Weinberg shows in his book on QFT that the effect of a rotation on a(adsbygoogle = window.adsbygoogle || []).push({});

massive particle's spin is the same as applying exp(-i \theta J.n) on

the state, regardless of particle's momentum. In other words just

rotate the spinor in its own representation. This makes calculations

very simple. But what about boosts? There is of course a general

approach described in Weinberg's book using little group elements; but

it is too difficult to apply it each time. Is there any genral theorem

for boost like that Weinberg presented for rotations? And also what

about massless particles?

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# Boosts on spinors

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