The Journey from Classical to Quantum Mechanics

Quantum mechanics is a paradigm-shift in our understanding of dynamics of particles in this universe. It took shape rapidly over the first couple decades of the twentieth century, following a sequence of more gradual developments starting in the mid-nineteenth century. In this article we describe the developments in physics, relevant to the formulation of quantum physics, since the mid-nineteenth century until around 1935, spanning the entire period of evolution of quantum mechanical ideas. We lay special emphasis on those aspects of the scientific and logical history of this period which we believe were the most significant milestones, but are currently not part of the modern pedagogy of the subject. After reading this article, we hope that a beginner-student, someone with even a cursory idea of classical mechanics in the Hamiltonian formalism, will be able to understand how quantum mechanical ideas originated—ideas like the wave function, operators, the quantum jumps, quantum state-collapse under measurement etc.