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 Post subject: Stable Nuclei
PostPosted: Wed Dec 07, 2011 4:09 pm 
Avisaru
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I only have a rudimentary understanding of the forces at work binding atoms together. But I was curious that if you tweaked the power of the strong force or perhaps tweaked the distance of its influence if you'd be able to make some ludicrously large nuclei.

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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 5:50 pm 
Sumerul
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If you have any system, and then go and tweak its variables, you can make some silly stuff.

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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 6:11 pm 
Sumerul
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Kvan wrote:
I only have a rudimentary understanding of the forces at work binding atoms together. But I was curious that if you tweaked the power of the strong force or perhaps tweaked the distance of its influence if you'd be able to make some ludicrously large nuclei.


That's what I was thinking. But I tend to think that the universal constants are fairly fickle.

I'd love to know more about quantum physics and the like, but it's not particularly my focus of interest in science.

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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 7:38 pm 
Lebom
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The strong interaction is already the strongest one known to science. The problem here is that you think of the strong interaction as in classical physics, or even non-relativistic quantum mechanics. The strength of the interaction does not only describe the attraction between two particles, but also the various transformations, desintegrations and emissions the particles may do (such as the emission of a gluon, the splitting of a gluon in two, quark-antiquark annihilation, etc).

To give you a simple example, the weak interaction is somewhere in strength between the strong one and electromagnetism, yet I can't think of any system of particles bound by the weak interaction (it might be possible, I don't know, as it is rather similar to electromagnetism in many ways).

The strong interaction one of the most complex one, because of some mathematical wizardry (non-commutative generators and all), and even a single proton is beyond the current scope of analytical results from quantum chromodynamics. Making the interaction stronger or weaker would produce uncertain results, as such (although making it extremely weaker would make it much easier to calculate).

Also, just for fun, you could consider neutron stars like ludicrously large nuclei (although that's not strickly true - the crust is regular nuclei packed together closely, the inside is neutron fluid with weird proton structures and electron gas and the core is god knows what).


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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 8:39 pm 
Osän
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Worse, the strong force isn't really its own force at all - it's a special case of the color force, the force that binds quarks together. Color-force interactions are what hold quarks into the groupings we call protons, neutrons, etc., and only as a residual effect of those particular quark configurations does it also cause these groupings to cling to each other too. (Other quark groupings, such as IIRC electrons, don't necessarily exhibit any "strong force", as they aren't configured in such a manner as to cause it.) So the strong force is not something you can properly manipulate as an independent variable: you cannot screw with it without screwing with the very structure that causes protons and neutrons to exist.


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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 9:06 pm 
Smeric
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Radius Solis wrote:
Worse, the strong force isn't really its own force at all - it's a special case of the color force, the force that binds quarks together. Color-force interactions are what hold quarks into the groupings we call protons, neutrons, etc., and only as a residual effect of those particular quark configurations does it also cause these groupings to cling to each other too. (Other quark groupings, such as IIRC electrons, don't necessarily exhibit any "strong force", as they aren't configured in such a manner as to cause it.) So the strong force is not something you can properly manipulate as an independent variable: you cannot screw with it without screwing with the very structure that causes protons and neutrons to exist.

Electrons are leptons, not quarks, nor are they made of quarks. That would be why they don't experience the strong force or any form thereof.

As for what would happen if the strong nuclear force or the color force had its properties changed, I have no idea how that would change things, and I don't think anyone does, besides some very basic assumptions.

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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 9:18 pm 
Osän
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Isn't quantum physics weird and unknown enough without messing with the numbers?

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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 9:34 pm 
Sumerul
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Aldwinkle wrote:
To give you a simple example, the weak interaction is somewhere in strength between the strong one and electromagnetism, yet I can't think of any system of particles bound by the weak interaction (it might be possible, I don't know, as it is rather similar to electromagnetism in many ways).
Weak is weaker than EM. I can't think of anything it does other than beta-minus decay and some stuff in other decay chains.

So.. Tweaking the weak force might change decay rates? The whole random-quantum-fluctuation-interaction stuff is a bit of a mystery.

What if strange quarks were stable? What we call Xi minus and zero could then act as nucleons; the zero would act something like a heavy neutron, but a negative nucleon would be ... different. What changes to laws/constants would this need?


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 Post subject: Re: Stable Nuclei
PostPosted: Wed Dec 07, 2011 11:05 pm 
Lebom
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Quote:
Worse, the strong force isn't really its own force at all - it's a special case of the color force, the force that binds quarks together.


That is what "The strong force" is. The force that binds nucleons together is sometimes referred to as the residual strong interaction, because of this.

Quote:
Weak is weaker than EM. I can't think of anything it does other than beta-minus decay and some stuff in other decay chains.


Ah yes. Mistake on my part.

