These probability amplitudes make up something called the "wavefunction" of the particle.
This wavefunction evolves in time by a fundamental equation called the Schrödinger equation.
However, at no time after 12:00 noon will the wavefunction evolving by the Schrödinger equation describe a situation in which the nucleus has defi- nitely decayed, or a situation in which it has definitely not decayed.11
Consequently, after a measurement the old wavefunction must be replaced by a new wavefunction that reflects the observer's newly acquired knowledge.
This change or replacement of the wavefunction is tradi- tionally called "the collapse of the wavefunction."
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