positive and negative helicity components, in ratios proportional to the mass of the particle. A treatment of the helicity of gravitational waves can be found in Weinberg. In summary, they come in only two forms: +2 and −2, while the +1, 0 and −1 helicities are "non-dynamical" (they can be removed by a gauge transformation). Little group In dimensions, the little group for a massless particle is the double cover of SE(2). This has unitary representations which are invariant under the SE(2) "translations" and transform as under a SE(2) rotation by . This is the helicity representation. There is also another unitary representation which transforms non-trivially under the SE(2) translations. This is the continuous spin representation. In dimensions, the little group is the double cover of SE() (the case where is more complicated because of anyons, etc.). As before, there are unitary representations which don't transform under the SE() "translations" (the "standard" representations) and "continuous spin" representations. See also Helicity basis Gyroball, a macroscopic object (specifically a baseball) exhibiting an analogous phenomenon Wigner's classification Pauli–Lubanski pseudovector References Other sources Quantum field theory