  This work will incorporate a few related tools for addressing the conceptual
difficulties arising from sewing together classical and quantum mechanics:
deterministic operators, weak measurements and post-selection. Weak
Measurement, based on a very weak von Neumann coupling, is a unique kind of
quantum measurement with numerous theoretical and practical applications. In
contrast to other measurement techniques, it allows to gather a small amount of
information regarding the quantum system, with only a negligible probability of
collapsing it. A single weak measurement yields an almost random outcome, but
when performed repeatedly over a large ensemble, the averaged outcome becomes
increasingly robust and accurate. Importantly, a long sequence of weak
measurements can be thought of as a single projective measurement. I claim in
this work that classical variables appearing in the macro-world, such as centre
of mass, moment of inertia, pressure and average forces, result from a
multitude of quantum weak measurements performed in the micro-world. Here
again, the quantum outcomes are highly uncertain, but the law of large numbers
obliges their convergence to the definite quantities we know from our everyday
lives. By augmenting this description with a final boundary condition and
employing the notion of "classical robustness under time-reversal" I will draw
a quantitative borderline between the classical and quantum regimes. I will
conclude by analyzing the role of macroscopic systems in amplifying and
recording quantum outcomes.
