Is there a quantum nuclear theory?

[RJMooreII]

I was re-watching Richard Feynman's 1979 New Zealand lectures on Quantum Electro-Dynamics, and one of the points he brings up is that there is not a satisfactory formulation of nuclear behavior in quantum equations. Has this since been resolved? Is there a satisfactory quantum formulation of nuclear forces and particles?

Note: This isn't a homework question, just personal interest.

 

[Kevin_Axion]

Yea, Quantum Chromodynamics is the most acceptable quantum field theory to understand quark and gluon behaviour: http://en.wikipedia.org/wiki/Quantum_chromodynamics.

 

[RJMooreII]

Yea, Quantum Chromodynamics is the most acceptable quantum field theory to understand quark and gluon behaviour: http://en.wikipedia.org/wiki/Quantum_chromodynamics.

Thanks, this had been bouncing around in my mind for a while now.

 

[inflector]

I was re-watching Richard Feynman's 1979 New Zealand lectures on Quantum Electro-Dynamics, and one of the points he brings up is that there is not a satisfactory formulation of nuclear behavior in quantum equations. Has this since been resolved? Is there a satisfactory quantum formulation of nuclear forces and particles

Quantum Chromodynamics describes quarks and gluons and the strong force which in residual form holds nucleons together but it does not model how nucleons are put together into any particular element's nucleus. We don't have a good theory for the nucleus itself at this point, in particular, the binding energies of the nucleus and why there are rises and drops in the binding energy curve as protons and neutrons are added to a nucleus. We know some of the factors involved but we don't have a good theory for why they are important.

The http://en.wikipedia.org/wiki/Semi-empirical_mass_formula" is reasonably accurate but it is only partially based on theory and it does not match measurements of binding energy to anywhere near the accuracy that say QED matches the phenomena it models.

 

[RJMooreII]

Quantum Chromodynamics describes quarks and gluons and the strong force which in residual form holds nucleons together but it does not model how nucleons are put together into any particular element's nucleus. We don't have a good theory for the nucleus itself at this point, in particular, the binding energies of the nucleus and why there are rises and drops in the binding energy curve as protons and neutrons are added to a nucleus. We know some of the factors involved but we don't have a good theory for why they are important.

The http://en.wikipedia.org/wiki/Semi-empirical_mass_formula" is reasonably accurate but it is only partially based on theory and it does not match measurements of binding energy to anywhere near the accuracy that say QED matches the phenomena it models.

Interesting. So there are still elements about the formation of the neutrons and proton interaction that aren't well explained.

What I recall Feynman saying is that, under certain circumstances, you can treat neutrons and protons as point-like particles and the normal rules for quantum mechanics apply very well, but in other situations - especially those over very small distances - they fall apart and give crazy answers like infinity or zero.

 

[Kevin_Axion]

That is true.

 

[inflector]

Interesting. So there are still elements about the formation of the neutrons and proton interaction that aren't well explained.

I first started getting interesting in physics again a few years back when I realized this was true. I was doing research on directions for fusion research and I was floored that we're spending billions on reactor prototypes like ITER but we still don't know very much about the nucleus itself and why it fuses.

To me it's as if the theoretical physicists had more interesting work once the experimentalists discovered the particle soup and the theorists just left the nucleus behind as it wasn't as sexy as quantum theory, QED, QCD, string theory and all the new unification and quantum theories.

That's why I find it interesting. I think there is still work to be done.

If we understood the mechanisms of the nucleus better we might find some way to finesse fusion by increasing the cross section for particular reactions.

 

[RJMooreII]

I first started getting interesting in physics again a few years back when I realized this was true. I was doing research on directions for fusion research and I was floored that we're spending billions on reactor prototypes like ITER but we still don't know very much about the nucleus itself and why it fuses.

To me it's as if the theoretical physicists had more interesting work once the experimentalists discovered the particle soup and the theorists just left the nucleus behind as it wasn't as sexy as quantum theory, QED, QCD, string theory and all the new unification and quantum theories.

That's why I find it interesting. I think there is still work to be done.

If we understood the mechanisms of the nucleus better we might find some way to finesse fusion by increasing the cross section for particular reactions.

Yes, unfortunate for us on Earth that we can't replicate the easy way to get nuclear fusion, extreme gravitational forces :P.
Of course, if we could generate extreme Gs in localized and controlled situations, it would probably be simpler to make a singularity, throw matter at it and catch the gamma rays that stream off it.

 

[Kevin_Axion]

The problem with fusion is the net energy return, it is less than what is used to create it.

 

[RJMooreII]

The problem with fusion is the net energy return, it is less than what is used to create it.

Right, because, as I understand it, it's quite difficult to keep the reaction going in a localized position? Obviously fusion bombs have more output than the fission reaction that gets them going, but such a boom isn't quite useful for power generation.

 

[Kevin_Axion]

Correct, fusion isn't a continuous uncontrollable process like a chain reaction. Where as fission is. It's also finding a more efficient way to create fusion or plasma, a lot of research is being done in lasers for this purpose. Also creating tritium is expensive but deuterium can be found in ore.

 

[RJMooreII]

Correct, fusion isn't a continuous uncontrollable process like a chain reaction. Where as fission is.

