1) Quantum mechanic
Found on
https://www.crosswordclues.com/clue/selection-rule
In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions in molecules. The selection rules may differ according to the technique used to observe the transition....
Found on
http://en.wikipedia.org/wiki/Selection_rule
in quantum mechanics, any of a set of restrictions governing the likelihood that a physical system will change from one state to another or will be ... [3 related articles]
Found on
http://www.britannica.com/eb/a-z/s/60
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