“People sometimes refer to the uncertainty principle in everyday contexts, outside of the equation-filled language of physics texts. So it’s important to emphasize what the principle does not say. It’s not an assertion that “everything is uncertain.” Either position or momentum could be certain in an appropriate quantum state; they just can’t be certain at the same time. And the uncertainty principle doesn’t say we necessarily disturb a system when we measure it. If a particle has a definite momentum, we can go ahead and measure that without changing it at all. The point is that there are no states for which both position and momentum are simultaneously definite. The uncertainty principle is a statement about the nature of quantum states and their relationship to observable quantities, not a statement about the physical act of measurement.”