And this is a problem the sun has 'solved', since it's gravity well continuously force feeds the plasma back into itself and creates sustained heat and compression, ja?

 

[Kevin_Axion]

I'm not fully sure on astronuclear processes, eventually the outwards pressure of nuclear fusion will strip it's atmosphere leaving an extremely dense white dwarf. Also plasma emissions are continuous by the Sun so it isn't a perpetual cycling.

 

[RJMooreII]

I'm not fully sure on astronuclear processes, eventually the outwards pressure of nuclear fusion will strip it's atmosphere leaving an extremely dense white dwarf. Also plasma emissions are continuous by the Sun so it isn't a perpetual cycling.

Well, obviously, I mean even if it could keep all it's plasma it would eventually break down due to loss of energy in the form of light. But what I mean is that the sun is, to a fair extent, able to keep itself going for quite some time. Of course, it also has absolutely enormous quantities of fuel.

 

[Kevin_Axion]

The typical fusion reaction in stars is: $$p+p+p+p \rightarrow \alpha+\overline{p}+\overline{p}+\nu+\nu + MeV(\gamma)$$

 

[RJMooreII]

The typical fusion reaction in stars is: $$p+p+p+p \rightarrow \alpha+\overline{p}+\overline{p}+\nu+\nu + MeV(\gamma)$$

I'm not always symbol savvy...this is a proton–proton chain reaction you've got here?

I need to take some intro physics in college :|. I'm good with a lot of the qualitative stuff, but I have never delved much into the math except for some limited fiddling with relativity and the easy stuff like some classical mechanics.

 

[granpa]

p is proton
p bar is positron

I had the impression that it was more a matter of absorbing electrons than emitting positrons

 

[RJMooreII]

p is proton
p bar is positron

I had the impression that it was more a matter of absorbing electrons than emitting positrons

Ah, okay.
Quick question, since positrons were brought up...a photon is it's own anti-particle, right? I mean, can't two photons hit each other and spit out, say, an electron and positron? I think I've heard this, but I've also heard that photons can't interact with one another. I mean, obviously they tend not to with a laser since they're more or less hyper-stacked on top of one another.

 

[granpa]

its called pair production.

it only occurs in the vicinity of pre-existing matter.

 

[RJMooreII]

its called pair production.

it only occurs in the vicinity of pre-existing matter.

Okay, so this, "A photon can decay into an electron positron pair in the Coloumb field of a nucleus when the center of momentum frame energy exceeds the rest mass of the nucleus plus two electrons"?
Which would mainly be gamma rays, right?

 

[RJMooreII]

Are there any good layman books on QCD? Most quantum mechanics related material I've read or watched online relates to QED and problems like quantum gravity. I've of course heard quarks mentioned, but no real detailed discussion of gluons, the various colors, how these relations manifest themselves in particles, etc.

 

[Kevin_Axion]

Get The Road to Reality, it's probably the best investment in mathematics and physics literature. I wouldn't get into dense QCD books unless you've taken a Quantum Mechanics course.

 

[Kevin_Axion]

I'm not always symbol savvy...this is a proton–proton chain reaction you've got here?

I need to take some intro physics in college :|. I'm good with a lot of the qualitative stuff, but I have never delved much into the math except for some limited fiddling with relativity and the easy stuff like some classical mechanics.

Like stated above
$$p=proton$$
$$\overline{p}=positron$$
$$\alpha=alpha particle$$
$$\nu=neutrino$$
$$MeV(\gamma) = energy$$

 

[Kevin_Axion]

$${\cal L}_{QCD} = \overline{\psi}_i(i\gamma^\mu(D_\mu)_{ij}-m\delta_{ij})\psi_j-\frac{1}{4}G^a_{\mu\nu}G^{\mu\nu}_a = \overline{\psi}_i(i\gamma^\mu\partial_\mu-m)\psi_i-gG^a_\mu\overline{\psi}_i\gamma^\mu\ T^a_{ij}\psi_j-\frac{1}{4}G^a_{\mu\nu}G^{\mu\nu}_a$$

Just because physics and mathematics is so awesome!

 

[cgk]

I first started getting interesting in physics again a few years back when I realized this was true. I was doing research on directions for fusion research and I was floored that we're spending billions on reactor prototypes like ITER but we still don't know very much about the nucleus itself and why it fuses.

To me it's as if the theoretical physicists had more interesting work once the experimentalists discovered the particle soup and the theorists just left the nucleus behind as it wasn't as sexy as quantum theory, QED, QCD, string theory and all the new unification and quantum theories.

One particular problem in theoretical nuclear physics is that you have to go into ultra-hardcore numerics mode just to get any answer at all... it doesn't even have to be good. Basically, there are immense amounts of work to do (both in derivation/computer implementation and actual numeric computations) before you can actually investigate/test any kind of physical insight. That's just the nature of highly-correlated fermionic many-body physics. Unfortunatelly, the required algebraic/numeric tools often do not exist, or not to the desired degree, and therefore theorists tend to search for other topics where experimentally verifiable answers can be obtained in a less laborious way.

The problems are in fact quite similar to what is found in Quantum Chemistry (arguably the branch of physics with the most advanced quantitative many-body methods), but the difference is that there are many more people interested in calculating molecules than there are people interested in calculating nuclear cores, so the former field is more advanced.