necessary in order to make it compatible with the experimentally verified violation of Bell inequalities, with the relativistic principle of locality. Models exist that attempt to generalize in a relativistic sense the GRW and CSL models, but their status as relativistic theories is still unclear. The formulation of a proper Lorentz-covariant theory of continuous objective collapse is still a matter of research. Tail problem In all collapse theories, the wave function is never fully contained within one (small) region of space, because the Schrödinger term of the dynamics will always spread it outside. Therefore, wave functions always contain tails stretching out to infinity, although their “weight” is smaller in larger systems. Critics of collapse theories argue that it is not clear how to interpret these tails, since they amount to the system never being really fully localized in space. Supporters of collapse theories mostly dismiss this criticism as a misunderstanding of the theory, as in the context of dynamical collapse theories, the absolute square of the wave function is interpreted as an actual matter density. In this case, the tails merely represent an immeasurably small amount of smeared-out matter, while from a macroscopic perspective, all particles appear to be point-like for all practical purposes. See also Interpretation of quantum mechanics Many-worlds interpretation Philosophy of information Philosophy of physics Quantum information Quantum entanglement Coherence (physics) Quantum decoherence EPR paradox Quantum Zeno effect Measurement problem Measurement in quantum mechanics Wave function collapse Quantum gravity Notes External links Giancarlo Ghirardi, Collapse Theories, Stanford Encyclopedia of Philosophy (First published Thu Mar 7, 2002; substantive revision Fri May 15, 2020) Interpretations of quantum mechanics Quantum measurement