Thursday, August 7, 2014

quantum mechanics - Why $2j+1$ number of states?


In this statement from Modern Quantum Mechanics by J.J. Sakurai:



If $j$ is an integer, all $m$ values are integers; if $j$ is a half-integer, all $m$ values are half-integers. The allowed $m$-values for a given $j$ are $$m = \underbrace{-j,-j+1,\ldots,j-1,j}_{2j+1 {~\rm{states}}}$$



It says that $m$ will have a total of $2j+1$ states. I do not see this, however. Perhaps it is obvious, but could someone explain or show me why if $m$ goes from $-j,\ldots,j$ it will give $2j+1$ number of states?




Answer



The $2j$ is from the positive and negative j values, and the additional $+1$ accounts for $j=0$.


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