You can't perfectly know a particle's position and speed
The Heisenberg uncertainty principle states that precisely measuring a particle's position makes its momentum uncertain, and vice versa, revealing a fundamental, probabilistic nature of reality.
In quantum mechanics, the Heisenberg uncertainty principle reveals a fundamental limit: you can't simultaneously know both a particle's exact position and its momentum (speed and direction). German physicist Werner Heisenberg formulated this in 1927, showing it's not about measurement flaws but an inherent property of nature. This principle arises because tiny particles, like electrons, behave as both waves and particles. The more precisely you measure one, the less certain you become of the other. This revolutionary idea overturned classical physics, explaining why atoms behave probabilistically and influencing fields from quantum computing to particle physics. It suggests the universe at its core is governed by probabilities, not absolute certainty.