- #1

- 861

- 42

## Main Question or Discussion Point

Weinberg considers (p.68 QFT Vol. 1) particles with mass M > 0. The Little Group is SO(3). He wants to calculate the rotation

W(Λ,p) ≡ L

k

W(Λ,p) ≡ L

^{-1}(Λp) Λ L(p). He says that for this we need to choose a standard boost L(p) which carries the four momentum fromk

^{μ}= (0,0,0,M) to p^{μ}. He then shows the expressions for the components of L(p). What I dont understand is that because the spatial components of k^{μ}all vanish, i.e. k^{i}= 0, why does it matter to choose a specific set of expressions for (L(p))^{i}_{ j}?