3 ms·

We're talking about stars here. The difference between 10 km and 14 km is pretty negligible at this scale, these are both specks.

ben_w 5y ago
If they neutron stars are bigger at any given mass, that implies black holes have different minimum sizes, which has implications for other things in cosmology.
- at_a_remove 5y ago
  Ah, no. Neutron stars have some kind of average density. That's ... not applicable to black holes in any real sense. Their sizes would not change and are pre-determined by a relatively simple formula in the case of non-rotating black holes without any significant charge.
  - treeman79 5y ago
    Well they gave a real density size and shape, it just doesn’t matter. Inside could be shaped like a dolphin a potted plant or a whale. Outside of horizon we couldn’t tell from outside.
  - ben_w 5y ago
    I’m not suggesting otherwise. I’m saying that “neutron stars have lower density than previously thought” means “neutron stars can be more massive before reaching constraint of Schwarzschild radius for given mass”.
- DougMerritt 5y ago
  As cletus said above, black holes have only three externally-observable properties: mass, angular momentum, and charge; see for instance [1]. Note that radius/size is not one of those three. The radius of a black hole derives from its mass under general relativity, and no amount of change found in the density nor size of neutron stars will change that. You're right that new understanding of neutron stars has implications for other things in cosmology, though. [1] https://phys.org/news/2020-12-black-holes-gain-powers-fast.html https://phys.org/news/2020-12-black-holes-gain-powers-fast.h...
  - adgjlsfhk1 5y ago
    What this changes is how much mass a neutron star can have before it collapses into a black hole. That limit is defined by when the neutron star's mass fits within the size of an equally massive black hole.
    - DougMerritt 5y ago
      Not exactly. Certainly it is a black hole once a neutron star's mass fits within a certain radius, but since it is then not a neutron star any longer, the question is how that condition came about. Neutron stars have an effective outward pressure that is caused by the Pauli exclusion principle [1]; two or more fermions such as neutrons cannot be in the same state (which includes location). A strong enough inward counter force from internal gravity (or from an external force) will cause a net motion inward, overcoming the outward force, such that enough matter (more accurately, enough mass-energy, not just matter) is within the critical region. See for instance [2] [1] https://en.wikipedia.org/wiki/Pauli_exclusion_principle https://en.wikipedia.org/wiki/Pauli_exclusion_principle [2] https://www.forbes.com/sites/startswithabang/2018/06/13/the-surprising-reason-why-neutron-stars-dont-all-collapse-to-form-black-holes/?sh=587230a7159c https://www.forbes.com/sites/startswithabang/2018/06/13/the-...

ben_w 5y ago

If they neutron stars are bigger at any given mass, that implies black holes have different minimum sizes, which has implications for other things in cosmology.

at_a_remove 5y ago
Ah, no. Neutron stars have some kind of average density. That's ... not applicable to black holes in any real sense. Their sizes would not change and are pre-determined by a relatively simple formula in the case of non-rotating black holes without any significant charge.
- treeman79 5y ago
  Well they gave a real density size and shape, it just doesn’t matter. Inside could be shaped like a dolphin a potted plant or a whale. Outside of horizon we couldn’t tell from outside.
- ben_w 5y ago
  I’m not suggesting otherwise. I’m saying that “neutron stars have lower density than previously thought” means “neutron stars can be more massive before reaching constraint of Schwarzschild radius for given mass”.
DougMerritt 5y ago
As cletus said above, black holes have only three externally-observable properties: mass, angular momentum, and charge; see for instance [1]. Note that radius/size is not one of those three. The radius of a black hole derives from its mass under general relativity, and no amount of change found in the density nor size of neutron stars will change that. You're right that new understanding of neutron stars has implications for other things in cosmology, though. [1] https://phys.org/news/2020-12-black-holes-gain-powers-fast.html https://phys.org/news/2020-12-black-holes-gain-powers-fast.h...
- adgjlsfhk1 5y ago
  What this changes is how much mass a neutron star can have before it collapses into a black hole. That limit is defined by when the neutron star's mass fits within the size of an equally massive black hole.
  - DougMerritt 5y ago
    Not exactly. Certainly it is a black hole once a neutron star's mass fits within a certain radius, but since it is then not a neutron star any longer, the question is how that condition came about. Neutron stars have an effective outward pressure that is caused by the Pauli exclusion principle [1]; two or more fermions such as neutrons cannot be in the same state (which includes location). A strong enough inward counter force from internal gravity (or from an external force) will cause a net motion inward, overcoming the outward force, such that enough matter (more accurately, enough mass-energy, not just matter) is within the critical region. See for instance [2] [1] https://en.wikipedia.org/wiki/Pauli_exclusion_principle https://en.wikipedia.org/wiki/Pauli_exclusion_principle [2] https://www.forbes.com/sites/startswithabang/2018/06/13/the-surprising-reason-why-neutron-stars-dont-all-collapse-to-form-black-holes/?sh=587230a7159c https://www.forbes.com/sites/startswithabang/2018/06/13/the-...