The world of physics works like a music box: there's a speed knob. Turn it slowly, and the melody is strict — balls roll along trajectories (classical mechanics). Speed it up — the notes blend into a chord: a particle is in many places at once, all probabilities (quantum regime).

The secret of switching is in the parameter κ. It intertwines the distribution of particles and the routes of their paths into a complex wave (a blend of real and imaginary, like a melody and accompaniment). When κ is small, Newton's laws naturally emerge from the equation. But when κ is not small, an equation arises, almost identical to Schrödinger's, and with it — all the quantum weirdness: superposition, uncertainty, randomness.

This unified approach shows that classical and quantum are merely different modes of one order, much as entropy unifies micro- and macrostates, and the Big Bang marks the beginning of everything from a singularity.

The irony is that the Hamilton-Jacobi equation from the 1830s, created for billiards and planets, already carried a quantum wave within it; physicists needed complex numbers and a dash of courage to turn classical into quantum.