By applying the commutator to an arbitrary state in either the position or momentum basis, one can easily show that: [ x ^ ^ , p ^ ^ ] = x ^ ^ p ^ ^ − − p ^ ^ x ^ ^ = i ℏ ℏ I , where I is the unit operator. The Heisenberg uncertainty principle defines limits on how accurately the momentum and position of a single observable system can be known at once. In quantum mechanics, position and momentum are conjugate variables.