[HN Gopher] Quark Stars
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Quark Stars
Author : chmaynard
Score : 103 points
Date : 2024-08-02 11:25 UTC (11 hours ago)
(HTM) web link (johncarlosbaez.wordpress.com)
(TXT) w3m dump (johncarlosbaez.wordpress.com)
| ndsipa_pomu wrote:
| As a layman, I find the idea of neutron and quark stars to be
| fascinating. What puzzles me though is what distinguishes a
| compressed mass of neutrons from unconfined quarks/gluons. I
| thought that the idea of a neutron having a "shell" with quarks
| inside it was just a visualisation tool, but the compressed
| neutrons must be providing some force to prevent them from
| collapsing further into unconfined quarks. That also raises the
| question of how the unconfined quarks/gluons provide a force to
| prevent collapse into black holes.
| itishappy wrote:
| You're looking for the concept of a degeneracy pressure,
| specifically neutron and quark degeneracy pressures. The short
| version is that the Pauli exclusion principle forbids fermions
| from occupying the same state, and this manifests as an actual
| physical force that resists attempts to bring them close
| together.
|
| https://en.wikipedia.org/wiki/Degenerate_matter
| ndsipa_pomu wrote:
| Thanks. It would appear that quark degeneracy pressure is the
| missing bit that I didn't know about and I don't really
| understand how that works, but then it does appear to be
| poorly understood in general.
| archgoon wrote:
| > short version is that the Pauli exclusion principle forbids
| fermions
|
| I'd like to point out (for others following along) that the
| Pauli exclusion principle isn't actually a separate rule
| (That is, something you'd have to apply after you do 'normal'
| physics). What is happening with the PEP is that if you start
| off with a wavefunction that has fermionic symmetry (that is,
| interchange of two particle swaps the sign of the
| wavefunction), the evolution via the Schroedinger equation
| will preserve that (much like it preserves the
| Integral(|psi|^2)=1 relation). Same for bosons.
|
| So if you're writing a Quantum physics simulator, you don't
| need to put in a "Pauli Exclusion Rule" step.[1]
|
| [1] Though depending on your representation you may toss one
| in for numerical stability.
| pdonis wrote:
| _> what distinguishes a compressed mass of neutrons from
| unconfined quarks /gluons._
|
| If the density and pressure get high enough, there is no longer
| a stable "neutron" state in which three valence quarks are
| bound together. You just have a soup of quarks. Calling the
| quarks "unconfined" is a bit of a misnomer since each of them
| _is_ restricted to a very small "cell" in space. But they are
| "unconfined" in the sense of not being bound together by the
| strong interaction; within their small "cell" they move more or
| less like free particles.
|
| _> the compressed neutrons must be providing some force to
| prevent them from collapsing further into unconfined quarks._
|
| "Collapse" isn't really a good word to describe this
| transition. If you add a small amount of mass to the object, it
| compresses a little more. If it compresses enough, there is
| something like a phase transition where the quarks stop being
| bound into neutrons; but the overall size of the object doesn't
| "collapse", it just gets a little smaller.
|
| _> That also raises the question of how the unconfined quarks
| /gluons provide a force to prevent collapse into black holes._
|
| This is just the Pauli exclusion principle, as has already been
| said in response to you. It's more or less the same whether the
| quarks are bound into neutrons or not.
| delichon wrote:
| ChatGPT just told me that ... the Schwarzschild
| radius for a quark-gluon plasma with a mass of 10^12 kg is
| approximately 1.485x10^-15 meters, which is extremely small.
|
| Extremely small is around the diameter of a proton. Which would
| mean that any quark star would a black hole. So this estimate
| must be wrong, right?
| danparsonson wrote:
| Leaving aside the calculation (I don't know if it's correct),
| and the sense or not of relying on a fancy word-association
| machine for such calculations (I wouldn't), your Schwarzchild
| radius is even smaller than that and you're not a black hole -
| an object is only a black hole if it is entirely inside its
| Schwarzchild radius.
| delichon wrote:
| If the Schwarzchild radius is reached within a larger mass,
| wouldn't that radius become inescapable and eventually
| consume the remainder while growing in proportion?
|
| Since I did a sanity check on the result which seems to have
| failed, why do you think that it's necessary to warn me not
| to rely on it?
