Thursday, 3 September 2015

homework and exercises - The variation of the Lagrangian density under an infinitesimal Lorentz transformation


I'm trying to introduce myself to QFT following these lectures by David Tong. I've started with lecture 1 (Classical Field Theory) and I'm trying to prove that under an infinitesimal Lorentz transformation of the form


Λμν=δμν+ωμν,


where ω is antisymmetric, the variation of the Lagrangian density L is


δL=μ(ωμνxνL).


Using L=L(ϕ,μϕ), I've tried computing δL directly using


δϕ=ωμνxνμϕ


[which I obtained earlier computing explicitly ϕ(x)ϕ(Λ1x)], however, I get δL=μ(ωμνxνL)L(μϕ)ωσμσϕ The extra term arises when I compute μ(δϕ)=ωσν[δνμσϕ+xνμσϕ]=ωσμσϕωσνxνμσϕ


[because I'm assuming μ(δϕ)=δ(μϕ)]; I thought I'd get rid of it just replacing ϕ with σϕ in (1.52), however μ(δϕ)=δ(μϕ) should still hold, ain't it? I also tried using (the previous expression to) 1.27 in the lectures, namely that the derivatives of the field transform as



μϕ(x)(Λ1)νμνϕ(Λ1x),


but I still get (to the first order in ω),


(Λ1)νμνϕ(Λ1x)=(δνμωνμ)νϕ(xσωσρxρ)=(δνμωνμ)[νϕ(x)ωσρxρσνϕ(x)]=μϕωσρxρσμϕωνμνϕ


I'm resisting the idea that ωνμνϕ=0, but I don't understand what I'm doing wrong.



Answer



Provided that L is a Lorentz scalar, the quantity L/(μϕ) has to carry an upper index. Since L is a function of ϕ and μϕ, the only object that can give such an index is μϕ. Hence L(μϕ)μϕ. Then, L(μϕ)ωσμσϕωσμσϕμϕ=ωσμσϕμϕ=0. The last expression vanishes because σϕμϕ is symmetric under the interchange of indices while ωσμ is antisymmetric.


I actually don't understand why Tong didn't simply write δL=ωμνxνμL. After all, L should have the same transformation rule as ϕ because they are both Lorentz scalars. One can verify the above equation by noting that δL=μ(ωμνxνL)=ωμνxνμLωμμL, and that ωμμ=ημρωμρ=0 because ημρ is symmetric and ωμρ is antisymmetric.


No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...