My QM text defines the position operator as follows: The position operator $X= (X_1,X_2,X_3)$ is such that for $j=1,2,3: \ X_j \psi(x,y,z)= x_j \psi(x,y,z)$. To me this can mean two things. 1) $X$ is a vector and acts as $X \psi(x,y,z)= (x \psi(x,y,z), y \psi(x,y,z), z \psi(x,y,z))$. But...

We all know that the asymmetry between matter and antimatter is a big puzzle in physics. But I don't know why one expects matter-antimatter symmetry in the first place? As in, is there a fundamental principle which suggests that matter and antimatter should have been produced in the same number a...

Traditionnal definition of speed in space-time If I understood it correctly a speed is defined relatively to space-time, i.e. we can talk of the speed of anything moving or propagating “inside” space-time. This speed is measured in m / s. The space-time has a structural “speed limit” which is th...

Wolfgang Demtröder writes this in his book on Experimental Physics, The future destiny of a microparticle is no longer completely determined by its past. First of all, we only know its initial state (location and momentum) within limits set by the uncertainty relations. Furthermore, the fina...