Hi, I would appreciate some help with this issue: I want to calculate the probability denstities and currents from the Schrödinger and Klein-Gordon equations, and I've found 2 ways so far, the one that gives the "standard" result (the one I've seen on my course, or wikipedia) but I don't understand, and the one that is obvious, but something I don't like about it (more later). The first solution is the one in Bjorken's book, and here: http://goo.gl/D04Xlo, one just multiplies the conjugate wave function to the left of the equation, or the wave function to the left of the conjugate equation, and substracts both, but I don't understand why, for example, ψ*(∂ψ/∂t)+ψ(∂ψ*/∂t)=∂(ψ*ψ)/∂t (I think they are implying that), or even why the m^2 terms in K-G dissapear, the same with the ∇ relations, to me those things do not commute, or is there something else happening? The second solution can be found here https://goo.gl/0Rq9nx and in many other sites, the difference is that one wave function is multiplied to the RIGHT of the equations, so the operations are straightforward, the m^2 terms obviously cancel out, but the formulas for the probability denstities and currents are not the same as before. So my questions are: what I am missing in the first case? Are the solutions on the second case somewhat equivalent to the first ones? why can I call both probability denstities and currents? Thanks!!