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]

@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)[sub]color[/sub]. A system of n quarks transforms as 3[sup]n[/sup]: 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 3[sup]n[/sup] left invariant by the action of the group. That is, decompose 3[sup]n[/sup] 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. 3[sup]n[/sup] = 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

3[sup]4[/sup]?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[sup]a rather awful lot[/sup]?

[Flander]

Um, yeah. What Mathochist said.

[Whack-a-Mole]

@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]

@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]

@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]

@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]

@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.

@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]

@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]

@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.:D) 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]

@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…

@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]

@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]

@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]

@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]

@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?

@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.