Paul Budnik email@example.com
Quantum mechanics is the most successful and the strangest theory in the history of physics. It is the theory that allows us to build postage stamp size computers that do billions of calculations in a second and to build nuclear weapons. It is strange because, unlike all previous fundamental physical theories, it does not model what happens physically. It only models how probabilities change over time. Yet no previous theory has come remotely close to the accuracy that quantum mechanics is at times capable of.
It is these two aspects of a particle that causes physicists to speak of the ``collapse'' of the wave function. Prior to observing a particle it could be located at anywhere the wave function is not zero. After observing it the particle is known to be within a much smaller region of space. The wave function has collapsed. Of course there cannot be a physical wave function that collapses because this would violate special relativity. However the wave function does at times seem to be a physical entity because wave functions from two particles can interfere with each other just as physical waves do.
More information is available in sections of my online book.