At this point, there is no agreement on whether collapse actually exists or if it is even needed. Relative state interpretations (aka many-worlds interpretations) do not require collapse. It assumes that a single quantum world following the unitary Schrödinger evolution exists. However, this quantum world is split into infinite noninteracting branches. The collapse is, thus, an illusion of an observer whose wave function is trapped in one of these branches. Decoherence is a fundamental piece of relative state interpretations, as it establishes the preferred basis for the branching. Epistemic interpretations also do not require collapse. QBism, for example, considers collapse "nothing but the updating of an agent's state assignment on the basis of her experience." The objective collapse theories share the core hypothesis that a wave function is a real object (rather than a way of interpreting reality ) and that a quantum superposition of states occasionally collapses into a single state. The unified unitary wave function plus collapse time evolution is achieved by including nonlinear terms in the Schrödinger equation. These nonlinear terms give rise to a collapse time inversely proportional to the system size, explaining why macroscopic objects are never observed in quantum superposition.