Hello, I'm sure most of you are already familiar with the book "Mechanics" by Landau and Lifshitz. There's a section that I do not understand. In section 4 towards the end they mentions that "It is easy to see that the mass of the particle cannot be negative." They then give the argument that the action integral will not have a minimum if the mass is negative as can be seen if the particle leaves point 1 rapidly and approaches point 2 rapidly. Can someone please elaborate on this?