Fuzzia

Meet Fuzzia, a quantum particle who can never be pinned down in position and momentum at the same time.

Werner Heisenberg introduced the uncertainty principle in 1927, and Earle Kennard proved the precise form the same year. Prepare many particles in the same state and measure position on some, momentum on others. The spread of the position results, Δx, times the spread of the momentum results, Δp, can never be less than ħ/2. This is not about clumsy instruments. It is built into the wave description: a wave packet narrow in space must mix many wavelengths, and so many momenta.

Example: an electron confined to about 0.1 nanometres, the size of an atom, must have a velocity spread of at least about 579 kilometres per second. That is why electrons in atoms cannot simply sit still on the nucleus.

The principle sets the size of atoms, explains the zero-point jiggle of oscillators and limits how precisely any measurement can track a quantum system.

For everyday objects ħ is so small that the limit is irrelevant. A similar-looking energy–time relation exists, but time is not an observable in the same way, so it is interpreted differently.