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billj
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What happens to the neutrons in a neutron star as it collapses Into a black hole?
Same thing as happens to ALL matter that gets into a black hole, it disappears into the singularity. Now this is not believed to be physical but it's what the current model shows. Expectations are that if/when loop quantum gravity becomes a solid theory we might understand what's REALLY happening, but for now we don't.billj said:What happens to the neutrons in a neutron star as it collapses Into a black hole?
billj said:What happens to the neutrons in a neutron star as it collapses Into a black hole?
billj said:What happens to the neutrons in a neutron star as it collapses Into a black hole?
Bernie G said:If some neutrons collapse in a neutron star do all neutrons collapse?
sevenperforce said:If the pressure at the center of a neutron star were to exceed the limits of neutron degeneracy pressure, then the neutrons would presumably start to collapse into a black hole.
What makes you think so?Bernie G said:That would only be true if the collapsed neutrons had a volume that approached zero.
Not necessarily. The density at the center of a neutron star is believed to exceed that of an atomic nucleus: 8e17 kg/m3. Of course, such high gravity is going to warp space pretty significantly, so Euclidean geometry doesn't exactly hold here...but taking the Euclidean approximation, a core which grows to 4.8 solar masses at this density will become a black hole in its own right without needing to collapse at all. If quark-degenerate matter starts to form at the core of a neutron star as neutrons begin to break down, then the density is expected to be around 1.7e18 kg/m3; such a quark-matter core would satisfy the condition for a black hole with just under 3.5 solar masses. A non-Euclidean formation would likely decrease these requirements significantly.Bernie G said:Let me rephrase the statement:
That would only be true if the collapsed neutrons had significantly less volume.
Jonathan Scott said:On the other hand, if they collapse to a form which is not sufficiently dense to cause an immediate black hole, then what happens beyond that would depend on the nature of that form and in particular the pressure it could support, but that form would also be certain to collapse to a black hole at a smaller mass than if it were able to remain as a neutron star because it would have greater density.
Indeed.Bernie G said:So far there are about 2000 observed neutron stars all with a maximum mass limit of about 2M☉. If neutron stars collapsed directly into black holes there should be black holes starting at 2M☉ but none have been observed yet. To me it looks like there is some kind of process intrinsic to neutron stars that limits their mass to about 2M☉.
In the case you quote, the energy comes from the collider.Bernie G said:Collider experiments show that when a nucleus collapses what is produced is from 1% quark type matter and 99% energy to 10% quark type matter and 90% energy.
So the binding energy between the quarks in quark-degenerate matter or a quark-gluon plasma is exactly identical to the binding energy between the quarks in a neutron? That doesn't quite make sense; breaking a bunch of neutrons down into quark-degenerate matter ought to release at least some of the strong-interaction-binding energy that kept the quarks in a baryonic configuration. Baryon number wouldn't be violated because you still have the same number of quarks, right?Jonathan Scott said:As far as I know, unless baryon number can be violated (which would be a non-mainstream assumption outside the scope of these forums), the effective rest energy (including internal kinetic energy) of the components of a neutron cannot be less than that of a proton, and quarks cannot be isolated, so very little additional kinetic energy can be obtained by breaking down a neutron into its components.
Jonathan Scott said:... so very little additional kinetic energy can be obtained by breaking down a neutron into its components.
This appears to be a personal theory of yours which you have already posted in some other threads, and I pointed out that you should start a new thread and provide acceptable references if you wished to continue to discuss it.Bernie G said:What if that new form was ultra relativistic quark matter? Ultra relativistic matter would either heat or escape the star.
sevenperforce said:So the binding energy between the quarks in quark-degenerate matter or a quark-gluon plasma is exactly identical to the binding energy between the quarks in a neutron? That doesn't quite make sense; breaking a bunch of neutrons down into quark-degenerate matter ought to release at least some of the strong-interaction-binding energy that kept the quarks in a baryonic configuration. Baryon number wouldn't be violated because you still have the same number of quarks, right?
Bernie G said:So are you saying when a 1000 MeV neutron disintegrates all we get out of it is some quarks with about 10 MeV rest mass?
