Scientists may finally detect hidden ripples in spacetime

These minute variations were first proposed by physicist John Wheeler and are expected to arise in several leading quantum gravity theories. However, different theories predict different types of fluctuations, which has made it difficult for experimental scientists to know exactly what signals to search for.

Turning Theory Into Measurable Signals

The new research, published in Nature Communications, tackles this problem by grouping spacetime fluctuations into three main categories based on how they behave across space and time. For each category, the team identified clear, measurable patterns that could be detected using laser interferometers -- ranging from large-scale systems like the 4km long LIGO to smaller experimental setups such as QUEST and GQuEST being developed in the UK (Cardiff University) and USA (Caltech) respectively.

Dr. Sharmila Balamurugan, Assistant Professor, University of Warwick and first author said: "Different models of gravity predict very different underlying trends in the random spacetime fluctuations, and that has left experimentalists without a clear target. Our work provides the first unified guide that translates these abstract, theoretical predictions into concrete, measurable signals.

"It means we can now test a whole class of quantum-gravity predictions using existing interferometers, rather than waiting for entirely new technologies. This is an important step towards bringing some of the most fundamental questions in physics firmly into the realm of experiment."

What the Study Revealed

The findings highlight several important insights about how different instruments can detect these fluctuations:

• Tabletop interferometers beat LIGO in bandwidth.
  Despite their much smaller size, systems like QUEST and GQuEST could offer more detailed information about spacetime fluctuations. Their broader frequency range allows them to capture all key signal patterns.
• LIGO is an excellent "yes/no" detector.
  Because of its long arm cavities, LIGO is extremely sensitive to whether spacetime fluctuations exist at all. However, the relevant frequencies fall outside the range currently available in public data.
• A long-running debate is resolved.
  The study addresses an ongoing question about whether arm cavities improve detection. The results show that they do enhance sensitivity, depending on the type of fluctuation being studied.

Dr. Sander Vermeulen, Caltech, co-author of the study said: "Interferometers can measure spacetime with extraordinary precision. However, to measure spacetime fluctuations with an interferometer, we need to know where -- i.e. at what frequency -- to look, and what the signal will look like. With our framework we can now predict this for a wide range of theories. Our results show that interferometers are powerful and versatile tools in the quest for quantum gravity."

A Flexible Tool for Fundamental Physics

An important strength of this framework is that it does not depend on any single explanation for how these fluctuations arise. Instead, it only requires a mathematical description of the proposed fluctuations and details about the measurement setup. This flexibility makes it useful not just for studying quantum gravity, but also for investigating stochastic gravitational waves, possible dark matter signals, and certain types of experimental noise.

Prof Animesh Datta, Professor of Theoretical Physics at Warwick concluded: "With this methodology, we can now treat any proposed model of spacetime fluctuations in a consistent, comparable way. In the coming years, we can use this to design smarter tabletop interferometers to confirm or refute possible theories of quantum or semiclassical gravity and even test new ideas about dark matter and stochastic gravitational waves."

This work was funded by the UK STFC "Quantum Technologies for Fundamental Physics" program (Grant Numbers ST/T006404/1, ST/W006308/1 and ST/Y004493/1) and the Leverhulme Trust under research grant ECF-2024-124 and RPG-2019-022.