Is There a Symmetric Form of the Dirac Lagrangian?

  • Context: Graduate 
  • Thread starter Thread starter ChrisVer
  • Start date Start date
  • Tags Tags
    Dirac Form Hermitian
Click For Summary
SUMMARY

The discussion focuses on the possibility of writing the Dirac Lagrangian in a symmetric form by utilizing symmetric derivatives. The original Dirac Lagrangian, expressed as L_{D}= \bar{\psi} (i γ^{\mu}∂_{\mu}-m) \psi, can be modified by adding a term involving a 4-divergence, specifically K^{\mu}=i\bar{\psi}\gamma^{\mu}\psi. This modification leads to a doubled Lagrangian, which raises questions about the implications for the equations of motion. The final expression presented is L_{D}=\frac{1}{2} \bar{\psi} (i γ^{\mu}[∂_{\mu}^{→}-∂_{\mu}^{←}] -2m) \psi + \frac{1}{2} i ∂_{\mu}(\bar{\psi}γ^{\mu}\psi), confirming the correctness of the approach.

PREREQUISITES
  • Understanding of the Dirac Lagrangian and its formulation
  • Familiarity with Hermitian operators in quantum mechanics
  • Knowledge of 4-divergence in field theory
  • Basic grasp of gamma matrices and their properties
NEXT STEPS
  • Research the implications of Hermitian conjugates in quantum field theory
  • Explore the role of 4-divergence in deriving equations of motion
  • Study the properties of gamma matrices in relativistic quantum mechanics
  • Investigate alternative formulations of the Dirac Lagrangian
USEFUL FOR

The discussion is beneficial for theoretical physicists, particularly those specializing in quantum field theory and particle physics, as well as graduate students exploring advanced topics in Lagrangian mechanics.

ChrisVer
Science Advisor
Messages
3,372
Reaction score
465
Is there any way to write the Dirac lagrangian to have symmetric derivatives (acting on both sides)? Of course someone can do that by trying to make the Lagrangian completely hermitian by adding the hermitian conjugate, and he'll get the same equations of motion (a 1/2 must exist in that case)...However same equations of motion, imply that there should be a connection between the two Lagrangians via a 4divergence, right? Unfortunately I cannot see what that could be.
L_{D}= \bar{\psi} (i γ^{\mu}∂_{\mu}-m) \psi
In particular choosing
L_{D}= \bar{\psi} (i γ^{\mu}∂_{\mu}-m) \psi + ∂_{\mu}K^{\mu}
seems to bring some result if I choose K^{\mu}=i\bar{\psi}\gamma^{\mu}\psi
but the initial lagrangian seems to get doubled...
 
Physics news on Phys.org
L_{D}=\frac{1}{2} \bar{\psi} i γ^{\mu}∂_{\mu} \psi + \frac{1}{2} \bar{\psi} i γ^{\mu}∂_{\mu} \psi -m \bar{\psi}\psi

L_{D}=\frac{1}{2} \bar{\psi} i γ^{\mu}∂_{\mu} \psi + \frac{1}{2} i ∂_{\mu}(\bar{\psi}γ^{\mu}\psi) - \frac{1}{2}i (∂_{\mu}\bar{\psi})\gamma^{\mu}\psi -m \bar{\psi}\psi

L_{D}=\frac{1}{2} \bar{\psi} (i γ^{\mu}[∂_{\mu}^{→}-∂_{\mu}^{←}] -2m) \psi + \frac{1}{2} i ∂_{\mu}(\bar{\psi}γ^{\mu}\psi)

Should the - exist?
 
Last edited:
Yes, your last expression is right.
 

Similar threads

  • · Replies 49 ·
2
Replies
49
Views
7K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 38 ·
2
Replies
38
Views
6K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 17 ·
Replies
17
Views
3K
  • · Replies 4 ·
Replies
4
Views
2K