Well, I have just studied this, from Landau's Mechanika: "It is easy to see that the mass of a particle cannot be negative. For, according to the principle of

Well, I have just studied this, from Landau's Mechanika: "It is easy to see that the mass of a particle cannot be negative. For, according to the principle of least action, the integral \[ S=\int_1^2 \frac{1}{2} m v^2 dt \] has a minimum for the actual motion of the particle in space from point 1 to point 2. If the mass were negative, the action integral would take arbitrarily large negative values for a motion in which the particle rapidly left point 1 and rapidly approached point 2, and there would be no minimum." Isn't Hamilton's principle also valid in quantum mechanics?