The mind-bending laws of quantum mechanics say we can't observe the smallest particles without affecting them. Physicists have now caused the smallest-ever disturbance while making a quantum measurement — in fact, almost the minimum thought to be possible.
This disturbance is called back-action, and it is one of the hallmarks of quantum mechanics, which governs the actions of the very small. It arises from the supposition that before a measurement is made, particles exist in a sort of limbo state, being neither here nor there while retaining the possibility of either.
Once an observer intervenes, the particle is forced to "choose" a state ? to settle on one possibility, eliminating the other options. Thus, the state of the particle is altered by the act of measuring it.
Near-Perfect Particle Measurement Achieved - Yahoo! News
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