quark stars

[tim314]

Quark Stars were mentioned in this thread, and I'd like to hear some more information about them, but didn't want to completely hijack that thread.

Here's an article about their apparent discovery.

My question is: where do these free quarks come from, and why do they remain free? I was under the impression that free quarks will hadronize almost instantaneously, and that it is impossible to remove quarks from hadrons because the binding force increases the further you pull them apart.

I'm also puzzled by this quote from the article linked to above:

Some scientists have suggested that if strange quark matter does exist, it could destroy ordinary matter by converting protons and neutrons to naked quarks, spreading through space like a cosmic wildfire.

The "cosmic wildfire" bit seems a bit outrageous (space is pretty darn big and things are pretty spread out -- you can't exactly start a wildfire with lightyears in between the trees), but how is it even possible to convert protons and neutrons into free quarks? Again, I would have thought the nature of the strong force makes this impossible.

[Mathochist]

Originally Posted by tim314

My question is: where do these free quarks come from, and why do they remain free? I was under the impression that free quarks will hadronize almost instantaneously, and that it is impossible to remove quarks from hadrons because the binding force increases the further you pull them apart.

Who said they're free (other than that conjecture about "strange quark matter", which is an awful name given the existance of "strange quarks")?

What the color theorem says is that observed states are color singlets. Basically, each quark carries a three-dimensional "color vector" which transforms under the regular representation 3 of SU(3)_color. A system of n quarks transforms as 3ⁿ: the nth tensor power of this representation (yes, tensor products and representation theory and group actions all wrapped up together. Seems someone was advocating this recently...). Observable states occupy the subspace of vectors in 3ⁿ left invariant by the action of the group. That is, decompose 3ⁿ as a direct sum of irreducible representations and pick out the 1-homogenous component. As a note: antiquarks add factors of the dual representation.

Now, 3?3' = 1?8, so there's a one-dimensional space of observable color states of a quark and an antiquark: mesons. 3ⁿ = 1?8?8?10, and the 1 gives a space of color states for three quarks: baryons.

Now, a year ago we already had discovered "pentaquarks", which in terms of color live in

3⁴?3 = (1?8)?(1?8?8?10)

Now I'm not going to distribute that tensor product over the direct sums on the right, but note that since each term has a 1 in it, there will be a direct summand of 1?1 = 1 in the result, leaving "room" for color-singlet pentaquark states. The puzzle was why none were ever seen. It was resolved when one was seen.

Now, why couldn't a quark star occupy a singlet state in the decomposition of 3^{a rather awful lot}?

[Flander]

Um, yeah. What Mathochist said.

[Whack-a-Mole]

Originally Posted by Flander

Um, yeah. What Mathochist said.

No kidding.

For those of us who were sick the day they taught Quantum Chromodynamics in Kindergarten is there a more layman's version? (I realize that dumbing it down for the unwashed masses loses perhaps important details but when those details are beyond us anyway I'm not sure it matters.)

[Exapno Mapcase]

The article quoted in the OP itself gives the version that's all that most of us, including me, will ever need:

But theory has long suggested that inside the superdense remains of dead stars, quarks might be forced out permanently into the open.

They would make up a radically different state of matter called strange quark matter, so dense that a teaspoonful of it would weigh billions of tonnes.

Supersqueezing gravity can produce neutron stars or squeeze even more and squeeze quarks out of neutrons.

But they're no more free than the neutronium talked about in other recent threads would be free outside of this gravity squeezing.

[Dr. Lao]

Originally Posted by Whack-a-Mole

For those of us who were sick the day they taught Quantum Chromodynamics in Kindergarten is there a more layman's version? (I realize that dumbing it down for the unwashed masses loses perhaps important details but when those details are beyond us anyway I'm not sure it matters.)

I think it boils down to this: a quark star would be like one big neutron. Instead of being made up of 3 quarks, it is made up of some fantasticly huge number of quarks.

[tim314]

Originally Posted by Whack-a-Mole

No kidding.

For those of us who were sick the day they taught Quantum Chromodynamics in Kindergarten is there a more layman's version? (I realize that dumbing it down for the unwashed masses loses perhaps important details but when those details are beyond us anyway I'm not sure it matters.)

Actually, I think I got what I wanted out of Mathochist's post.

Basically, they're not free quarks -- the star just becomes like one big hadron.

That "cosmic wildfire" gibberish still strikes me as kind of odd, but I'm not losing any sleep over it.

[Mathochist]

Originally Posted by Whack-a-Mole

No kidding.

