In Zee's textbook, the photon and "graviton" propagators are derived as the m->0 limit of massive spin 1,2 particles; this works for the photon, but not the graviton. Weinberg adds the caveat that we obtain electrodynamics from the massive theory only if we couple the particles to a conserved source. In Peskin and Schroeder, the Weyl equations are presented as the Dirac equation in the limit of vanishing mass, and as far as I know nothing goes wrong.(adsbygoogle = window.adsbygoogle || []).push({});

Are there any general rules as to when it is and is not possible to derive the physics of a massless particle from the massive case?

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# The m->0 limit

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