 Wave function collapse

Wave function collapse is a key concept in quantum mechanics that describes the process by which a quantum system transitions from a superposition of states to a single state when a measurement is made. In quantum mechanics, particles such as electrons exist in a superposition of multiple possible states, each with a certain probability. The wave function is a mathematical function that encodes all the information about the system and describes these probabilities.

Similar to quantum jump in the Bohr model, but for the Schrödinger equation.

The idea the the wave function of a small observed system collapses "obviously" cannot be the full physical truth, only a very useful approximation of reality.

Because then are are hard pressed to determine the boundary between what collapses and what doesn't, and there isn't such a boundary, as everything is interacting, including the observer.

The many-worlds interpretation is an elegant explanation for this. Though it does feel a bit sad and superfluous.