"I think I can safely say that nobody understands quantum mechanics."
- Richard Feynman
We all have heard about quantum mechanics and all its implications: The many-world hypothesis, the Heisenberg uncertainty principle, Schrodinger's equation, Schrodinger's cat... The list would never end. But how did it all begin? Why was there the need to abruptly change our world view from a deterministic universe to a universe where randomness rules?
Once again, our story begins with the 23 year old English gentleman who saw an apple fall from a tree. The same gentleman, Isaac Newton, also derived the laws of optics and he concluded that light was made up of particles just like matter. However, an experiment from Thomas Young in 1801 concluded that light could undergo interference just like waves would do. Therefore, Newton's idea of the particulate nature of light was abandoned. A few decades later, Maxwell's equations showed that indeed light was just a wave, an electromagnetic wave.
In 1887, Heinrich Hertz observed a phenomenon that could not be explained by the wave properties of light, the photoelectric effect. In short, the photoelectric effect is what causes the emission of electrons from a material by the action of light. It was only in 1905, that Albert Einstein published a paper to explain the phenomenon and it implied that light behaved like particles, which he called photons. Einstein was later awarded the Nobel prize for this discovery. It is often assumed that quantum mechanics was born in 1905 with Einstein's paper on photoelectric effect, however, quantum mechanics was born on Friday 14th of December 1900 in a paper by German physicist Max Planck.
Indeed, the first problem that quantum mechanics helped solved was not about the nature of light but was rather about the black body radiation. A black body is a body that absorbs all frequencies of electromagnetic radiations. And when a black body is in thermal equilibrium with its surrounding it emits radiations called black body radiations which are dependent only on the temperature. The physics of black bodies was studied by numerous physicists including Boltzmann and Kirchhoff. The accepted theory of black body radiations was called the Rayleigh-Jeans law named after Lord Rayleigh and Sir James Jeans. But it had a huge problem, called the ultraviolet catastrophe. If the sum of all the energies emitted by a black body was calculated, an infinite amount was obtained. That was a violation of the law of conservation of energy.
On the 14th of December 1900, Max Planck published a paper, Zur Theorie des Gesetzes der Energieverteilung im Normalspektrum (On the Theory of the Law of Energy Distribution in Normal Spectrum). If energy could be considered to be discrete, that is, distributed in small packets called quanta, then that would solve the problem of the ultraviolet catastrophe. The Rayleigh-Jeans law was replaced by the Planck law and in 1923, Max Planck received the Nobel prize for it. Radioactivity was discovered four years earlier, light was made up of particles, the electron was discovered in the same era. All the ingredients were here, for the birth of quantum mechanics.