The noun ken means "range of vision or comprehension." If quantum mechanics is beyond your ken, you don't understand it, or it is beyond your scope of knowledge.
Ken is rarely used today outside of the phrase, "beyond one's ken." It goes all the way back, however, to Proto Indo-European, the reconstructed ancestor of most European, Near Eastern, and South Asian languages. Coming from the root *gno- "to know," ken has many relatives in modern English such as incognito, cunning, and know itself.