Weigh the vacuum

Why the library suggests it

Enrico Calloni and seventeen colleagues, Carlo Rovelli among them, propose cycling high-temperature superconducting plates through their transition so the cavity's reflectivity — and therefore its vacuum energy — switches on and off, and watching a balance for a force of about ten to the minus sixteen newtons (The Archimedes experiment, 2015). The prototype is built: a half-metre aluminium arm read by a Michelson interferometer, running closed-loop in the seismically quiet Sos Enattos mine in Sardinia, reaching a torque noise of about seven times ten to the minus thirteen newton-metres per root hertz — its own suspension's thermal limit, which is exactly the level the final cryogenic balance was designed around (Thermal noise-limited beam balance as prototype of the Archimedes vacuum weight experiment, 2024). The theoretical stake is set out by Ulf Leonhardt, who argues that doing the same renormalisation cosmology skips — the one laboratory Casimir physics does routinely — leaves a cosmological constant of the right size, and that an expanding universe plays the part a moving mirror plays in the dynamical Casimir effect (Casimir cosmology, 2022).

The experiment or build

The full balance is a large collaboration in a deep mine. But the enabling problems are separable and every one of them is a thesis: the superconducting sample stack, the transition-cycling drive, the thermal-noise budget of the suspension, the seismic isolation. The settling measurement is the force in newtons on the balance as the sample is cycled through its superconducting transition, against a non-superconducting dummy of identical mass and geometry cycled through the same temperature range. That control is the whole experiment: the dummy separates a change in vacuum energy from a change in temperature.