Quantum Mechanics (QM), in very simple language, can be regarded as the science which deals with the behaviour of matter and light on the atomic and subatomic scale. It describes and accounts for the properties of molecules, atoms and their constituents—electrons, protons, neutrons and even quarks and gluons! Also, it deals with how these constituents interact with each other and with Electromagnetic Radiation. Thus, it basically deals with the aspects of nature where classical mechanics fails. Three of the most basic features of QM that show how unique it is are as follows:
- All quantities of a bound system like energy, momentum, angular momentum, etc. are quantized (they have discrete values).
- Wave-particle duality: this simply means everything is made of waves and also particles! In other words, we can describe every object as a virtue of its wave or particle property.
- QM is probabilistic! It’s impossible to predict with absolute certainty the outcome of a single experiment on a quantum system. When scientists perform some experiment, the prediction always takes the form of a probability for finding each of the particular possible outcomes.
One can always go ahead and read more on topics like Quantum Interference, Quantum Entanglement and so on! The topic is never-ending. If General Relativity can be called one of the pillars of Modern Physics, QM or Quantum Theory would definitely be the other.