In quantum mechanics, what term describes a system being in a specific state due to observation?

In quantum mechanics, the phenomenon where a system enters a specific state due to observation is referred to as wavefunction collapse. When a quantum system is not being observed, it exists in a superposition of all possible states, described mathematically by its wavefunction. Upon measurement or observation, this wavefunction collapses, resulting in the system taking on a definite state.

This principle is crucial because it illustrates the role of the observer in determining the state of a quantum system. The act of measurement impacts the system in such a way that multiple potential outcomes are reduced to one observable outcome. This concept is foundational to understanding various quantum phenomena and highlights the differences between quantum and classical mechanics.

The other terms, while relevant to quantum mechanics, refer to distinct concepts. Measurement theory deals more broadly with how measurements are implemented and interpreted, quantum entanglement describes a phenomenon where particles become interconnected and the state of one affects the state of another, and quantum stability concerns the persistence of certain states over time. None of these capture the specific process of a wavefunction collapsing into a definite state upon observation, which is why the correct answer is wavefunction collapse.