|
| The hostility on this site for using LLMs as a tool for
| exploration is confusing to me. I also learn a lot by asking
| questions of highly unreliable humans. LLMs don't seem to be
| much worse than that.
| AnimalMuppet wrote:
| > If the Schwarzchild radius is reached within a larger
| mass, wouldn't that radius become inescapable and
| eventually consume the remainder while growing in
| proportion?
|
| If I understand your question correctly, no. If I have a
| mass M that corresponds to a Schwarzchild radius r, but the
| mass occupies a larger radius R, no, it will not create a
| black hole, since the mass m within r is less than the
| critical mass M. The portion of M that is within r can
| still escape that radius.
| javaunsafe2019 wrote:
| This is maybe cause ppl don't want to rely on some machine
| hallucinating wrong answer when trying to answer
| fundamental questions but rather use real wisdom instead.
| bell-cot wrote:
| It's fairly well-accepted that tiny black holes are quite
| unstable:
|
| https://en.wikipedia.org/wiki/Hawking_radiation
|
| On a larger scale, kinda yes - a small-ish black hole would
| generally "feed" from any near-enough object. Though that
| feeding will generally be extremely energetic and messy,
| and fling most of the victim away at high velocity.
| commodoreboxer wrote:
| I don't have a problem with your post, but the hostility I
| see is mostly out of a real concern that people posting LLM
| comments could degrade discussion. It's probably also
| association, because a lot of people have started posting
| comments that just say "ChatGPT says..." and nearly the
| entirety of the body is just LLM output.
|
| Honestly, a lot of people are just sick of seeing anything
| about LLMs and don't like seeing it at all in threads that
| aren't already about it.
| queuebert wrote:
| Exactly. It's not appropriate for this type of question.
| LLMs can summarize the global internet sentiment about
| something pretty well, but they aren't the right
| architecture for answering precise scientific questions,
| nor will they ever be. It's like asking an old,
| experienced physicist to guess answers to things instead
| of solving equations.
| ben_w wrote:
| They're already better than I'd have expected possible,
| so I wouldn't say "never" -- but also yes I agree with
| you, currently I'd compare their output to an off-the-
| cuff answer from an experienced professional (or an
| undergraduate trying hard).
| turzmo wrote:
| As a physicist I can say that unless ChatGPT is asked a
| definitional question, or a very common question (whose
| solution is likely described many places on the internet
| anyway), it is very likely to be wrong. Personally more
| than 90% of the questions I've asked it have been flat-
| out wrong so I stopped using it entirely.
| layer8 wrote:
| You only have a black hole and an event horizon if the mass
| whose Schwarzschild radius you are calculating is
| completely contained within said radius.
| noslenwerdna wrote:
| As others have said, that's not what a Schwarzschild radius
| means. You would only get a black hole if all the matter is
| inside the radius.
|
| The Schwarzschild radius comes from the free space solution
| to Einstein's equation. It only holds in free space. So if
| you want to use it here, all the matter would need to be
| within Schwarzschild radius, and then you could calculate
| what happens outside that radius (again, in the matter free
| space outside). If you want to know how gravity behaves
| inside a star you have to use a different tool.
| itishappy wrote:
| Neutron stars weigh in at about 1.4 solar masses, or 10^30kg,
| which gives a Schwarzschild radius of about 3mi.
|
| A neutron star can be viewed as a mass-just-shy-of-a-black-
| hole, so I'd expect it to be relatively similar in scale.
| pdonis wrote:
| _> Neutron stars weigh in at about 1.4 solar masses_
|
| Observed neutron stars actually have a range of masses, from
| the 1.4 solar mass point (which is the maximum mass for a
| white dwarf, so one would expect to see neutron stars of
| about that mass that were just over the white dwarf limit) up
| to, IIRC, almost 3 solar masses for the largest one that has
| been observed.
|
| _> A neutron star can be viewed as a mass-just-shy-of-a-
| black-hole_
|
| Not in general, no. A neutron star just under the maximum
| mass limit for neutron stars, which is believed to be about 3
| solar masses, could be sort of viewed this way, but even then
| it's not really correct, since there is nothing that forbids
| a black hole with a mass _smaller_ than that from existing.
| It 's just extremely unlikely that such a black hole could be
| formed by the collapse of a star, since such a collapse would
| be expected to stop at the neutron star stage (or at the
| white dwarf stage if the star was less than 1.4 solar
| masses).
