A Millimetre of Gravity: What the New Clocks Have Already Weighed

A clock used to be a machine for telling the time. The new optical clocks have quietly stopped being only that. At Boulder, a single cloud of strontium atoms, one millimetre tall, was read closely enough that its top and its bottom were seen ticking at different rates — the gravitational redshift, resolved inside one atomic sample, with a measured frequency gradient of −9.8(2.3) × 10⁻²⁰ per millimetre against a predicted −1.09 × 10⁻¹⁹ per millimetre. In the same town, three clocks built on three different atoms were compared as a network, their frequency ratios fixed with uncertainties of 6 to 8 parts in 10¹⁸ in 2021 and at or below 3.2 × 10⁻¹⁸ in the successor campaign. At Braunschweig, two transitions of a single ytterbium ion, watched for years, bound any present-day drift of the fine-structure constant to 1.8(2.5) × 10⁻¹⁹ per year — no drift, at a level where a part in 10¹⁸ of change per year is the whole allowance. And turned around, the same redshift that the clocks confirm has become a measuring instrument in its own right: a centimetre of height is 1.09 × 10⁻¹⁸ of frequency, and clock comparisons have carried geopotential differences across hundreds of kilometres. This paper prints each of those capabilities as what it is — a receipt, with its number on it — and prices the one door still closed: the redefinition of the second, whose roadmap, on the record of the 2026 General Conference, still has unfulfilled mandatory criteria and a decision dated 2030 at the earliest. The claim is narrow: the clocks have already become instruments of gravitational and fundamental physics, and the receipts, not the promise, are the proof. Grade: **RECEIPTS STAND — INSTRUMENTS, NOT JUST TIMEKEEPERS.** This paper stands outside my corpus's vacuum ledger; it neither spends that ledger nor repairs it, and it touches no claim of that program.