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.

Definitions of ken
  1. noun
    range of what one can know or understand
    “beyond my ken
    synonyms: cognizance
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    type of:
    a clear and certain mental apprehension
  2. noun
    the range of vision
    synonyms: sight
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    type of:
    compass, grasp, range, reach
    the limit of capability
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