| itishappy wrote:
| Excellent points! Galaxies have more mass than either, but
| behave very differently. I would correct my statement by
| saying it's the concentration of mass (density) that
| defines neutron stars and black holes.
| pdonis wrote:
| _> it 's the concentration of mass (density) that defines
| neutron stars and black holes._
|
| Not really, no. What defines a neutron star is that it is
| in hydrostatic equilibrium supported by neutron
| degeneracy pressure. If we include the quark-gluon plasma
| phase, we can just amend that to being supported by quark
| degeneracy pressure (and noting that "neutron" is just a
| special case of "quark" for this purpose). Whereas a
| normal star is in hydrostatic equilibrium supported by
| thermal pressure (with fusion reactions providing the
| heat source). And a galaxy is not in hydrostatic
| equilibrium at all, it's composed of stars in free-fall
| orbits. The average densities of the objects in all three
| of these cases are consequences of the above.
|
| A black hole is defined by having an event horizon and
| being vacuum. There is no well-defined concept of
| "density" for a black hole, nor is there "concentration
| of mass" in the sense of the hole being made of matter;
| it's not, it's vacuum. The "mass" of a black hole is a
| global feature of its spacetime geometry.
| itishappy wrote:
| Right, but are those phenomenon not also consequences of
| density? If we compressed stellar or even terrestrial
| material down the density of a neutron star, would we not
| get neutronium as a result?
|
| I agree that the density of a black hole will depend on
| choice of observer, but nothing stops us from picking a
| reasonable one, such as a Schwarzschild observer. Mass
| (as you've mentioned) and volume (Schwarzschild radius)
| are both we enough for us to define a global average
| density. It does not behave intuitively though,
| increasing mass actually decreases density, which does
| seems to contradict my earlier point...
| pdonis wrote:
| _> are those phenomenon not also consequences of
| density?_
|
| No, density is a consequence of the object's structure.
| Hydrostatic equilibrium supported by degeneracy pressure
| leads to higher density than hydrostatic equilibrium
| supported by thermal pressure.
|
| _> I agree that the density of a black hole will depend
| on choice of observer_
|
| That's not what I said. I said there is no well-defined
| density of a black hole at all. That's because the hole
| is vacuum and has no well-defined spatial volume.
|
| _> nothing stops us from picking a reasonable one, such
| as a Schwarzschild observer_
|
| There are no Schwarzschild observers inside a black hole.
| Schwarzschild _coordinates_ in the interior of a black
| hole give an infinite answer for the "spatial volume",
| which, of course, is not well-defined.
|
| _> volume (Schwarzschild radius)_
|
| The Schwarzschild radius is derived from the surface area
| of the hole's horizon. It does not mean the hole has a
| well-defined volume. It doesn't.
| itishappy wrote:
| > No, density is a consequence of the object's structure.
|
| Which is a consequence of the gravitational pressure, no?
| Quark degenerate matter without a confining potential
| would not be at equilibrium.
|
| > There are no Schwarzschild observers inside a black
| hole.
|
| Of course not, but the definition of the Schwarzschild
| radius does not require a black hole, just mass. For
| example, we define the Schwarzschild radius of the Sun to
| be about 3km. Outside of black hole dynamics, this
| defines a volume of space, and we call an object whose
| radius is smaller than its Schwarzschild radius a black
| hole. I understand that black holes are fundamentally
| dynamic processes, but can we not define the behavior at
| the limit?
| chasil wrote:
| The neutron star mass limit has a formal name.
|
| https://en.m.wikipedia.org/wiki/Tolman-Oppenheimer-
| Volkoff_l...
| mr_mitm wrote:
| The result is correct. But I don't understand your conclusion.
| Where does that number 10^12 kg come from?
| cryptonector wrote:
| Quark stars might even be a stage on the way to collapse from
| neutron star to black hole. The collapse is fast, but not
| instantaneous, so what would those neutrons become once their
| degeneracy pressure is exceeded?
