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
Using integration by parts on the second term (assuming χ vanishes on ∂U), diving by ε on both sides and letting ε→0 this becomes a variation in direction χ
δS[Φ][χ]=∫Uχ(x)[∂L∂Φ(Φ(x),∂μΦ(x))−∂μ(∂L∂(∂μΦ)(Φ(x),∂μΦ(x)))]d4x
By requiring variations in all directions equal zero we obtain
∂L∂Φ−∂μ(∂L∂(∂μΦ))=0
(arguments the same as always, so omitted).
Massive scalar field example
Consider Lagrangian density L=12ημν∂μΦ∂νΦ−12m2Φ2
ημν∂μ∂νΦ+m2Φ=◻Φ+m2Φ=0
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