In the beginning of their book, Peskin & Shroeder say that replacing non-relativistic energy [itex]p^2/(2m)[/itex] with the relativistic one [itex]\sqrt{p^2 c^2 + (mc^2)^2}[/itex] does not remove infinite propagation speeds given by the propagator(adsbygoogle = window.adsbygoogle || []).push({});

[tex]

\int\frac{d^3p}{(2\pi\hbar)^3} e^{-i(E_p t - p\cdot(x-y))/\hbar}

[/tex]

Does this claim also appear in earlier literature of quantum theory, or is it a new one?

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# Propagation speeds in literature

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