Forgive me if this is an elementary or obvious question, but why can't quarks released by the collapsing neutrons be bound in quark-degenerate or strange matter? Would that violate baryon conservation, or would that somehow constitute "quark isolation" and thus be prevented?Jonathan Scott said:Baryon conservation and the fact that quarks can't be isolated together mean that per original neutron the internal kinetic energy of the bound systems of quarks plus the rest mass of any components with rest mass cannot add up to less than the mass of a proton.
sevenperforce said:Forgive me if this is an elementary or obvious question, but why can't quarks released by the collapsing neutrons be bound in quark-degenerate or strange matter? Would that violate baryon conservation, or would that somehow constitute "quark isolation" and thus be prevented?
Naturally.Jonathan Scott said:I don't see any reason why alternative forms should be prevented. Baryon number conservation doesn't prevent the quarks being arranged in other ways or being excited to other levels such as strange quarks (with the same baryon number). However, any bound group of quarks and gluons could only be isolated if the total baryon number is a whole number (which implies groups of three plus optional particle / antiparticle pairs).
sevenperforce said:Naturally.
So what, then, is to prevent a gravitationally-bound collection of neutrons from collapsing into a soup of strong-interaction-bound quark matter with matching baryon number but lower binding energy, for a net exothermic process? I'm assuming that 21 quarks bound together in quark-degenerate plasma is going to have a lower binding energy than 7 neutrons...
I guess it would only be possible if strangelets were stable.Jonathan Scott said:If that was possible and you took that 21-quark unit out of that environment without adding energy, it couldn't decay back to protons and neutrons without adding energy, so either it or some decay product would be stable but have a mass less than the corresponding number of protons. I don't find that plausible.
Jonathan Scott said:As most of the energy per particle is simply derived from gravity, the only way for anything other than electromagnetic radiation and neutrinos to escape from the surface is if there is some effect such as a significant fusion explosion of accumulated matter which generates a huge amount of energy over a very short time. That could then result in a flash of neutron star surface material being ejected into space, as a cloud or shell containing traces of elements such as iron.
Continuous or frequent fusion would not produce enough energy per particle, but if material builds up for a while before a fusion chain reaction, then the resulting shock wave might well propel a small amount of material to escape velocity.Bernie G said:Fusion reactions do not produce enough velocity for nuclei to escape a neutron star's surface.
billj said:What happens to the neutrons in a neutron star as it collapses Into a black hole?
sevenperforce said:If the pressure at the center of a neutron star were to exceed the limits of neutron degeneracy pressure, then the neutrons would presumably start to collapse into a black hole. If this black hole were small enough (e.g., on the order of a few thousand tonnes), then the radiation pressure from Hawking radiation could potentially be high enough to arrest further collapse.
sevenperforce said:such high gravity is going to warp space pretty significantly, so Euclidean geometry doesn't exactly hold here
sevenperforce said:breaking a bunch of neutrons down into quark-degenerate matter ought to release at least some of the strong-interaction-binding energy that kept the quarks in a baryonic configuration
sevenperforce said:why can't quarks released by the collapsing neutrons be bound in quark-degenerate or strange matter?
sevenperforce said:I'm assuming that 21 quarks bound together in quark-degenerate plasma is going to have a lower binding energy than 7 neutrons
Bernie G said:Maybe core neutrons collapse into 1% quark matter with 99% energy that result in super intense X-rays which could result in intense positron/electron production.
Sounds good to me.PeterDonis said:It might be helpful to take a step back and look at the starting premise of this thread:
We have to find a plausible scenario for a neutron star collapsing into a black hole. One such scenario would be a neutron star that is below the maximum mass limit, but not by much, accreting enough mass onto it to push it over the limit (for example, the neutron star could be in a binary system with a massive companion and material from the companion could fall onto the neutron star). If that scenario seems ok to everyone, then further discussion can be based on it.
Ah. Quark-degenerate matter is a higher-energy state than baryonic matter. Got it!PeterDonis said:This reasoning would be valid if quark-degenerate matter were a possible state of matter at zero temperature, as baryonic configurations are. But it isn't; it can only exist to begin with at very high temperatures. Which means that the transition from baryonic configurations to quark-degenerate matter requires an input of energy; it is not a transition that will release energy, whether it's "binding energy" or anything else.
