Submitted by SlyusHwanus t3_120g3km in askscience
As the mass rises for a neutron star, it reaches a point where that mass at that diameter no longer allow light to escape the surface. as mass increases the event horizon expands. is this just a continuum? Could black holes basically have a neutron star in the middle and be more or less identical except being over the event horison threshold?
do all neutron stars collapse into a black hole?
No. Most don't. But a neutron star that continues to accumulate mass can. For example if 2 neutron stars collide they can form a black hole. We detected one of these mergers on August 17, 2017.
One that doesn't accumulate mass will, in theory, slowly lose mass until it explodes into a white dwarf in about 10^(38) years (if proton decay is real). Since the universe is much younger than that, this hasn't happened yet.
how will it become a white dwarf during proton decay? isn't that the phase when all stars die?
The idea is, if protons do indeed randomly decay (over extremely long periods of time), then a neutron star will very slowly lose mass via this process, with protons in its thin outer crust very occasionally evaporating. After about 10^(38) years, enough mass will have been lost that the neutron star finally reaches a tipping point where it is light enough to not be a neutron star anymore. So it explodes into a white dwarf.
>After about 10^(38) years,
Where does this figure come from, given that we don't yet know whether protons decay or not, let alone their half life?
There is a lower bound on the half-life, so I’m assuming this number is using that lower bound. The correct statement would then be after at least 10^38 years
oh so it will take a long time for protons to decay and the universe go back to a soup of subatomic particles (this time the soup is cold). then the black holes take over because these abnormalities don't care about atomic structures, as long as it got mass or energy it's gonna be eaten
They will take over only in the sense that they will still be around, there won't be matter left for them to capture - it will all be spread out too far at that time.
Wait, but Neutron Star, proton decay or not, losses mass anyway, every second, because all the emitted EM equals loss of mass/energy. I'm pretty sure it would lose a sizeable chunk by 10^38, or by the time it cools down (if ever)
That emission comes out of their heat and angular energy; they'll slow, cool, and dim long before that point.
Not all Neutron stars will expand to the mass/density required to collapse into Black Holes. These ones will exude visible light for about one billion years before cooling down below visible light, then live on for about 1e+38 years before decaying completely into a kind of neutron white dwarf which will then decay further for another 1e+38 years before becoming a neutron black dwarf which will be there to witness the elder years of the universe.
I watched something that posited that the neuron star still exists in the black hole. I know the math goes to singularity but is it not possible the mass is still there?
Anything inside the event horizon must reach the singularity in finite proper time (that is, time from its own point of view). However, events inside the horizon can never be in the (causal) past of an outside observer, so there is never a time at which an outside observer could say objectively that the neutron star is no longer there. Maybe that's what the claim was?
No the claim was that the victim would collide with the neutron star, assuming they weren't ripped to shreds. Basically all matter would join what was left of the star.
That's more like saying relativistic effects mean that any matter that fell in less than infinity years before you is going to be between you and the actual singularity. It's not that the neutron star is 'in' there, it's that it can't ever finish falling in before you catch up with it's trailing edge...I think?
Kinda way out on a limb there. Can anyone help out if that's totally wrong?
The singularity is a finite distance from the event horizon which means it's a finite time away (as measured in both time and space and the distinction could matter in this case) when inside the event horizon. You will catch up to something and that something should be a singularity.
The acceleration felt by the atoms or neutrons in a neutron star is > the speed of light. Nothing can resist ending up in the infinitely dense center. There is no physical mechanism we know of that could resist it.
So, really, you're one layer of an infinite relativistic spacetime-baklava of matter approaching the singularity...
I guess. The mathematical models we are using to guess at what is going on already break with what we've got.
Really as someone who intends to never enter an event horizon, mathematically there is no difference between the matter of a black hole being in a singularity, or being evenly spread just under the event horizon. The stuff that comes out of a black hole, gravity and charge, don't care so in a very real sense there isn't a difference.
In fact it might not even be defined in the same sense that many things in quantum mechanics aren't defined until measured. And since it can't be measured as far as the external universe is concerned, why would reality care to pick?
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According to GR no, once inside the event horizon the singularity is no longer a point in space, it is a point in time. It is unavoidable, just like “next Wednesday” is unavoidable. The matter that composed the neutron star must collapse and encounter the singularity at some point in its future.
More speculative theories offer other solutions. String Theory, for example, proposes “Fuzz Balls”, so called because the event horizon would be “fuzzy” at the smallest scales. The interior of the BH would be a degenerate matter composed of the fundamental strings, not empty space. Obviously very speculative.
We need a theory of Quantum Gravity to better understand the interior of a BH.
The short answer is no, the neutron degeneracy pressure for a neutron star of mass greater than around 2.16 time the mass of our sun (Tolman–Oppenheimer–Volkoff limit) is insufficient to stop it collapsing.
This is well below the mass density where an even horizon would form, thus a neutron star cannot simultaneously be a black hole.
If though there were a state of matter at higher density than neutron matter (perhaps a quark plasma or quark stranglet) that could stabilize the collapse before the critical radius is reached then there may be something denser than a neutron star. But this would not be a black hole.
But according to our current understanding of science, once the collapsing sphere is denser than the the Schwarzschild radius an even horizon forms & within that radius no known state of matter is sufficient to withstand it ultimately forming a singularity.
It would be very difficult with a non-rotating black hole to prove otherwise anyway as the event horizon forms an information boundary to external observers & also for infalling instrumentation.
There's a whole Einsteinian General Relativity answer here that I'll let other people give. But your question does remind me of a Newtonian idea. Basically after the speed of light was discovered but before Einstein came along, people asked your exact question: "What if a star is so dense that it's escape velocity exceeds the speed of light?" The answer is Dark Stars (AKA Newtonian Black Holes). To be clear, these objects don't exist in the real world, but they are fun to think about.
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To take a somewhat less abstract approach to this than those mentioned so far, an object like a neutron star needs some repulsive force to counteract gravity and prevent further collapse, and that force must be conveyed by particles that cannot exceed light speed. If no light or other particle can move outwards, no force can be conveyed, and there's nothing stopping the mass from collapsing eternally inwards.
Aseyhe t1_jdihufb wrote
To the extent our current best theory of gravity (general relativity) is accurate, it is not possible for a static extended structure to exist inside the event horizon of a black hole. Gravity there is so strong that even outgoing photons move toward the center; that's why there's an event horizon. If a static structure existed, its material would have to move outward faster than light, which is impossible.
Also: > As the mass rises for a neutron star, it reaches a point where that mass at that diameter no longer allow light to escape the surface.
This would happen for an idealized rigid body, but it's not really what happens to a neutron star. As the neutron star's mass rises, the repulsive interaction between neutrons is no longer able to support the star against gravity, and it collapses (see the TOV limit). That's when the black hole forms.
(If a neutron star could maintain its structure up until the event horizon enveloped it, the maximum mass of a neutron star would be at least 4 solar masses. Instead it's 2-3 solar masses.)