A distant orchestra plays a symphony, but the recording is full of noise. The melody is written for two instruments, yet a third voice seems to break through the static — quiet and unfamiliar. This is how astrophysicists listen to gravitational waves, trying to catch the slightest deviations from Einstein's predictions.

In 2017, the LIGO and Virgo detectors caught a signal from the merger of two neutron stars — an event named GW170817. Along with the wave came a gamma-ray burst that astronomers tracked for weeks. This burst allowed them to precisely determine the wave's orientation in space: its polarization. Previously, this angle remained unknown, but now it became a strict constraint for any theories attempting to improve gravity.

The researchers added three new parameters to the standard signal shape. One accounted for a scalar 'breathing'—an extra pulsation that changes volume rather than shape of space. The other two modified the familiar tensor modes. It turned out that the scalar addition is barely noticeable, but the overall picture still favors simplicity: the pure Einsteinian signal explains the data better than the extended model.

The main takeaway is not that Einstein was right again. The powerful tool is the joint analysis of gravitational and electromagnetic signals. The constraint on the polarization angle obtained from the afterglow improved the precision of the non-standard parameter estimates by 30–60%. Future observations, where both the wave and light from a single event are detected, will be able to test the boldest hypotheses about the nature of gravity. Then, the third voice from the distant orchestra will either become clearer or finally dissolve into the noise.