Despite the fact that it has been known since the time of Heisenberg that quantum operators obey a quantum version of Newton's laws, students are often told that derivations of quantum mechanics must necessarily follow from the Hamiltonian or Lagrangian formulations of mechanics. Here, we first derive the existing Heisenberg equations of motion from Newton's laws and the uncertainty principle using only the equations F=dtdP, P=mdtdV, and [X,P]=i. Then, a new expression for the propagator is derived that makes a connection between time evolution in quantum mechanics and the motion of a classical particle under Newton's laws. The propagator is solved for three cases where an exact solution is possible 1) the free particle 2) the harmonic oscillator 3) a constant force, or linear potential in the standard interpretation. Such a picture may be useful for students as they make the transition from classical to quantum mechanics and help solidify the equivalence of the Hamiltonian, Lagrangian, and Newtonian formulations of physics in their minds.
This chapter deals with the all-important notion of state as it is central to all parts of physics. For this book, most important are, on the one hand, the notions of state by Newton and Euler – on the other hand, the notion of state by Lagrange and Laplace used nowadays. Surprisingly enough, Newton and Euler’s notions of state are closer to quantum mechanics than the nowadays’ one. Newton’s axiomatic contains solely the conservation of states (1st and 3rd axioms) and the change of states (2nd axiom). In contrast, the equation of motion is not part of the axiomatic. Euler’s axiomatic contains solely the conservation of states. The change of state (and subsequently the equation of motion) is to be examined according to the problem under consideration. In contrast to Newton’s axiomatic and correcting Bohr’s corresponding claim, Euler’s principles of state change for classical bodies are formulated such, that they can be translated even to quantum-mechanical motion. Their power is also demonstrated through an alternative derivation of Hamilton’s equation of motion.