For those of us who were sick the day they taught Quantum Chromodynamics in Kindergarten is there a more layman's version? (I realize that dumbing it down for the unwashed masses loses perhaps important details but when those details are beyond us anyway I'm not sure it matters.)

I'm not really sure you can explain quark confinement (which is what tim314 about: specifically if it was violated) without representation theory.

Basically, you've got three states a quark can be in, commonly called "red, green, blue". There's a symmetry acting that exchanges these states among themselves. Something might swap red and green, another swaps red and blue, the composition shuffles them all around (red->green->blue->red). The only states that you're allowed to see in the real world are ones where the symmetry action does nothing.

Let's label a quark state (fc), where f denotes a flavor (up or down, say) and c a color. The state (ur,ug,db), for instance, is one red up quark, one green up quark, and one blue down quark. If we swap green and blue, we get (ur,ub,dg), which is not the same state, so (ur,ug,db) isn't allowed. However, we can superpose states. Consider

(ur,ug,db) + (ur,ub,dg) + (ug,ur,db) + (ug,ub,dr) + (ub,ur,dg) + (ub,ug,dr)

Now any shuffling of the colors merely swaps around terms in the sum, and so gives back the same state. This is an allowed state, which we call the proton.

Now a quark star can be composed of an awful lot of quarks with various flavor and color states such that the symmetry swapping around the colors leaves the total state invariant, but not one expressible as a simple collection of a bunch of nucleons (the distinction is subtle, but important). This would be seen as a blob composed of quarks as opposed to neutrons, but the state as a whole would not violate quark confinement.

If you grok neutron stars, thinking about one as sort of a giant atomic nucleus, you're halfway there. A quark star isn't just a giant nucleus, but more like a giant nucleon: something like a neutron blown up to a macroscopic scale. I think the most interesting thing about this would be that (at least as it seems to me) it's essentially a quantum mechanical object at a macroscopic scale. In particular it has noticeable gravitational effects and may well be an useful object in studying the interaction between quantum mechanics and general relativity.

[Mathochist]

Originally Posted by Mathochist

I'm not really sure you can explain quark confinement (which is what tim314 about: specifically if it was violated) without representation theory.

Gr. Something went wrong. Change that parenthetical to "which is what tim314 was asking about"

[MikeS]

This particular subfield of physics isn't my specialty, but let me give it a shot:

There are two "stable" particles that are made out of quarks that exist on Earth: The neutron and the proton. Both are made out of quarks called "up" and "down" quarks; the neutron has one up quark and two down quarks, the proton two ups and one down. There are four other types of quarks, originally discovered in particle accelerators, called (in order of increasing mass) the "strange", "charm", "bottom", and "top." These heavier quarks will, in general, decay via the weak nuclear force into the lighter up and down quarks.

Now, it was conjectured in the mid-80s (actually earlier, but the mid-80s was when people started paying attention to the possibility) that under sufficient temperature and pressure, you might be able to make a mix of up, down, and strange quarks stable. These wouldn't be grouped into particles as they are in neutrons, but instead you would have a "soup" of quarks running around whose color charges happened to cancel (this is essentially what Mathsochist was saying). It was further conjectured that the necessary conditions of temperature and pressure would be met in collapsing stars: a quark star would be even denser than a neutron star.

Here's the fun part: it has also been conjectured that this "strange quark matter" is even more stable than "conventional quark matter" (i.e. neutrons and protons), even at low temperatures and pressures. Under this hypothesis, the only reason that we're still made of protons and neutrons is that there need to be a certain number of strange quarks around for SQM to be stable, and there just aren't that many on Earth. But if a sufficiently large chunk of SQM were to hit the Earth (for example), it would start a chain reaction that would convert the entire Earth into SQM. A close analog would be how water can be supercooled: liquid water can exist below 0 C, but if a "seed" is added for the ice crystals to start forming on, it'll immediately freeze.

I think this is all correct, but like I said, this isn't my specialty. I welcome any corrections/additions/bitchslaps that someone more knowledgeable than me might give.

Originally Posted by Mathochist

Who said they're free (other than that conjecture about "strange quark matter", which is an awful name given the existance of "strange quarks")?

Actually, it's quite an apt name, because "strange quark matter" is characterized by the presence of strange quarks (as opposed to "conventional" quark matter, which only has up and down quarks.)

[Mathochist]

Originally Posted by MikeS

Actually, it's quite an apt name, because "strange quark matter" is characterized by the presence of strange quarks (as opposed to "conventional" quark matter, which only has up and down quarks.)

