Can We Test Quantum Gravity? Physicists Lay Out an Experimental Pathway

https://www.youtube.com/watch?v=5uBBz0upYoY

A leading question in modern physics—whether gravity itself is a quantum force—moved from philosophical debate toward experimental plausibility in a recent World Science Festival conversation between Brian Greene and Professor Vlatko Vedral. The discussion charted both the conceptual objections that have long kept gravity outside the quantum fold and the concrete experimental ideas now being proposed to settle the issue.

Historically, some eminent physicists argued that gravity might never need quantization: Freeman Dyson suggested that known physics could remain consistent even if gravity stayed classical, a position that has kept portions of the field skeptical about chasing a quantum gravity experiment. Opposing that view, many theorists see strong reasons—consistency with quantum field ideas and the success of quantization elsewhere—to expect gravity to have quantum properties, but until recently this expectation lacked accessible empirical tests.

Roger Penrose has offered an alternative perspective, proposing that gravity could play a role in objective wave-function collapse—an idea that links quantum measurement to spacetime curvature rather than to purely quantum interactions. Vedral and Greene placed Penrose’s proposals alongside other collapse and decoherence frameworks, clarifying how different interpretations lead to different experimental signatures and why collapse models remain testable hypotheses rather than settled dogma.

The heart of the conversation described a feasible experimental strategy: create two nearby quantum systems that can be put into spatial superposition and look for entanglement caused only by their mutual gravitational interaction. In practice this could mean two tiny masses placed into spatially separated states inside neighboring interferometers; if gravity can transmit quantum information, the masses should become entangled in a way that cannot be explained by purely classical gravitational interaction. Vedral explained how such setups—sometimes sketched as an updated Schrödinger’s-cat thought experiment—could distinguish quantum gravity from classical alternatives by testing for gravitationally induced entanglement.

Practical progress gives reason for optimism: experiments with increasingly massive quantum systems (including split Bose–Einstein condensates and interferometry with larger objects) have pushed the boundary of what can be coherently superposed, and technological advances in isolation, cooling, and precision control are narrowing the gap between concept and implementation. Vedral and Greene estimated that within roughly the next five to ten years critical experimental milestones could be reached if current technical progress continues and focused experimental efforts are mounted.

Beyond the lab, the implications are profound. If gravity is shown to be quantum by these experiments, it would support the view that spacetime and gravity are emergent from or tightly linked to quantum information (some speakers framed reality in terms of “Q numbers” or information-theoretic building blocks). Conversely, a failure to detect gravitational entanglement under conditions where it should appear would validate alternative frameworks—such as objective collapse models or new physics—forcing a major conceptual reassessment. Greene and Vedral also discussed interpretational issues—many-worlds probabilities, the role of observers, and whether living systems could play a role in collapse—showing how an experimentally resolved status for gravity would ripple into foundations of quantum mechanics.

Illustrative example:

One proposed experiment places two micron-scale masses in adjacent interferometers; each mass is prepared in a superposition of two positions and allowed to interact solely via gravity. After an interaction interval, measurements on each interferometer would reveal whether the two masses became entangled—clear evidence that gravity can mediate quantum correlations.

References:

World Science Festival, “Can We Test Quantum Gravity? | Rethinking Reality” video discussion with Vlatko Vedral and Brian Greene [video]. Available from YouTube; program description and chapter timestamps available through the World Science Festival posting