Gravitational Decoherence Experiment
Massive objects placed in two places at once — to ask whether gravity itself is what eventually picks one.
Tests proposals — most famously by Diósi (1989) and Penrose (1996) — that the gravitational field of a quantum object cannot itself superpose, so a sufficiently massive spatial superposition collapses spontaneously on a timescale set by its own gravitational self-energy. The experiments use opto-mechanical resonators, levitated nanospheres, matter-wave interferometers, or torsion balances to look for excess decoherence beyond environmental sources, in regimes where the predicted Diósi–Penrose timescale could be reached.
Two main strategies. (1) Matter-wave interferometry: split a massive object's centre-of-mass wave function over a separation Δx, hold it there for time τ, recombine, and read out fringe contrast. Diósi–Penrose predicts an additional decay of contrast as exp(-τ/τ_DP) where τ_DP scales inversely with the gravitational self-energy difference between the two branches. (2) Opto-mechanical: drive a levitated dielectric nanoparticle or membrane resonator into a quantum-coherent state, watch its phase-space distribution as a function of trap configuration, and look for excess noise beyond standard quantum mechanics. Both approaches require: cryogenic environments (typically <50 mK for opto-mechanics, ultra-high vacuum for levitated systems), magnetic and acoustic isolation, and exquisitely characterised baseline decoherence (gas collisions, blackbody photons, charge noise) so that anything left over could be attributed to gravity. As of 2024–2025 no experiment has reached the regime where Diósi–Penrose effects must appear, but several proposals (notably MAQRO, satellite-based; and ground-based ~10⁻¹⁵ kg superpositions) are within an order of magnitude.
In plain terms
Imagine a marble that's somehow in two places at once on a table. Most of physics says the marble stays in two places forever, until someone looks. A few stranger theories say gravity itself eventually picks one — like an invisible referee tapping the marble down. Gravitational decoherence experiments are the rules-of-the-game for catching that referee in the act.
Also called: GDE, Diósi–Penrose test, macroscopic-superposition gravity probe.
Why it matters
- One of very few laboratory probes of where (and whether) quantum mechanics and general relativity must meet.
- Related models (CSL, Diósi–Penrose, Adler) make different predictions for the same hardware — experiments can rule classes in or out.
- Drives ultra-quiet opto-mechanics, levitated-particle tech, and atom-interferometry — capabilities that flow into gravimetry, navigation, and dark-matter searches.
- Provides a physics motivation for satellite-borne quantum-mechanical tests (MAQRO) and large-mass ground interferometry.
Where it lives
- MAQRO (proposed ESA mission, levitated 10⁻¹⁵–10⁻¹⁰ kg interferometry)
- MAGIS-100 (Fermilab, 100 m atom interferometer)
- Aspelmeyer group levitated-particle experiments (Vienna)
- LIGO-style suspended-mirror tests of macroscopic coherence
Key numbers
- Predicted DP timescale (10 µg, 10 nm): ~0.5 s (Penrose, 1996)
- Largest atom-interferometer arm: 54 cm (Stanford-MAGIS-class, single-atom paths)
- Levitated-particle ground-state mass: 10⁻¹⁸ kg (Aspelmeyer group, 2020s)
- Required vacuum: <10⁻¹¹ mbar (To suppress gas-collision decoherence)
Surprising facts
- Penrose's original 1996 estimate predicted a 10 µg sphere in superposition over 10 nm should collapse in roughly half a second purely from gravitational self-energy.
- Levitated-nanoparticle groups have reached ground-state cooling of ~10⁻¹⁸ kg objects — the largest objects ever cooled to their motional quantum ground state.
- Atom interferometers have produced spatial superpositions of single cesium atoms separated by 54 cm — a macroscopic distance for a single quantum entity.
- If the Diósi–Penrose mechanism is real, it sets an absolute upper bound on the size of any future quantum computer that uses massive coherent states.
Open questions
- Excess opto-mechanical heating at low pressure: Several levitated-particle experiments report a residual heating rate of trapped nanospheres that does not scale cleanly with background gas pressure or laser intensity. The remainder, after well-characterised decoherence sources, sits within the right order of magnitude for collapse-model predictions but is consistent with mundane stray-charge noise. The interpretation remains open. We label any 'detection' claim of Diósi–Penrose collapse as speculative; no experiment has confirmed it.
- R₀ regularisation parameter ambiguity: Both the Diósi and Penrose models contain a smearing length R₀ that prevents the gravitational self-energy diverging for point particles. Different choices of R₀ shift the predicted collapse rate by orders of magnitude. The community has no first-principles way to fix it, which makes a firm experimental refutation harder.
A short history
Lajos Diósi proposed gravitationally induced wavefunction collapse in 1989; Roger Penrose independently formulated a closely related model in 1996, motivated by his concerns about general relativity and quantum measurement. The first proposed table-top tests (Marshall, Simon, Penrose, Bouwmeester 2003) used a tiny mirror reflecting a single photon. Levitated-nanosphere and atom-interferometer routes emerged in the 2010s. The MAQRO mission concept, proposed in 2010 and refined ever since, would carry an interferometer with 10⁻¹⁰-kg test masses to a Lagrange point to escape ground-based seismic and thermal noise. As of 2026, no Diósi–Penrose-regime test has been completed and no claim of detection is supported by the community.
Sources
- Diósi, Phys. Lett. A (1989) — gravitational localization model (https://www.sciencedirect.com/science/article/pii/0375960189902660)
- Penrose, Gen. Rel. Grav. (1996) — gravity's role in quantum state reduction (https://link.springer.com/article/10.1007/BF02105068)
- Marshall et al., PRL (2003) — towards quantum superpositions of a mirror (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.91.130401)
- Kaltenbaek et al., MAQRO mission proposal (2016) (https://link.springer.com/article/10.1140/epjqt/s40507-016-0043-7)