# Klein paradox and causality

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Suppose we're trying to localize particle obeying relativistic wave equation in space. As we know, then pairs of particles and antiparticles will occur, unlike the situation with non-relativistic theory. From the other side, antiparticles are needed in Poincare-covariant theory for restoring the causality of the theory.

Therefore I want to ask: is there any direct relation between the Klein paradox and the causality?

asked Sep 9, 2017

As we know from the right results of relativistic quantum electrodynamics, we always deal with multi-particle systems and the "classical mechanical" results are inclusive QED pictures where a sum on soft photons is carried out. No wonder that Poincaré covariance is about relativistic systems with multi-particle contents. Soft photons have no threshold to be created/absorbed, massive particles have, but in a relativistic limit this threshold is negligible and a truly relativistic system is inevitably multi-particle, so is its mathematical description.

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