Since translation operators all commute with each other (see above), and since each component of the momentum operator is a sum of two scaled translation operators (e.g. p ^ ^ y = lim ε ε → → 0 i ℏ ℏ ε ε ( T ^ ^ ( ( 0 , ε ε , 0 ) ) − − T ^ ^ ( ( 0 , 0 , 0 ) ) ) ), it follows that translation operators all commute with the momentum operator, i.e. T ^ ^ ( x ) p ^ ^ = p ^ ^ T ^ ^ ( x ) This commutation with the momentum operator holds true generally even if the system is not isolated where energy or momentum may not be conserved.