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Oxford physicists Chiara Marletto and Vlatko Vedral have proposed a feasible experiment called gravitationally-induced entanglement (GIE) that could provide the first evidence that gravity has quantum properties, potentially falsifying Einstein’s classical general relativity. The experiment works at low energies using today’s technology, overturning the long-held belief that probing quantum gravity would require a particle accelerator the size of the solar system.
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The core idea: If two massive particles, each in a quantum superposition of different locations, become entangled solely through their gravitational interaction, that entanglement acts as a “witness” that gravity is non-classical. This relies on the General Witness Theorem (developed using constructor theory), which shows that no classical mediator can create entanglement between two quantum systems — only something with quantum-like information-processing capacity can.
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Quantum gravity vs. reconciliation: A true quantum theory of gravity treats gravity as capable of processing quantum information (superposition, entanglement), extending quantum mechanics to gravity itself. By contrast, semi-classical approaches (like Penrose’s collapse model or Oppenheim’s model) keep gravity classical and try to reconcile it with quantum matter by having quantum effects “stop” at some scale. GIE would rule out these semi-classical descriptions.
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What the experiment tests — and what it doesn’t: GIE tests whether gravity can mediate entanglement (a non-classical property), but it does not demonstrate that gravity itself can be placed in a superposition of configurations — that would require a direct interference experiment on gravitons, which remains practically impossible. The experiment also does not distinguish between specific quantum gravity models (e.g., field-theory approaches vs. loop quantum gravity).
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Experimental design: The leading proposal uses microcrystals (nanogram-scale masses, roughly 10⁻¹² to 10⁻¹⁴ kg — much lighter than the Planck mass of 10⁻⁸ kg). Each crystal has a spin degree of freedom used as a handle. A Stern-Gerlach-type setup uses magnetic fields to place each crystal in a spatial superposition. The two crystals interact only gravitationally (other forces are shielded), and if entanglement is detected afterward, gravity must be non-classical.
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Key experimental challenges: Maintaining superposition of massive objects long enough for gravitational interaction (seconds are needed and have been demonstrated); isolating gravitational interaction from electromagnetic and other noise; and closing the Stern-Gerlach interferometer with objects massive enough for the effect. A 2023 Bell-test experiment using similar nanocrystals separated by 1.5 km provides confidence in the underlying technology.
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Relation to prior proposals: The classic COW (Colella-Overhauser-Werner) experiment used a single neutron in superposition and is consistent with both classical and quantum gravity. Page and Geilker tested a narrow semi-classical model. The Bose-Sugato proposal uses the same GIE principle but with a specific nanocrystal setup and a less general theoretical argument; Marletto and Vedral’s version uses constructor theory to make the argument as assumption-free as possible.
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Feasibility and funding: Back-of-the-envelope estimates suggest the experiment is achievable with current technology, likely within 5–10 years, at a cost of roughly £5–10 million — a fraction of large particle-physics budgets. The main bottleneck is sustained funding for fundamental physics, since funding agencies tend to favor applied research. The team is conducting feasibility studies and seeking support through Oxford’s New Frontiers Hub and private foundations (Templeton, Gordon and Betty Moore, Utopia).
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Constructor theory’s role: Marletto’s framework of constructor theory (developed with David Deutsch) provided the tools to generalize DeWitt’s earlier argument about quantum-classical incompatibility. It allowed them to formulate the witness theorem in a way that does not assume gravity obeys quantum mechanics — making the argument convincing even to a skeptic who believes gravity is classical.
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On the state of physics: Both guests expressed disappointment that the recent Nobel Prize in Physics went to AI-related work, arguing it sends the wrong signal about what matters in fundamental physics. They emphasized that foundational problems — quantum gravity, cosmology, foundations of quantum field theory, the physics of life and consciousness — remain wide open and under-discussed, partly because of funding and career incentives that favor incremental over transformative research.
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Advice to young researchers: Forget fashions, follow genuine curiosity, and give yourself permission to spend years on difficult, uncertain problems. Creativity in physics requires the same freedom and passion that drives any artistic or intellectual pursuit.