Well, in this case, we'd obviously have a collapse happening, so it's definitely not a static system.PeterDonis said:No, this won't work. It is true that, if we look at the event horizon in a spacetime where an object like a neutron star (or an ordinary star) is collapsing to a black hole, the horizon forms at the center, ##r = 0##, and moves outward until it reaches the Schwarzschild radius associated with the total mass of the object. But that does not mean the mass of the black hole starts at zero and slowly grows; what it means is that, until all of the matter in the object has collapsed below the event horizon, there is no clean way to separate the "black hole" from "the rest of the object".
What's more, you can't even have a static system with a radius just a little bit larger than the Schwarzschild radius associated with its mass. There is a theorem called Buchdahl's theorem which says that the minimum radius that any static system can have is 9/8 of the Schwarzschild radius associated with its mass. That means there is a finite "gap" between an object being stable in a static configuration and an object being a black hole; there is no continuous sequence of static configurations with gradually increasing mass that suddenly turns into black holes without any collapse in between.
Indeed. I know that Hawking's equations were "set" using the model of a stable black hole in a vacuum, but there is no vacuum (since the CMBR is always causing SOMETHING to fall into the black hole) and there are no stable black holes (because, Hawking radiation).When the event horizon forms at ##r = 0## and starts moving outward, it won't be producing Hawking radiation (at least according to our best understanding of Hawking radiation), for at least two reasons. First, the horizon is not in vacuum--it is embedded in the collapsing matter. The derivation of Hawking radiation being emitted from a horizon assumes vacuum. Second, the horizon is not a trapped surface--in other words, its area is not constant. The area of the horizon grows until all the collapsing matter has fallen inside it. The derivation of Hawking radiation, if you look at the details, assumes that the horizon is a trapped surface--that its area is not growing.
So this proposed mechanism for stopping a black hole from forming, at least if we use the current understanding of Hawking radiation, won't work. However, it should be noted that our current understanding of Hawking radiation and how it is produced might not be correct.
sevenperforce said:I'd also note that the Schwarzschild radius won't "form" at the center and grow outward
sevenperforce said:a neutron star already has a pretty significant Schwarzschild radius
sevenperforce said:When such a neutron star collapses, the Schwarzschild radius will remain constant while the outer layers fall into it
sevenperforce said:I'd love to get your input over there.
sevenperforce said:there is no vacuum (since the CMBR is always causing SOMETHING to fall into the black hole)
sevenperforce said:applying Hawking's predictions to a Planck-mass model
Sure, I get that.PeterDonis said:The Schwarzschild radius remaining constant is just another way of saying the externally measured mass remains constant. (This assumes that no radiation is emitted during the collapse process, which is highly unlikely in the real world, but we can assume it for this thought experiment.) It does not mean that there is an event horizon sitting there waiting for things to fall in.
I'm not quite sure how this would change the scenario. If there is a particular event at ##r = 0## such that an outgoing light signal emitted from that event would intersect the surface of the collapsing core just at the Schwarzschild radius corresponding to the mass of the core, then you have a core-mass black hole already inside the collapsing neutron star. Similarly, if there is a particular event at ##r = 0## such that an outgoing light signal emitted from that event would intersect the inner-core/outer-core boundary just at the Schwarzschild radius corresponding to the mass of the inner core, then you have an inner-core-mass black hole at the center of the collapsing core.Actually, though, this way of putting things can be misleading. A better way to put it starts with recognizing the definition of the event horizon: it is the boundary of the region of spacetime from which light signals cannot escape. So what is actually happening is that there is a particular event at ##r = 0## such that, if an outgoing light signal is emitted from that event, it will intersect the surface of the collapsing star just at the Schwarzschild radius corresponding to the mass of the system, and will then be trapped there forever, unable to move any further outward.
I would advise rethinking the rest of your proposed scenarios (with varying densities of core vs. outer parts, etc.) in the light of the above.
Well, any analysis might be completely pointless if Hawking radiation predictions break down at a larger scale, but if they don't, then there might be a useful statistical analysis of what would happen as the Planck scale is approached, even if we're not dealing specifically with the Planck mass. For instance, trying to derive the minimum mass by looking at where the math would no longer make sense, like when the peak wavelength of emitted radiation would correspond to a particle energy exceeding half the energy of the object.Hawking's prediction for a Planck mass black hole is that it will evaporate immediately, with no time lapse--i.e., that such a hole can't really exist since it will evaporate as soon as it is formed. This is not something that can be usefully analyzed statistically, as far as I can see.