Register to reply 
EL Equations for the modified electromagnetic field Lagrangian 
Share this thread: 
#1
Apr1612, 10:51 AM

P: 47

Hi,
I'm trying to work through something and it should be quite simple but somehow i've gotten a bit confused. I've worked through the Euler Lagrange equations for the lagrangian: [tex] \begin{align*} \mathcal{L}_{0} &= \frac{1}{4}(\partial_{\mu}A_{\nu})(\partial^{\mu}A^{\nu}) \\ &= \frac{1}{4}F_{\mu\nu}F^{\mu\nu} \end{align*} [/tex] getting: [tex]\Box A_{\nu}  \partial^{\nu}\partial_{\mu}A^{\mu} = 0[/tex] I'm ok with this. Then considering the modified lagrangian: [tex]\mathcal{L}_{\xi} = \mathcal{L}_{0} + \frac{\lambda}{2}(\partial_{\sigma}A^{\sigma})^2[/tex] I'm trying to work out the EL equation components and as part of one of these calculations, I've to calculate: [tex] \begin{align*} \frac{\partial}{\partial(\partial_{\mu}A_{\nu})} \left[ \frac{\lambda}{2} (\partial_{\sigma}A^{\sigma})^2 \right] &= \frac{\lambda}{2} \frac{\partial}{\partial(\partial_{\mu}A_{\nu})} \left[ ( \partial_{\sigma}A^{\sigma} ) ( \partial_{\rho}A^{\rho} ) \right] \\ &= \frac{\lambda}{2} \frac{\partial}{\partial(\partial_{\mu}A_{\nu})} \left[ ( \partial_{\sigma}A_{\alpha} \eta^{\sigma \alpha} ) ( \partial_{\rho}A_{\beta} \eta^{\rho \beta} ) \right] \\ & = \frac{\lambda}{2} \eta^{\sigma \alpha} \eta^{\rho \beta} \frac{\partial}{\partial(\partial_{\mu}A_{\nu})} \left[ ( \partial_{\sigma}A_{\alpha} ) ( \partial_{\rho}A_{\beta} ) \right] \\ & = \frac{\lambda}{2} \eta^{\sigma \alpha} \eta^{\rho \beta} \left[ ( \partial_{\sigma}A_{\alpha} ) \left( \frac{\partial}{\partial(\partial_{\mu}A_{\nu})} ( \partial_{\rho}A_{\beta} ) \right) + \left( \frac{\partial}{\partial(\partial_{\mu}A_{\nu})} ( \partial_{\sigma}A_{\alpha} ) \right) ( \partial_{\rho}A_{\beta} ) \right] \\ & = \frac{\lambda}{2} \eta^{\sigma \alpha} \eta^{\rho \beta} \left[ ( \partial_{\sigma}A_{\alpha} ) \delta^{\mu}_{\rho} \delta^{\nu}_{\beta} + \delta^{\mu}_{\sigma} \delta^{\nu}_{\alpha} ( \partial_{\rho}A_{\beta} ) \right] \\ & = \frac{\lambda}{2} \eta^{\sigma \alpha} \eta^{\rho \beta} ( \partial_{\sigma}A_{\alpha} ) \delta^{\mu}_{\rho} \delta^{\nu}_{\beta} + \frac{\lambda}{2} \eta^{\sigma \alpha} \eta^{\rho \beta} \delta^{\mu}_{\sigma} \delta^{\nu}_{\alpha} ( \partial_{\rho}A_{\beta} ) \\ & = \frac{\lambda}{2} \eta^{\sigma \alpha} \eta^{\mu \nu} ( \partial_{\sigma}A_{\alpha} ) + \frac{\lambda}{2} \eta^{\mu \nu} \eta^{\rho \beta} ( \partial_{\rho}A_{\beta} ) \\ & = \frac{\lambda}{2} \eta^{\mu \nu} \left[ ( \partial_{\sigma}A^{\sigma} ) + ( \partial_{\rho}A^{\rho} ) \right] \\ & = \lambda \eta^{\mu \nu} ( \partial_{\sigma}A^{\sigma} ) \\ \end{align*} [/tex] Now I was hoping to get: [tex] \lambda \partial^{\nu} A^{\mu} [/tex] as ultimately I need the EL equations to give me: [tex] \begin{align*} \frac{\partial \mathcal{L}_{\xi}}{\partial A_{\nu}}  \partial_{\mu} \left( \frac{\partial \mathcal{L}_{\xi}}{\partial (\partial_{\mu} A_{\nu})} \right) &=\Box A^{\nu}  \partial^{\nu} ( \partial_{\mu} A^{\mu} )  \lambda \partial^{\nu}(\partial_{\mu} A^{\mu}) \\ &= \Box A^{\nu}  ( 1 + \lambda ) \partial^{\nu} ( \partial_{\mu} A^{\mu} ) \\ &= 0 \end{align*} [/tex] Can anyone show me where i've gone wrong? I didn't stick this in the homework section as it's not homework. I'm just trying to work through the through missing steps from the text i'm reading. Thanks in advance 


#2
Apr1612, 10:51 AM

P: 47

edit... now i'm finished stating the question.



#3
Apr1612, 04:20 PM

Sci Advisor
Thanks
P: 4,160

Irrational, You've got exactly what you need. Well, up to a minus sign anyway. You've got λ η^{μν}(∂_{σ}A^{σ}). So plug this into the EulerLagrange equation:
∂_{μ}(λ η^{μν}(∂_{σ}A^{σ})) = λ ∂^{ν}(∂_{σ}A^{σ}) = λ ∂^{ν}(∂_{μ}A^{μ}) 


#4
Apr1612, 04:54 PM

P: 47

EL Equations for the modified electromagnetic field Lagrangian
you have no idea how thick i feel right now. thanks for filling in the gap.



Register to reply 
Related Discussions  
Lagrangian for electromagnetic field  Classical Physics  2  
Electromagnetic lagrangian  Advanced Physics Homework  1  
Electromagnetic Lagrangian  Advanced Physics Homework  3  
Electromagnetic lagrangian  Classical Physics  1  
Why is the Electromagnetic Field Tensor in the QED Lagrangian?  Quantum Physics  5 