Physicists get tired of carrying c and ħ around, so they simply SET THEM TO 1 — measuring speed in fractions of light and action in quanta. Then the units collapse: time and length become the same thing, mass and energy become the same thing, and a length is just ONE OVER an energy. It’s choosing the universe’s own ruler. Slide an energy and read the length that falls out.
Natural units set c = ħ = 1 (and often kᴸ = 1). Then [length] = [time] = [energy]⁻¹ and [mass] = [energy]: a single scale (an energy) fixes everything. The bridge back to SI is ħc ≈ 0.1973 GeV·fm, so 1 GeV⁻¹ = 0.197 fm = 1.97×10⁻¹⁶ m — a length falls straight out of an energy. This is why particle physics quotes masses, lengths, and times all in eV: the constants aren’t lost, they’re set to the universe’s own units. A fail-loud self-check throws unless 1 GeV⁻¹ converts to 0.197 fm. ◆ real physics, node-verified.
The conversions are exact (they ARE the constants, just re-based); ‘natural’ is a convention, not a claim the SI units are wrong — it’s the same physics in the universe’s own ruler, where a length is one over an energy.