  A thought experiment is discussed to clarify the concept of decoherence.
Superposition of states consisting of ground state of a single hydrogen atom
and its excited state after a huge amount of time is discussed to show that the
decoherence of the state, more precisely the loss of coherence, is literally
the loss of ability of interference. The loss of superposition of states is
just apparent or even superfacial. Applications of this example in its relation
to many-world interpretation of observation of a quantum state, decoherence in
usual cases, and weak collapse of wavepackets are discussed.
