First order in time=>"time cannot go backwards"? I have had numerous professors mention, but not explain, the differences between PDEs that are second order and first order in time. For example, in the regular wave equation, they say that "time can go backwards," or something to that effect. In order to avoid this in the schrodinger equation, it was made first order, but imaginary. Can you guys explain how reducing the order implies that time cannot flow backward (if that is indeed what my professors meant and not a misunderstanding on my part), why this was implemented into the schrodinger equation but not the wave equation, and how this is supposed to be better than complex solutions? Thanks in advance.