| sigmoid10 wrote:
| >what would those neutrons become once their degeneracy
| pressure is exceeded
|
| That kind of depends on what the equation of state looks like
| for quark matter. If you look at black hole formation from a
| theoretical perspective (e.g. as a collapsing shell of matter),
| it is very much possible that neutrons transition directly into
| a black hole before entering a new state. Unfortunately, we
| have no idea about the equation of state and this stuff is very
| far beyond anything that could be studied experimentally in a
| collider. It's admittedly a fascinating topic, but there is
| very little rigorous science surrounding it.
| cryptonector wrote:
| > it is very much possible that neutrons transition directly
| into a black hole before entering a new state
|
| At least the neutrons that start out being outside the
| Schwarzschild radius of the black-hole-to-be cannot
| transition directly into being in a black hole state because
| they have to first move from where they are to the event
| horizon, and that has to take some non-zero amount of time
| since they can't exceed the speed of light on the way to the
| event horizon.
|
| It's possible that with the degeneracy pressure exceeded they
| just don't become something else, but instead simply move...
| through each other? That makes little sense. Since they are
| made up of quarks what makes sense instead is that they
| become quark soup on the way to the event horizon. Though
| there are other possibilities too. Maybe all the neutrons
| turn to photons going towards the incipient black hole and
| without becoming quark soup first, they they can travel
| through each other.
|
| > Unfortunately, we have no idea about the equation of state
| and this stuff is very far beyond anything that could be
| studied experimentally in a collider. It's admittedly a
| fascinating topic, but there is very little rigorous science
| surrounding it.
|
| The best we could do is check different theories for
| consistency, but we can't test them _unless_ those theories
| make predictions about electromagnetic emissions or gravity
| wave emissions from a neutron star collapse that we might be
| able to observe and which could be used to test those
| predictions.
| sigmoid10 wrote:
| >they have to first move from where they are to the event
| horizon
|
| No. I mean, from their point of view it would seem they
| kind of have to do, but there are some serious open
| questions about what happens here exactly. Regardless, as
| an outside observer, you would never see them actually move
| behind the horizon, you would only see the black hole grow
| beyond the point where they used to be. For a detailed
| description of all this you can check out chapter 32 of
| MTW's _Gravitation._
|
| >Since they are made up of quarks what makes sense instead
| is that they become quark soup on the way to the event
| horizon
|
| Again, this depends on the equation of state and the
| existance and location of transition points. We don't know
| any of those things - but we do know that any realistic
| model must include general relativity, because quantum
| effects alone are no longer sufficient to describe what
| happens here. At that level all bets are off.
|
| >The best we could do is check different theories for
| consistency
|
| That is one thing. But it is also kind of moot since this
| is where string theory has been stuck for half a century
| now.
| Modified3019 wrote:
| Off topic, but the reference to critical point of water reminded
| me of this lovely video from ages ago:
| https://www.youtube.com/watch?v=2xyiqPgZVyw
|
| The original link is now gone, and archive.org doesn't have the
| other two videos, but I'm pretty sure I have them in my archive:
| https://web.archive.org/web/20080416125148/http://www.scienc...
| csours wrote:
| I wonder how this would interact with neutrinos?
|
| It seems like a quark core would be predominated by the weak
| interaction, so it might be more opaque to neutrinos.
| rbanffy wrote:
| I'd imagine it as more opaque just because of the density.
| ThouYS wrote:
| nice! just like the strong interaction material from the three
| body problem!
| edem wrote:
| If you are interested in this topic then i __highly__ recommend
| the PBS Spacetime channel on YouTube:
| https://youtu.be/1Ou1MckZHTA?si=enfHtWOSa9BwYRZ-
|
| where they discuss this topic and so much more. it is truly a
| gold mine on this topic!
| gigatexal wrote:
| 100% can attest I love this channel
| ryandvm wrote:
| Love Spacetime, but I find there's a point about 2/3 of the way
| through every video where I realize I am totally lost in the
| explanation and I'm just letting the science talk spritz over
| me like a light summer rain. Vaguely hoping that I'm picking up
| some sort of osmotic education. I think it's like ASMR for
| wish-they-could-have-been physicists.
| quarkw wrote:
| This is an exceptionally fun read for me since my name is Quark!
| The terms "quark star" and "quark matter" are completely new to
| me so I'll have to do some reading!
| rbanffy wrote:
| > It's not every day we find quintillions of tonnes of a new
| state of matter
|
| In terms of size, it'd still be quite unimpressive. About the
| volume of a million matchboxes. Would fit in a truck.
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