Quantum mechanics (cried quantum physics or quantum theory an aa) is a branch o physics which deals wi physical phenomena at microscopic scales, whaur the action is on the order o the Planck constant. It departs frae classical mechanics primarily at the quantum realm o atomic an subatomic length scales. Quantum mechanics provides a mathematical description o much o the dual pairticle-lik an wave-lik behavior an interactions o energy an matter. It is the non-relativistic leemit o quantum field theory (QFT), a theory that wis developed later that combined quantum mechanics wi relativity.