Well, yes it must have strange quarks to keep from decoupling into nucleons (like the pentaquark needed strange quarks to keep from decoupling into a nucleon and a pion). Still, there could be "strange quark matter" states completely devoid of strange quarks: swap out all the strange for bottom quarks. Yes, it wouldn't be as stable locally, but I think it shows the unfortunateness of the name. "Unconventional quark matter" I could get behind.

[tim314]

Originally Posted by MikeS

Here's the fun part: it has also been conjectured that this "strange quark matter" is even more stable than "conventional quark matter" (i.e. neutrons and protons), even at low temperatures and pressures. Under this hypothesis, the only reason that we're still made of protons and neutrons is that there need to be a certain number of strange quarks around for SQM to be stable, and there just aren't that many on Earth. But if a sufficiently large chunk of SQM were to hit the Earth (for example), it would start a chain reaction that would convert the entire Earth into SQM.

So it doesn't require the gravitational force of a star to make "strange quark matter" stable? If that's the case, then why can't we produce SQM in accelerators? (Other than our desire not to destroy the Earth, I mean.) We can certainly make strange quarks.

[Whack-a-Mole]

It may be that SQM will not eat the earth (or whatever it comes across) since there is speculation that some has hit the earth and we are all still here.

Did quark matter strike Earth?

Admittedly more assumption than fact but interesting nonetheless.

Is a Quark-Gluon Plasma (QGP) synonymous with Strange Quark Matter (SQM)?

[Chronos]

I think the most interesting thing about this would be that (at least as it seems to me) it's essentially a quantum mechanical object at a macroscopic scale. In particular it has noticeable gravitational effects and may well be an useful object in studying the interaction between quantum mechanics and general relativity.

No doubt there's a lot of interesting physics to be gotten from study of quark stars, but unfortunately I don't think this is among it. All this would give us is quantum mechanics in the presence of a gravitational field, which we think we already have a pretty good handle on. Experiments have been done on neutron interference in a gravitational potential, and Hawking radiation is a quantum effect in curved space as well. What's really the Holy Grail for quantum gravity, though, is a system where the spacetime itself would be subject to quantum mechanical effects.

[MikeS]

Originally Posted by tim314

So it doesn't require the gravitational force of a star to make "strange quark matter" stable? If that's the case, then why can't we produce SQM in accelerators? (Other than our desire not to destroy the Earth, I mean.) We can certainly make strange quarks.

You're not alone in worrying about this. The experiment went ahead, and as far as I can tell, the earth's still here, although I don't believe that this experiment conclusively detected SQM. So there are three possibilities, not necessarily exclusive: either (a) you need a larger amount of SQM to cause the transition than we've produced as yet, (b) SQM requires a larger anmount of energy to produce than our accelerators can yield as yet, or (c) the SQM really does require the higher temperature & pressure to be stable. I think most scientists would lean towards (c), for the simple reason that there are very high-energy processes that happen naturally (higher than we can currently produce here on Earth), so if it could happen, it would have already happened in one of those processes. It's not an iron-clad argument, though...

Originally Posted by Whack-A-Mole

Is a Quark-Gluon Plasma (QGP) synonymous with Strange Quark Matter (SQM)?

No. A QGP is a large number of quarks that have given enough energy (however briefly) to dissociate and run around freely before cooling off and recondensing into hadrons (and, possibly, SQM.) A very close analog would be the way electrons and protons are dissociated in the plasma in the sun (the analog of the gluons in this example would be the photons that would necessarily be running around.) In practice, you produce QGP by smashing heavy nuclei together (in this case, lead.)

[Mathochist]

Originally Posted by Chronos

No doubt there's a lot of interesting physics to be gotten from study of quark stars, but unfortunately I don't think this is among it. All this would give us is quantum mechanics in the presence of a gravitational field, which we think we already have a pretty good handle on. Experiments have been done on neutron interference in a gravitational potential, and Hawking radiation is a quantum effect in curved space as well. What's really the Holy Grail for quantum gravity, though, is a system where the spacetime itself would be subject to quantum mechanical effects.

Well, if this is more massive than a neutron star but less than a black hole, it allows us to get that much closer to the unobservable.

Actually, I just thought of something that might shoot this in the foot: is any curvature large enough to be interesting guaranteed to be behind an event horizon?

[Mathochist]

Originally Posted by Whack-a-Mole

It may be that SQM will not eat the earth (or whatever it comes across) since there is speculation that some has hit the earth and we are all still here.

Did quark matter strike Earth?

Hmm. Does anyone think this might be what Jenkoul is periodically on about?

[Whack-a-Mole]

Originally Posted by Mathochist

Hmm. Does anyone think this might be what Jenkoul is periodically on about?

