A Short History of Quantum Mechanics

Classical mechanics was based on the idea that the world is deterministic. That is, if given enough information, you would be able to know the past and predict the future of a system and its constituents. As we shall see in just a moment, this is not always true.

Physics in the $20^{th}$ century stood at a crossroads. Experimental evidence from technologically advanced apparatus started contradicting the established rules of classical mechanics. Experiments involving particles on the scale of electrons and photons started highlighting the chinks in the armor of classical mechanics. To avoid this becoming an expose on physics, we shall briefly describe the key takeaways from quantum mechanics.

Quantum mechanics, as the name suggests, is the study of infinitesimal particles (or quanta) that permeate everything around us. Photons, electrons, quarks, gluons, and the like. You may have noticed the italicization of the word ‘particles’ in the previous sentence. This follows Einstein’s discovery of the Photoelectric Effect and Thomas Young’s Double-Slit Experiment. Einstein’s discovery showed that light is made up of particles and earned him the Nobel Prize in Physics. Thomas Young, on the other hand, showed that light also exhibited wave-like behavior. De Broglie later built upon this and proved that every particle has a wavelength associated with it.

These shockingly contradictory results laid the foundations of what is known today as the wave-particle duality. In some situations, quanta behave like particles, while in other situations quanta behave like waves. The two formulations of quanta are inseparable from each other and making peace with the duopoly aligned the wave and particle-like nature of quanta.

Subsequent work by Schrödinger and others also highlighted the non-deterministic nature of quanta. The world of photons, electrons, etc., is founded upon probabilities and uncertainty. This new way of understanding the world, called quantum mechanics, definitively struck the final nail in the coffin of classical mechanics and took its place as the correct explanation of the infinitesimal world.

In the past lessons, we have discussed superposition as one of the peculiarities of quantum mechanics. In the subsequent lessons, we shall now focus on entanglement and interference. But before we can do that, a discussion of the three postulates of quantum mechanics is necessary.