The weak interaction does actually much more than that, though rarely at regular energies. It can do many of the same things as the photon - you can have e+ e- pairs annihilating into a Z (and the opposite process of pair creation), absorption or emission of a Z, etc (hence why the whole electroweak idea - the Z boson is very similar to the photon in the standard model). W bosons are a bit more complicated but in essence they do the same kind of things.

Quote:
So.. Tweaking the weak force might change decay rates? The whole random-quantum-fluctuation-interaction stuff is a bit of a mystery.


Tweaking the coupling constant (that is the actual name of "the force of an interaction") would indeed change the decay rate. This one at least is rather simple as, from memory, the decay rate is proportional to the square of the constant.

Quote:
What if strange quarks were stable? What we call Xi minus and zero could then act as nucleons; the zero would act something like a heavy neutron, but a negative nucleon would be ... different. What changes to laws/constants would this need?


Hard to say. You don't really speak about the stability of the quark themselves, as they are never free. The fact is, only the proton is stable (or at least has a decay rate currently too long to measure). The neutron is not stable, with a half life of about 17 minutes, and it's all downhill from here. Why the neutron is stable inside a nucleus is a complicated issue, but the most intuitive notion I've heard on it is that because of the constant pion exchange (the carrier particle for the residual strong interaction), the protons and neutrons constantly change in the nucleus - a proton emits a pi+ and becomes a neutron, and a neutron absorbs it, etc.

The interaction responsible for the decay of baryons is (I think) usually the strong interaction, with the weak one in a smaller measure (from a cursory glance, anyway - less than 1% of decay products include neutrinos, which are a pretty good sign of weak decay). But the problem isn't so much the strength of the interactions as it is the fact that the proton is the lightest one. On a purely energy perspective, any baryon will end up as protons + electrons and neutrinos, because then, it cannot decay further (well, it can, but it is pretty unlikely).


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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 12:33 am 
Smeric
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Drydic Guy wrote:
Isn't quantum physics weird and unknown enough without messing with the numbers?


No. I see your c and raise you 1,000,000,001.8 m/s.

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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 9:37 am 
Sumerul
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Aldwinkle wrote:
Hard to say. You don't really speak about the stability of the quark themselves, as they are never free. The fact is, only the proton is stable (or at least has a decay rate currently too long to measure).
Ah -- I'd been reading about decay processes and was thinking of neutron decay in terms of one of the downs decaying to an up.

So a proton (uud) can't decay into a delta++ (uuu) due to both exclusion and conservation of isospin -- but apparently conservation of mass-energy too. What is the delta doing with that 294 extra MeV/c^2? Some sort of "binding energy"? Excited quarks? Hmm. Interesting.


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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 11:36 am 
Lebom
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Quote:
So a proton (uud) can't decay into a delta++ (uuu) due to both exclusion and conservation of isospin -- but apparently conservation of mass-energy too. What is the delta doing with that 294 extra MeV/c^2? Some sort of "binding energy"? Excited quarks? Hmm. Interesting.


As said, the exact processes of the strong interaction aren't that well understood. What is called "three quarks" is actually just the average over all fields - it's actually an infinity of quarks and antiquarks in a field of gluons. The mass of a proton (about 938 MeV) is way higher than three quarks (about 5 x 3 MeV), and indeed most of it is the "binding energy" (that sea of quarks and gluons). Why the mass of a delta ++ (about 1200 MeV) is higher than a proton, I'm not sure there's a simple answer, because of this.


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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 11:55 am 
Sumerul
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Aldwinkle wrote:
What is called "three quarks" is actually just the average over all fields - it's actually an infinity of quarks and antiquarks in a field of gluons.
Argh -- I need to get the Bohr model out of my head.. it's far too easy to think of everything as little balls clumped together or moving in regular orbits.


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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 3:00 pm 
Avisaru
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Bob Johnson wrote:
Aldwinkle wrote:
To give you a simple example, the weak interaction is somewhere in strength between the strong one and electromagnetism, yet I can't think of any system of particles bound by the weak interaction (it might be possible, I don't know, as it is rather similar to electromagnetism in many ways).
Weak is weaker than EM. I can't think of anything it does other than beta-minus decay and some stuff in other decay chains.

So.. Tweaking the weak force might change decay rates? The whole random-quantum-fluctuation-interaction stuff is a bit of a mystery.

What if strange quarks were stable? What we call Xi minus and zero could then act as nucleons; the zero would act something like a heavy neutron, but a negative nucleon would be ... different. What changes to laws/constants would this need?


So if the Xi minus and Xi zero were stable as well as the proton and (bound) neutron then theoretically/speculatively we could have an even more robust/insane periodic table, with the elements we know "proton-neutron"-nuclei, and the "xi minus-xi zero"-nuclei, and even some cross over "xi-minus-proton", "xi-minus-neutron", "xi zero-proton", "xi minus-xi zero-proton" "xi minus-xi zero-proton-neutron" "xi minus-xi zero-neutron" or perhaps even "xi zero-neutron" nuclei. But someone brought up something about pion exchange and how it leads to stability of the neutron, so I'm unsure as to whether or not a neutral atom would be stable. But then again this is almost all speculation and crazy left field conjecture. I'm just thinking it might be interesting for a background of a story with Chemists and Alchemists in my setting.

