Measurement in quantum mechanics

The measurement problem in quantum mechanics concerns the interaction of a macroscopic measurement apparatus with an observed quantum mechanical system, and the so-called "collapse" of the wavefunction upon measurement from a superposition of possibilities to a defined state. A review can be found in Zurek, and in Riggs.

The most famous illustration of this problem is Schrödinger's cat, in which a random quantum event like a radioactive decay is set up to kill a cat in a box. In the microscopic description, the cat is described by a superposition of "alive" and "dead" possibilities, and we have the peculiar result that we are in a state of suspense until we open the box to see what has happened. Is this uncertainty about us (the observers), or the state of the cat?