QED could precisely predict the magnetic moment of electron

[vincentchan]

I have a little question about QED..
How come QED could precisely predict the magnetic moment of electron and muon, but can't predict the lifetime of neutron with the same accuracy?

 

[Berislav]

Because the strong force holds the neutron together, the strong force is described by QCD.

 

[vincentchan]

how about the life time of muon, can QED predict the lifetime of muon with a high accuracy?

 

[Norman]

how about the life time of muon, can QED predict the lifetime of muon with a high accuracy?

Actually the decay of the muon is mediated by weak force (this can be seen by the large lifetime of the muon). So electroweak theory would govern this decay, not plain QED.
Cheers,
Ryan

 

[dextercioby]

Yes,both neutron & muon decay are weak processes.Incidentaly,they're both wonderfully treated in [1].And in the same chapter,i.e.chapter 10.

Daniel.

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[1]David J.Griffiths,"Introduction to Elementary Particles",Harper & Row,1987.

 

[vincentchan]

So, Do we fully understand the weak interection?
I mean, if we understand weak interaction, we should able to predict the lifetime of muon, right?
But it seems like weak interaction is not as good as QED... can anyone tell me why...?

 

[dextercioby]

They're both good.It's just that the calculations involved in QED are much simpler.

Daniel.

 

[Haelfix]

Fundamentally the weak interaction is harder to calculate (even with a computer), b/c the coupling constant for QED is so small it allows many orders of perturbation series to reliably be calculated, before asymptotic renormalon behaviour starts corrupting your data. The Fermi coupling being larger doesn't give you that reliability.

Of course there are other issues in the calculational difficulty, some of which are based on the accuracy of experiment, and some technicalities in the weak sector that are quite challenging. First of all the propagator is massive, which adds extra stuff to the calculation, second of all the process is now distinctively axial in nature. Essentially this makes drawing feynmann diagrams for V-A interactions an enormous chore, best left for grad students =)

(note I am pulling an all niter grading papers, so forgive the brevity and conceptual mistakes I am likely making)

(incidentally I hate the weak interaction. I find it remarkably unbeautiful compared to QED or QCD. I also hated attending conferences with the electro weak experamentalists, as invariably some bayesian statistics debate would rear its ugly head and everything would degenerate)

 

[Berislav]

Yes,both neutron & muon decay are weak processes.

Yes, of course. I never meant to imply anything else by my post.