Uncertainty principle
Definition
The rule that you cannot know both a particle's exact position and its exact momentum at the same time. The more precisely you pin down one, the fuzzier the other becomes.
Unit: joule-seconds (J·s) for the product Δx·Δp
Technical definition
Heisenberg's principle: the product of the uncertainties in position (Δx) and momentum (Δp) can never be smaller than ħ/2, where ħ is the reduced Planck constant. This is a fundamental property of quantum systems, not a limitation of instruments.
Example
Confining an electron to an atom-sized region (Δx ≈ 10^-10 m) forces a momentum uncertainty of roughly 5×10^-25 kg·m/s — which is why electrons cannot sit still inside atoms and instead occupy spread-out orbitals.
This is not the 'observer effect' — it is not about clumsy measurements disturbing the particle. Even with perfect instruments, a quantum particle genuinely does not possess an exact position and momentum simultaneously. Uncertainty is built into nature, not into our tools.