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Never mind, I answered my own question...
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Thank you. My problem was in convincing myself that J_3 leaves the state invariant. I will look at Weinberg when I canmeopemuk said:By definition
[tex] W_0 = \mathbf{P} \cdot \mathbf{J} [/tex]
Applying this operator to [itex]| p \rangle [/itex] we obtain
[tex] W_0 | p \rangle = P_3 J_3 | p \rangle [/tex]
[itex]J_3 [/itex] is a generator of the "little group" which leaves this vector invariant (up to a constant factor), so (see eq. (2.5.39) in Weinberg's "The quantum theory of fields")
[tex] W_0 | p \rangle = P_3 \lambda| p \rangle = p_0 \lambda| p \rangle [/tex]
where [itex] \lambda[/itex] is helicity.
The Poincare group is a mathematical concept used in physics to describe the symmetries of spacetime. It includes translations, rotations, and boosts (changes in velocity) and is a fundamental part of the theory of relativity.
Massless representations refer to particles that have zero mass. In physics, particles are classified as either massive or massless, and this distinction has important implications for their behavior and interactions.
Massless representations are a subset of the Poincare group, specifically the representations that describe particles with zero mass. This means that they exhibit the same symmetries as the Poincare group, including translations, rotations, and boosts.
Massless representations are important in physics because they include particles such as photons, which are the carriers of electromagnetic energy and play a crucial role in many physical phenomena. They also have special properties, such as traveling at the speed of light, that make them essential to our understanding of the universe.
Massless representations are studied and used in physics through various theoretical and experimental approaches. Theoretical physicists use mathematical models and equations to describe the behavior of massless particles, while experimental physicists use particle accelerators and other tools to observe and measure the properties of these particles in action.