The observer effect is the theory that the mere observation of a phenomenon inevitably changes that phenomenon.
One of the most famous experiments in physics is the double-slit experiment. It demonstrates, with unparalleled strangeness, that little particles of matter have something of a wave about them, and suggests that the very act of observing a particle has a dramatic effect on its behavior.
Aspects of the double-slit experiment (such as the collapse of the wavefunction upon observation) bring into question the possibility of consciousness interacting with matter. Nevertheless, experimental evidence for the interaction of consciousness and matter or the non-locality of consciousness is generally ignored by mainstream science, which limits its investigations to the physical world. The missing link in mainstream science is consciousness. (See Dean Radin, Ph.D., below). Scientists believe that non-local consciousness might be modeled by quantum entanglement. Quantum mechanics is a fundamental and well-established theory.