𝗪𝗵𝗲𝗻 𝗟𝗶𝗴𝗵𝘁 𝗦𝗹𝗼𝘄𝘀, 𝗧𝗶𝗻𝘆 𝗚𝗿𝗮𝘃𝗶𝘁𝗮𝘁𝗶𝗼𝗻𝗮𝗹 𝗙𝗼𝗿𝗰𝗲𝘀 𝗕𝗲𝗰𝗼𝗺𝗲 𝗘𝗮𝘀𝗶𝗲𝗿 𝘁𝗼 𝗠𝗲𝗮𝘀𝘂𝗿𝗲.

Gravity is the weakest of the four fundamental forces.

Measuring the pull of a millimetre-scale object is therefore a demanding precision experiment.

Researchers have demonstrated a sensor combining a 𝘁𝗼𝗿𝘀𝗶𝗼𝗻 𝗽𝗲𝗻𝗱𝘂𝗹𝘂𝗺 with an 𝗼𝗽𝘁𝗶𝗰𝗮𝗹 𝗺𝗶𝗰𝗿𝗼𝗰𝗮𝘃𝗶𝘁𝘆.

The principle begins with Newton’s law:

𝗙₍g₎ = 𝗚𝗺₁𝗺₂/𝗿²

A small source mass exerts a minute gravitational force on the suspended pendulum, causing it to twist. The experiment used a 𝟭.𝟮-𝗴𝗿𝗮𝗺 source mass, just 𝟲 𝗺𝗺 in diameter.

How does the sensor detect such a tiny movement?

A silica microsphere attached to the pendulum sits close to a tapered optical fibre, with a gap of roughly 𝟭𝟬𝟬 𝗻𝗺. When gravity moves the pendulum, that gap changes. This alters how light couples between the fibre and microcavity, changing the optical signal.

The key is the cavity’s 𝘀𝘁𝗼𝗽𝗽𝗲𝗱-𝗹𝗶𝗴𝗵𝘁 𝗿𝗲𝗴𝗶𝗺𝗲. A coupled photon–phonon resonance reduces light’s group velocity to about 𝟮,𝟬𝟬𝟬 𝗺/𝘀 and creates a steep change in optical phase with frequency. This makes the readout unusually sensitive to tiny shifts in the pendulum’s position. Light is not simply “held still”; its interaction with mechanical vibrations produces the sharp optical response used for sensing.

The team periodically moved the source mass so the gravitational signal appeared at a known frequency, helping distinguish it from background noise. They reported a minimum detectable force change of 𝟯.𝟬𝟮 × 𝟭𝟬⁻¹⁶ 𝗡 and a displacement sensitivity of 𝟳.𝟴𝟱 𝗽𝗺/√𝗛𝘇.

This is a laboratory demonstration, not yet an everyday instrument. Still, it shows how photonics and precision mechanics can measure exceptionally weak forces, with possible future applications in microscale gravimetry and tests at the boundary of gravity and quantum physics.

𝗦𝗼𝘂𝗿𝗰𝗲: Zhu et al., Nature Nanotechnology (2026), DOI: 10.1038/s41565-026-02298-8

Based on the research published in Nature Nanotechnology. Research summary: Phys.org.

Image Credit - ChatGPT