How to go from Quantum World to Classical World? [duplicate]

Question

Possible Duplicate:
Is it possible to recover Classical Mechanics from Schrödinger’s equation?
Classical Limit of the Feynman Path Integral

In the quantum world we don't have specific trajectories, the particle so to speak goes through all possible paths. In the classical and macroscopic world we have definite paths, and usually one specific trajectory is assigned to a body's motion.

How would you go from a trajectoryless world to trajectoried world?

Are there any theories about this bridge between the two worlds?

I guess there should be such a theory, cause one world is the building block of the other.

Answer 1

There are classical systems without trajectories with the particles 'going through' all possible classical paths. Check for instance Poincaré resonances and the limits of trajectory dynamics.

The concept of trajectory is an approximation both in quantum and classical mechanics (check above ref.); we recover trajectories when the states are localized $\sigma \rightarrow \delta$.