Classical statistical mechanics requires the existence of h (but does not define its value). Eventually, following upon Planck's discovery, it was recognized that physical action cannot take on an arbitrary value. Instead, it must be some multiple of a very small quantity, the "<answer>quantum of action</answer>", now called the Planck constant. Classical physics cannot explain this fact. In many cases, such as for monochromatic light or for atoms, this <answer>quantum of action</answer> also implies that only certain energy levels are allowed, and values in between are forbidden.
What is the Planck constant formerly known as?
quantum of action