**Quantum mechanics** (also known as **quantum physics** or **quantum theory**), as a branch of physics, describes the behaviors of objects at very small scales and low energy levels, i.e. atoms and subatomic particles. Classical physics, on the other hand, deals with matter and energy on a scale familiar to human experience. However, classical mechanics can be derived from quantum mechanics as an approximation valid only at macroscopic scales. From this point of view, it can be said that quantum mechanics is the general theory and classical mechanics is its manifestation at macroscopic scale (though there are physicists who oppose this view). At very large scale and high energy limit, Einstein’s theory of relativity is needed.

#### About jlw

Junling got his PhD in Materials Engineering from University of Maryland, College Park. He is currently a Professor at Nanyang Technological University, Singapore, teaching undergraduate courses related to Quantum Mechanics.