We show that the formulations of non-relativistic quantum mechanics can be
derived from the principle of least observability. Observability is a concept
introduced here to measure the distinguishability (or traceability) that a
physical object exhibits during its dynamics. To quantify observability, we
assume that the Planck constant defines the discrete amount of action a
physical object needs to exhibit in order to be observable. Then, observability
is calculated by 1.) dividing the action variable along the classical path by
the Planck constant, and 2.) adding information metrics on distinguishability
due to vacuum fluctuations. The least observability principle not only recovers
quantum formulations including the uncertainty relation and the Schr\"{o}dinger
equation in both position and momentum representations, but also brings in new
results on two fronts. At the conceptual level, we find that the information
metrics for vacuum fluctuations are responsible for manifesting entanglement
effects without underlying physical interactions, implying that entanglement
effects are non-causal. At the mathematical level, defining the information
metrics for vacuum fluctuations using more general definitions of relative
entropy results in a generalized Schr\"{o}dinger equation that depends on the
order of relative entropy. The least observability principle is a new
mathematical tool, and we expect other advanced quantum formulations can be
obtained from it.Comment: 17 pages, 1 figure. Revised Section I and II to clarify the concept
of observability; Further improved the mathematical notation