QED could precisely predict the magnetic moment of electron

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Discussion Overview

The discussion revolves around the predictive capabilities of Quantum Electrodynamics (QED) in relation to the magnetic moments of particles like the electron and muon, and contrasts this with the challenges in predicting the lifetimes of neutrons and muons, which are governed by weak interactions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why QED can predict the magnetic moment of the electron and muon accurately but struggles with the neutron's lifetime.
  • Another participant notes that the strong force, described by Quantum Chromodynamics (QCD), is responsible for holding the neutron together.
  • A participant asks whether QED can predict the muon's lifetime accurately, suggesting that muon decay is mediated by the weak force, thus requiring electroweak theory rather than QED.
  • It is mentioned that both neutron and muon decays are weak processes, and a reference to a textbook is provided for further reading.
  • One participant expresses uncertainty about the understanding of weak interactions and questions why they seem less predictable than QED.
  • Another participant argues that both interactions are effective, but calculations in QED are simpler due to the small coupling constant, allowing for more reliable perturbation series calculations.
  • It is pointed out that the weak interaction is fundamentally harder to calculate due to factors like the massive propagator and the axial nature of the process, complicating Feynman diagram construction.
  • A participant expresses a personal dislike for the weak interaction, describing it as less aesthetically pleasing compared to QED or QCD.

Areas of Agreement / Disagreement

Participants generally agree that both neutron and muon decays are weak processes, but there is no consensus on the predictive capabilities of weak interactions compared to QED, with differing opinions on the complexity of calculations involved.

Contextual Notes

The discussion highlights limitations in the understanding of weak interactions and the challenges in calculations, but does not resolve these complexities or provide definitive answers.

vincentchan
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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?
 
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Because the strong force holds the neutron together, the strong force is described by QCD.
 
how about the life time of muon, can QED predict the lifetime of muon with a high accuracy?
 
vincentchan said:
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
 
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.

--------------------------------------------------------------
[1]David J.Griffiths,"Introduction to Elementary Particles",Harper & Row,1987.
 
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...?
 
They're both good.It's just that the calculations involved in QED are much simpler.

Daniel.
 
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)
 
Last edited:
Yes,both neutron & muon decay are weak processes.

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

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