Are there fundamental limits for observing quantum phenomena from within quantum theory?

Does there exist a limit for the applicability of quantum theory for objects of large mass or size, or objects whose states are of large complexity or dimension of the Hilbert space? The possible answers range from practical limitations due to decoherence within quantum theory to fundamental limits due to collapse models that modify quantum theory. Here, we suggest the viewpoint that there might be also fundamental limits without altering the quantum laws. We first demonstrate that for two quantum spins systems of a given spin length, no violation of local realism can be observed, if the measurements are sufficiently coarse-grained. Then we show that there exists a fundamental limit for the precision of measurements due to (i) the Heisenberg uncertainty relation which has to be applied to the measuring apparatus, (ii) relativistic causality, and (iii) the finiteness of resources in any laboratory including the whole universe. This suggests that there might exist a limit for the size of the systems (dimension of the Hilbert space) above which no violation of local realism can be seen anymore

Macroscopic realism and spatiotemporal continuity

Macroscopic realism, as introduced by Leggett and Garg, is the world view in which properties of macroscopic systems exist independent of and are not influenced by measurement. Motivated by classical physical laws such as Newtonian mechanics or Maxwell's electrodynamics, in this work we add the restrictive postulate that the observables of macroscopic objects are evolved continuously through space and time. Quantum theory violates both macroscopic realism and the continuity assumption. While decoherence or collapse models (e.g. due to a universal noise background or gravitational self energy) can restore macroscopic realism, we show that a continuous spatiotemporal description does not become possible in general. This shines new light on the question how the classical world arises out of the quantum realm