my pay grade. Almost certainly based on models rather than direct observation, though informed by the latter. I'm struggling to find a strong source, but this page, by M. Coleman Miller at the University of Maryland, describes properties suggesting modelling over observation. Specifically it describes "the guts of a neutron star": Even further down, you mainly have free neutrons, with a 5%-10% sprinkling of protons and electrons. With an subsequent 'graph: Yes, you may say, that's all very well for keeping nuclear theorists employed, but how can we possibly tell if it works out in reality? Well, believe it or not, these things may actually have an effect on the cooling history of the star and their spin behavior! What follows is more description of theory with a few points based in what is directly observable, largely spin (via radio astronomy) and some temperature observations largely in X-ray observations, and the occasional gamma-ray burst. <https://www.astro.umd.edu/~miller/nstar.html https://www.astro.umd.edu/~miller/nstar.html> Miller's bio at UMD emphasizes his theoretical work: Cole Miller's research in the last few years has focused on theory and modeling of high-energy radiation from neutron stars and black holes. <https://www.astro.umd.edu/people/miller.html https://www.astro.umd.edu/people/miller.html>
  - abakker 2y ago
    Does that mean that in some ridiculous sense, a neutron star is a unique element?
    - dredmorbius 2y ago
      I've seen similar suggestions, but it's ... probably not an especially useful conceptualisation. The conceptualisation of elements is useful to us because physical characteristics of atoms are (mostly) stable over time, with atoms having distinctive masses, atomic numbers, and most importantly, electron shells. The latter account for most of what we consider to be "chemistry", along with some other effects, most notably the van der Waals force. Neutron stars ... lack most of this. They're in constant flux, they (probably) don't have stable masses (if only due to constant accretion) or atomic numbers, they probably don't have anything resembling an electron shell, and in interactions with other objects any electromagnetic forces would probably be overwhelmed by gravity or, say, spin-induced magnetism. If a measure of a concept or model's usefulness is how much it explains behaviours, "neutron-star-as-element" doesn't buy you much. Though there's possibly some truth or conceptual validity to it.