Quiz: Remember the definition of mal de mer?
alteration of a physical system from one state, or condition, to another. In atomic and particle physics, transitions are often described as being allowed or forbidden (see selection rule). Allowed transitions are those that have high probability of occurring, as in the case of short-lived radioactive decay of atomic nuclei. In three-millionths of a second, for instance, half of any sample of unstable polonium-212 becomes stable lead-208 by ejecting alpha particles (helium-4 nuclei) from individual atomic nuclei. Forbidden transitions, on the other hand, are those that have a high probability of not occurring. A strictly forbidden transition is one that cannot occur at all