Heisenberg's principle of uncertainty depends upon a critical insight, namely that when studying minute subatomic particles the act of observing them can affect their properties.

Naturalists who study wild animals take great concern that their presence in the field does not cause undue stress for their subjects, or alter the environment significantly. They worry about everything from abandoned trash altering the eating habits of scavengers to foreign diseases decimating the native populations, or nosy watchers inhibiting the mating habits of shy creatures.

For physicists, the situation is even more complex. When we observe an object the size of an atom or smaller, even the beam of light the we shine on it serves as a source of energy and produces a perturbation (change). We characterize objects by their three-dimensional position, and their three-dimensional momentum (how fast they are moving, given their mass). By observing a particle, we may change its position (by shifting it in space) or its momentum, by causing it to absorb or emit energy (in the form of photons). In summary, the act of observation alters the fabric of reality.