Certainly seems more plausible than a mini black hole but still seems rather far fetched compared to it simply having been an asteroid or comet. CalMeacham mentions looking over seismic records that would show what the article I linked to showed and said nothing like that is in the record.

[Chronos]

Actually, I just thought of something that might shoot this in the foot: is any curvature large enough to be interesting guaranteed to be behind an event horizon?

Yes. Either you have a curvature singularity at the center, which is certainly large enough to match any standard of "interesting", or you have some quantum gravity effect which prevents a singularity from forming, in which case the curvature is large enough to produce that interesting effect. However, there is no reason to suppose that there's any interesting curvature immediately inside the horizon of a black hole: That's all happening down in the center, and for a stellar-mass object, the curvature near the Schwartzschild radius would be rather moderate.

[MikeS]

Originally Posted by Mathochist

Actually, I just thought of something that might shoot this in the foot: is any curvature large enough to be interesting guaranteed to be behind an event horizon?

Originally Posted by Chronos

Yes. Either you have a curvature singularity at the center, which is certainly large enough to match any standard of "interesting", or you have some quantum gravity effect which prevents a singularity from forming, in which case the curvature is large enough to produce that interesting effect. However, there is no reason to suppose that there's any interesting curvature immediately inside the horizon of a black hole: That's all happening down in the center, and for a stellar-mass object, the curvature near the Schwartzschild radius would be rather moderate.

This is true for the "physically reasonable" spacetimes that folks working in general relativity have been able to come up with thus far (there are some spacetimes with so-called "naked singularities", such as the superextremal Reissner-Nordstrøm solution or the negative-mass Schwarzchild solution, which nobody expects would actually exist in the real world.) More generally, though, what Mathochist is asking isn't too far from the question, "Does the cosmic censorship conjecture hold?" This is still an open question in GR, largely because it's not easy to formulate the question in a mathematically tractable way.

[Mathochist]

Originally Posted by MikeS

More generally, though, what Mathochist is asking isn't too far from the question, "Does the cosmic censorship conjecture hold?"

Well, I want to be clear that I'm specifically not asking that, but something even less well-defined: "is everywhere that curvature is large enough to make quantum gravitational effects significant hidden behind an event horizon?

[FriendRob]

Well, a Planck mass black hole would have its Schwartzschild radius at the Planck length, so any less massive BH would have a Schwartzschild radius less than the Planck length. That should be enough to produce "interesting" effects. Unfortunately, we can't probe distances that small, so all the interesting effects will be hidden, anyway....

Hey, maybe that's what elementary particles are! They're mini-BHs, and things like spin, charge, and color are the quantum gravity effects of twisted spacetime! Now, where can I publish....?

[Mathochist]

Originally Posted by FriendRob

Hey, maybe that's what elementary particles are! They're mini-BHs, and things like spin, charge, and color are the quantum gravity effects of twisted spacetime! Now, where can I publish....?

Unfortunately I don't think it's new. A similar thing happened when I first grokked GR and thought that maybe just like there's no "force" of gravity, that possibly all other "forces" can be reformulated as curvatures. This wasn't the first time the idea came up, but unfortunately the quantum effects mean needing a quantized theory of geometry that isn't really worked out yet and everyone ran off to thinking about other solutions while I was seduced by algebra.

The road not taken, eh?

[bonzer]

No, it's not new: 't Hooft was giving seminars ten years back about some highly speculative scheme where elementary particles were black holes, though I'm not sure he ever published anything on it. And I've forgotten the details.

[Whack-a-Mole]

Originally Posted by Mathochist

"is everywhere that curvature is large enough to make quantum gravitational effects significant hidden behind an event horizon?

I can't be arsed to dig up the cite right now but somewhere in my reading of Quark Stars they mentioned the gravity well could be sufficicient to make light orbit it (some would still get away though I believe).

Considering that going not much further gets you a Black Hole I think that is about as close as you will get. Someone else will have to tell us if that is a sufficiently large curvature to make quantum garvitational effects sans an event horizon.

[KarlGauss]

This is a complete hijack, but since the thread topic is somewhat related, and since the people gathered here may be best positioned to answer, let me ask:

Could the tidal forces near a black hole be sufficient to separate quark pairs into free quarks?

[most likley this is one of those questions that results from a naive "understanding", or a totally inaccurate concept of what quarks (and other subatomic particles) really are]

[Mathochist]

Originally Posted by KarlGauss

Could the tidal forces near a black hole be sufficient to separate quark pairs into free quarks?

As I understand it, no. Outside the event horizon it's no more intense gravity than any other object of that mass would have at that distance from the center.

