Give me a mass and I’ll give you a length: the COMPTON WAVELENGTH, λ = h / (mc). It’s the scale below which you can’t pin a particle down without making a copy of it — the fuzzy size a mass has just by being quantum. For the electron it’s 2.4×10⁻¹² m. Slide the mass and watch the length shrink.
The Compton wavelength λ₊ = h/(mc) is the length scale a rest-mass sets by itself: probe a particle at distances below λ₊ and the energy needed (by ΔxΔp ≥ ħ/2) exceeds mc², enough to pair-produce another particle — so it bounds how well a mass can be localized. For the electron λ₊ = 2.426×10⁻¹² m (the Bohr radius is λ₊/(2πα), larger). Heavier mass → smaller λ₊. A fail-loud self-check throws unless the electron value lands between 2.3 and 2.5 ×10⁻¹² m. ◆ real physics, node-verified.
Exact from mass and constants; the ‘can’t localize below it’ reading is the standard heuristic from the uncertainty principle + pair production. A length falling out of a mass, no ruler.