Friday, 7 August 2015

lagrangian formalism - The Euler-Lagrange equation in special relativity


How can I derive the Euler-Lagrange equations valid in the field of special relativity? Specifically, consider a scalar field.



Answer



General approach


First recall that Euler-Lagrange equations are conditions for the vanishing of the variation of action S. For a scalar field Φ with Lagrangian density L on some open subset U we have


S[Φ]=UL(Φ(x),μΦ(x))d4x


Consider a variation of the field in direction χ and compute


S[Φ+εχ]=ML(Φ(x)+εχ(x),μ(Φ(x)+εχ(x)))d4x

Then using Taylor expansion S[Φ+εχ]S[Φ]=U[εχ(x)LΦ(Φ(x),μΦ(x))+ε(μχ(x))L(μΦ)(Φ(x),μΦ(x))+O(ε2)]d4x


Using integration by parts on the second term (assuming χ vanishes on U), diving by ε on both sides and letting ε0 this becomes a variation in direction χ



δS[Φ][χ]=Uχ(x)[LΦ(Φ(x),μΦ(x))μ(L(μΦ)(Φ(x),μΦ(x)))]d4x


By requiring variations in all directions equal zero we obtain


LΦμ(L(μΦ))=0


(arguments the same as always, so omitted).


Massive scalar field example


Consider Lagrangian density L=12ημνμΦνΦ12m2Φ2

By using the E-L equations we have just derived we obtain Klein-Gordon equation.


ημνμνΦ+m2Φ=Φ+m2Φ=0


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...