Historical research guide · 0902 · 2015
The Archimedes experiment
Enrico Calloni · S. Caprara · Martina De Laurentis · Giampiero Esposito · M. Grilli · Ettore Majorana · G. P. Pepe · S. Petrarca · Paola Puppo · P. Rapagnani · Fulvio Ricci · Luigi Rosa · Carlo Rovelli · P. Ruggi · N. L. Saini · Cosimo Stornaiolo · Francesco Tafuri
In one minute
Quantum theory fills empty space with energy. General relativity says energy has weight. Put the two together and you get one of the sharpest open questions in physics — does the vacuum’s own energy fall in a gravitational field? Enrico Calloni and seventeen colleagues, Carlo Rovelli among them, propose to answer it by weighing the vacuum on a balance. A Casimir cavity is two plates close enough that only some light waves fit between them, so the gap holds less vacuum energy than the space around it. If that shortfall has weight, the cavity is pushed upward — the same bookkeeping as a body displacing water, which is where the experiment gets its name. Archimedes, funded by Italy’s INFN as a two-year pathfinder, would cycle high-temperature superconducting plates through their transition so the cavity’s reflectivity, and therefore its vacuum energy, switches on and off, and watch a balance for a force of about ten to the minus sixteen newtons. This paper reports the three hard problems the pathfinder was built to solve.
Connection
Chapter 2 says the vacuum is a real medium carrying a real energy density, and chapter 3 says that energy is what gravity and inertia are made of. Archimedes is the experiment that turns the join between those two chapters into a laboratory reading rather than an argument — and chapter 1 cares that the collaboration published the noise budget, the force it is chasing and the deadline before building anything. The 2024 prototype that reached its designed noise floor in a Sardinian mine is on the site at /library/stm-2915006685.
From the source
Whether the enormous vacuum-fluctuation energy predicted by quantum field theory contributes to gravity is one of the main unsolved questions of modern physics: the density quantum theory suggests is enormously larger than the value general relativity allows for a universe of the observed size and accelerated expansion, there is no agreement on the theoretical side, and no experiment has yet been performed to verify or discard the assumption.Sect. 1, Introduction
A rigid Casimir cavity holds less vacuum energy than the surrounding space, because the modes that do not satisfy its boundary conditions are expelled. If vacuum energy interacts with gravity, the lack of weight of those expelled modes produces an upward force on the cavity, in similarity with the Archimedes buoyancy of a fluid — a force equal to minus the cavity’s vacuum energy divided by the square of the speed of light, times the local gravitational acceleration.Sect. 2, The weight of the vacuum, Eq. (1)
The measurement strategy is to modulate the reflectivity of the plates so that vacuum energy is periodically expelled from the cavity and its weight is modulated with it. Driving a superconducting transition changes the reflectivity, and the change in Casimir energy could be particularly large for layered type-II high-temperature superconductors such as the cuprates, which behave as natural multiple Casimir cavities because their planes are very poorly conductive in the normal state. Order-of-magnitude expectations put the modulated force at about ten to the minus sixteen newtons.Sect. 2, The weight of the vacuum
The favoured detection band for a thermally driven normal-to-superconducting transition is 1 to 100 millihertz, which makes a balance the more suitable instrument rather than a gravitational-wave detector. A seismic attenuator built on an inverted pendulum with a tuned resonator hung from it meets the isolation requirement, read out by an optical lever or an interferometric sensor, with thermal and seismic noise the two dominant limits and an integration time of several months giving a good signal-to-noise ratio.Sect. 3, The Experiment, and Sect. 3.1, Fig. 2
The sample’s temperature must be modulated by radiative heat exchange alone, so that it is isolated from any external interaction that could add energy other than the vacuum one. Finite-element analysis obtained a temperature modulation of about one kelvin on a disc 150 millimetres across and half a millimetre thick — a shape compatible with present yttrium-barium-copper-oxide technology and sufficient to put the signal inside the expected sensitivity.Sect. 3.2, Thermal modulation study
The pathfinder project is defined by three crucial deliverables: a balance able to measure forces at the level of ten to the minus sixteen newtons, a thermal modulation system using only radiative heat exchange, and a detailed study of high-temperature superconducting systems as the main source of the vacuum energy to be weighed.Sect. 4, Conclusion
The way in
https://arxiv.org/abs/1511.00609How to cite it
Enrico Calloni, S. Caprara, Martina De Laurentis, Giampiero Esposito, M. Grilli, Ettore Majorana, G. P. Pepe, S. Petrarca, Paola Puppo, P. Rapagnani, Fulvio Ricci, Luigi Rosa, Carlo Rovelli, P. Ruggi, N. L. Saini, Cosimo Stornaiolo, Francesco Tafuri (2015) The Archimedes experiment arXiv:1511.00609