Physics
Uncertainty Principle
Quick fact
The uncertainty principle isn't about clumsy measurements: it's woven into the fabric of reality. The more precisely you pin down a particle's location, the more wildly its momentum fluctuates—and the product of these uncertainties can never be less than about 10^{-34} joule-seconds (Planck's constant divided by 4π).
Why this is interesting
You can measure the position and speed of a moving ball as accurately as you like—but try that with an electron, and nature pushes back. Why can't we know exactly where an electron is and how fast it's going at the same time?