A theoretical team proposes a near-field experiment capable of probing gravitational interactions at frequencies up to megahertz, well beyond the current limit of 1.7 kHz. The central idea is to test whether gravity really acts instantaneously between nearby masses or whether a small delay exists in the coupling. So far, available measurements have only bounded such a possible delay at about 10⁻⁵ s.

The proposal relies on two magnetically levitated microrotors. One of them modulates a mass quadrupole as it spins, while the second rotor detects the resulting gravitational tidal field as a torque. Because the detector rotor also acts as a mechanical local oscillator, the megahertz signal is downconverted to a band of a few hertz, easy to measure. A key advantage is that, at fixed geometry, neither the torque nor the integration time depends on the rotation rate: reaching higher frequencies costs no sensitivity.

With parameters within reach of magnetically levitated NdFeB rotors, a signal at 2 MHz would yield a torque of about 2·10⁻²⁵ Nm, detectable in under an hour at a temperature of 10 mK. Interpreting a possible departure from instantaneous coupling as a field relaxation time, the measurement could constrain it to about 10⁻⁸ s, improving on existing experimental bounds by more than three orders of magnitude.

As a byproduct of the analysis, the authors also calculate the sensitivity of a rotor operated as a tunable high-frequency gravitational-wave detector. They estimate that a noise-equivalent strain around 10⁻²² Hz⁻¹/² could be achievable in the megahertz band, a range so far almost unexplored for gravitational-wave detection.