complementarity

Complementarity is Niels Bohr's principle that a quantum object has pairs of properties that are both real and yet can never be displayed at the same time. The wave aspect and the particle aspect are the headline example: an experiment arranged to reveal interference, the wave face, automatically forbids you from learning which path was taken, the particle face, and vice versa. The two are complementary — each is needed for a full account, but only one can ever be seen at once.

Bohr's deeper point was that the very meaning of a quantum property depends on the whole experimental arrangement used to ask about it. There is no point insisting an electron 'really is' a wave or 'really is' a particle independent of any measurement; the question is only answerable inside a concrete setup, and different setups answer mutually exclusive questions. The apparatus is part of the phenomenon, not a neutral window onto it.

Complementarity is not a cop-out but a disciplined refusal to smuggle classical pictures where they do not belong. It is closely tied to the uncertainty principle, which forbids sharp simultaneous values for complementary quantities like position and momentum. Together they say that nature, at its root, does not hold all the classical attributes at once that our intuition expects of it.