  We consider the quantum-to-classical transition for macroscopic systems
coupled to their environments. By applying Born's Rule, we are led to a
particular set of quantum trajectories, or an unravelling, that describes the
state of the system from the frame of reference of the subsystem. The
unravelling involves a branch dependent Schmidt decomposition of the total
state vector. The state in the subsystem frame, the conditioned state, is
described by a Poisson process that involves a non-linear deterministic
effective Schrodinger equation interspersed with quantum jumps into orthogonal
states. We then consider a system whose classical analogue is a generic chaotic
system. Although the state spreads out exponentially over phase space, the
state in the frame of the subsystem localizes onto a narrow wave packet that
follows the classical trajectory due to Ehrenfest's Theorem. Quantum jumps
occur with a rate that is the order of the effective Lyapunov exponent of the
classical chaotic system and imply that the wave packet undergoes random kicks
described by the classical Langevin equation of Brownian motion. The
implication of the analysis is that this theory can explain in detail how
classical mechanics arises from quantum mechanics by using only unitary
evolution and Born's Rule applied to a subsystem.
