Weigh the vacuum
National lab for the full experiment; university for the useful contribution. · 1 min read
What it proposes
Quantum theory fills empty space with energy. General relativity says energy has weight. Put a Casimir cavity on a balance and ask whether the shortfall of vacuum energy inside it is pushed upward, the same bookkeeping as a body displacing water. If it is, the join between the site's chapter on the vacuum and its chapter on gravity stops being an argument and becomes a laboratory reading.
Who it is forPrecision-mechanics engineersLow-noise metrology engineersCryogenic materials peopleSeismologists
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.
Where it stands
On the bench now — a funded collaboration, a prototype that has reached its designed noise floor, and a named next machine.
Take it up
- The measurement that settles it
- 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.
- What it costs to start
- National lab for the full experiment; university for the useful contribution.
- The engineer it grows
- It is also the most direct answer this field can give to the question a sceptical reader is entitled to ask: does any of this weigh anything.
What it rests on
Where it sits in the curriculum
Casimir physics and vacuum-force engineeringInertia and gravity from the vacuumWhat the vacuum isThe vacuum at cosmic scale