How uncertain is uncertainty? So much so that, according the Heisenberg uncertainty principle, you can't measure everything you'd like to. Particles have certain pairs of properties (such as momentum/location, or spin/angular position, or energy/time) that are ``complementary.'' In practice, that means you can't make accurate measurements of both. The more you know of one, the less you can know of the other.

Classical physics had a lot of uncertainty, too--lots of measurements weren't particularly accurate. But physicists assumed the problem lay in the imprecision of the instruments--and that some day, with better instruments, you'd get more precise data. Instead, the Heisenberg principle says that uncertainty is not a mistake but a law of nature.

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