[KarlGauss]

I guess the way I was looking at was that for sufficiently small radius, the tidal force should be infinite. No?

[Mathochist]

Originally Posted by KarlGauss

I guess the way I was looking at was that for sufficiently small radius, the tidal force should be infinite. No?

But the Schwartzchild radius is proportional to the mass inside. That is, as you get in closer you have to drop the mass to stay outside the event horizon. Maybe at a low enough mass you'd get the curvature high enough outside the radius to matter, but at that point you'd be looking at something the size of a subatomic particle anyhow.

[KarlGauss]

Ah, that makes a lot of sense. Thank you.

[FriendRob]

Originally Posted by KarlGauss

This is a complete hijack, but since the thread topic is somewhat related, and since the people gathered here may be best positioned to answer, let me ask:

Could the tidal forces near a black hole be sufficient to separate quark pairs into free quarks?

[most likley this is one of those questions that results from a naive "understanding", or a totally inaccurate concept of what quarks (and other subatomic particles) really are]

The other part of the answer to this question is that, no matter HOW you try to pull quarks apart, the energy you put into pulling eventually becomes enough to pop a new quark-antiquark pair into existence. You don't end up with free quarks, you end up with a couple of extra mesons.

{I won't go into the problems of even defining a "particle" in an accelerated reference frame....}

[RiverRunner]

First, a few observations:
1) Do not, under any circumstances, misread the word "hadron" as "hardon" in sentences such as tim314's ". . . the star just becomes like one big hadron." It only leads to confusion.

2) Since I'm sure someone else will point it out anyway, "Strange Quark Matter" would be a good name for a rock band.

3) There is no such place as Meson, Arizona.


Now, regarding this contribution:

Originally Posted by Whack-a-Mole

It may be that SQM will not eat the earth (or whatever it comes across) since there is speculation that some has hit the earth and we are all still here.

Did quark matter strike Earth?

Admittedly more assumption than fact but interesting nonetheless.

From the article:

. . . strange quark particles would dash through Earth with dramatic effect: a one-tonne spec would release the energy of a 50-kilotonne nuclear bomb, spread along its entire path through the Earth.

How big would a one-tonne speck of SQM be?


RR

[MikeS]

Originally Posted by RiverRunner

How big would a one-tonne speck of SQM be?

I haven't been able to come across any figures for the density of SQM, but a 1-tonne speck of neutron-star matter would be about 15 microns across (this would be a blithe extrapolation from the neutron star's mass, of course; without the gravity to hold it together, this chunk of neutronium would simply fly apart.) I assume that since SQM is supposed to be even denser than neutronium, it would be even smaller. If I find any specific numbers on the subject I'll post them here.

[Mathochist]

Originally Posted by FriendRob

{I won't go into the problems of even defining a "particle" in an accelerated reference frame....}

Someone's forgetting their GR: there's no such thing as an accelerated reference frame.

Then again, this only transfers the difficulty to defining a particle in GR.

[Chronos]

Well, I want to be clear that I'm specifically not asking that, but something even less well-defined: "is everywhere that curvature is large enough to make quantum gravitational effects significant hidden behind an event horizon?

Ah, I'm sorry, I misunderstood you. I thought you were asking, given a black hole, is there guaranteed to be large curvature inside.

But a similar answer applies: Yes, you can have significant curvature outside of a black hole. Take a black hole, and let it evaporate for a long time. Eventually, one of two things will happen: Either it will stop evaporating, or it will pass the Planck mass. If it passes the Planck mass, then curvature at or a little outside the horizon will be Planck-scale curvature, which should be enough for something "interesting". And if it stops evaporating before them, it would have to have been a quantum gravity effect which stopped it, in which case also the curvature at or outside the horizon would be interesting.

However, there is still no reason to suppose that there would be significant (in a quantum gravity sense) curvature in the vicinity of a quark star.

Also, the statement "there are no accelerated reference frames" is inconsistent with the statement "there is no gravitational force". A non-freefalling reference frame must be said to either be accelerating, or to have a gravitational force. But in any case, defining a particle in a non-freefalling reference frame is nontrivial.

[Mathochist]

Originally Posted by Chronos

Also, the statement "there are no accelerated reference frames" is inconsistent with the statement "there is no gravitational force". A non-freefalling reference frame must be said to either be accelerating, or to have a gravitational force. But in any case, defining a particle in a non-freefalling reference frame is nontrivial.

How can you define accelerated reference frames without nonaccelerated reference frames? All frames are "of the same type". That's what general covariance means, at least to a differential geometer.