672. In high school physics, we learned that matter and energy can neither be created nor destroyed. Is that true in quantum mechanics? What is quantum mechanics and how did the field come about? — JE, College Station, TX
While modern physics continues to maintain that matter and energy can't be created or destroyed, the picture is a little more complicated than it was before the discovery of relativity and quantum mechanics. First, relativity ties matter and energy together so that matter can become energy and energy can become matter in certain circumstances. As a result, it's only the sum of matter and energy that can't be created or destroyed. Second, there are situations in which that sum of matter and energy can change temporarily in an isolated system. Quantum mechanics and its famous "uncertainty principle" permit brief but important violations of the conservation of mass/energy. The shorter a particular violation, the worse it may be. These violations are never directly observable because all observations are done on long time scales. But there are indirect indications of these temporary violations and they're critical to much of modern high energy and particle physics.
Quantum mechanics developed at the beginning of this century to explain several strange experimental observations, particularly the photoelectric effect and the black-body radiation spectrum. Einstein received his Nobel Prize for explaining the photoelectric effect in terms of quantum mechanics, not for any of his work on relativity.