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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 3:51 pm 
Sumerul
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Kvan wrote:
So if the Xi minus and Xi zero were stable as well as the proton and (bound) neutron then theoretically/speculatively we could have an even more robust/insane periodic table
Yes.

Kvan wrote:
I'm unsure as to whether or not a neutral atom would be stable.
I'm unsure as well, but the total EM charge shouldn't matter (except for electron binding); nucleons are bound overwhelmingly by the strong interaction. It's really strong. (The catch is that it's short range only, unlike EM and grav which are both 1/r^2 over infinite range.)


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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 4:08 pm 
Lebom
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Quote:
But someone brought up something about pion exchange and how it leads to stability of the neutron, so I'm unsure as to whether or not a neutral atom would be stable.


That's not really the problem. There are mesons that would do the equivalent transformation for Xi baryons (I can't think of their names off hand but you just need the correct quark content and spin). The real problem is the stability.

Of course, there may be conditions where such things are stable. Again here, neutron stars have rather unique conditions when it comes to the strong interaction. (If you want a cool book based on conworlding in a neutron star, I advise reading Flux by Stephen Baxter).

Quote:
I'm unsure as well, but the total EM charge shouldn't matter (except for electron binding); nucleons are bound overwhelmingly by the strong interaction. It's really strong. (The catch is that it's short range only, unlike EM and grav which are both 1/r^2 over infinite range.)


Strong interaction is also theoretically infinite (it has the same basic characteristics as the electromagnetic interaction). The problem here is asymptotic freedom - the further you go from a quark, the stronger the force is, despite the fact that the intensity should just be in 1/r². The reason is vacuum polarization - pairs of quark-antiquark pairs are born from the gluon field, and those change the properties of the field around the particle.

This also exists for electromagnetism, but because of different properties, has the opposite effect - the charge becomes much weaker the further you go. And because of this, if two quarks try to separate, the binding energy will become stronger, until you just have enough energy to create new quarks from it.

Of course, residual strong interaction, on the other hand, is done by massive mesons. Hence the force will not be in 1/r², but in e^(-mr)/r², which is much easier to understand why it has finite range.


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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 5:35 pm 
Smeric
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Kvan wrote:
So if the Xi minus and Xi zero were stable as well as the proton and (bound) neutron then theoretically/speculatively we could have an even more robust/insane periodic table (…)

All this would have zero effect on the periodic table, actually. From the chemistry POV, the nucleus is a point charge/mass without any other interesting properties. An atom with a hypothetical stable Δ⁺⁺ as its nucleus would be just a fancy isotope of helium, frex.

Drastic mass differences would lead to atomic radii fluctuations, tho. A humungous nucleus with a charge of +1 would lead to teeny-tiny "ball-bearing" hydrogen atoms (not that hydrogen atoms aren't tiny to begin with); small, heavy oxide anions would be harder to get to participate in reactions than regular-sized.

—Cosmologically however, Ξ⁰ and Ξ⁻ being stable would mess things up quite a bit: negative nuclei would form positron-based atoms, leading to "faux antimatter" regions; also, neutral nuclei would start piling up (nothing's stopping them from escaping from stellar atmospheres) and form "transparent" nebulae — not quite dark matter, but uneffected by radiation with everyday energies.

If you want to do some con-chemistry, what you need to mess with are just the element abundances (which all this con-nuclear physics of course also yields). Imagine, say, a planet composed mainly of boron, fluorine, sodium, and copper (rather than our oxygen, magnesium, silicon, and iron)… it'd be quite a different place. Possibly reducting rather than oxidizing, for example: free oxygen might be about as unstable as free sodium is over here, while less metallic elements like silicon might freely occur bare.

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 Post subject: Re: Stable Nuclei
PostPosted: Thu Dec 08, 2011 6:14 pm 
Lebom
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Quote:
—Cosmologically however, Ξ⁰ and Ξ⁻ being stable would mess things up quite a bit: negative nuclei would form positron-based atoms, leading to "faux antimatter" regions


Not sure that really adds up. Positrons are not present in most of the universe because (maybe) of a fundamental asymmetry in physical laws. The fact that they can't bind with protons has little to do with it. Of course it would be easier for positrons to survive in such a thing, because then they wouldn't form the dreaded POSITRONIUM (an "atom" made of electron and positron) that usually leads them to their doom, since they would be in neutral compounds, but they would mostly have to be from primordial antimatter as it occuring naturally (from say, beta decay encountering a random xi) would be quite rare, and even then they might not survive easily (anything too dense would just collapse and ionise it and then they risk just recombining with ordinary matter, something like that perhaps). So really it would just be diffuse isolated clouds.

(maybe)

Also the obvious possibility is simply a proton /xi- type of atom, which would be much easier to form.


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