  Rapid experimental progress has recently allowed the use of light to prepare
macroscopic mechanical objects into nearly pure quantum states. This research
field of quantum optomechanics opens new doors toward testing quantum
mechanics, and possibly other laws of physics, in new regimes. In the first
part of this paper, I will review a set of techniques of quantum measurement
theory that are often used to analyze quantum optomechanical systems. Some of
these techniques were originally designed to analyze how a classical driving
force passes through a quantum system, and can eventually be detected with
optimal signal-to-noise ratio --- while others focus more on the quantum state
evolution of a mechanical object under continuous monitoring. In the second
part of this paper, I will review a set of experimental concepts that will
demonstrate quantum mechanical behavior of macroscopic objects --- quantum
entanglement, quantum teleportation, and the quantum Zeno effect. Taking the
interplay between gravity and quantum mechanics as an example, I will review a
set of speculations on how quantum mechanics can be modified for macroscopic
objects, and how these speculations --- and their generalizations --- might be
tested by optomechanics.
