Decoherrence

Quantum Decoherrence occurs as a particle becomes more entangled with it's environment; removing it from a quantum super position, and "forcing" the particle to behave in a "classical" manner. Dechoerrence is what prevents large objects from exhibiting quantum behavior. The more linked a particle becomes with it's environment (the more information it has given off in the form of photon radiation, or collisions with other particles) the more absolute it becomes. Larger objects cannot maintain a quantum super position because to much decoherrece will have necessarily occurred.

from Decoding the Universe