We show that, contrary to the statements made by many authors, the backflow is not a nonclassical effect. The backflow is a characteristic feature of solutions of the wave equations: quantum and classical. We present simple solutions of the Dirac equation, Maxwell equations and equations of linearized gravity where the backflow phenomenon is clearly seen. In this work we describe backflow in relativistic theories but this phenomenon occurs in the solutions of all kinds of wave equations.

We present further arguments which show that the backflow has a universal character. It is not restricted to quantum theory and it appears in many theories (quantum or classical). It is a general property of waves propagating in any number of dimensions.

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