At the centers of many galaxies lurk black holes — regions with gravity so strong that even light cannot escape. Stars often orbit them, and sometimes one of those stars is a pulsar, a cosmic lighthouse sending out strictly periodic radio pulses. When such a pulsar moves behind a black hole, its signals reach us with a delay. This isn't just a delay from a detour in space — the spinning black hole itself twists spacetime around it, like a spoon stirring thick honey, and drags the signal along.

The authors of the study used the STVG theory (scalar–tensor–vector gravity)—a variant of modified gravity that tries to explain galaxy rotation without invoking dark matter. In this theory, a black hole has an extra parameter α that controls the strength of gravitational vector forces. The scientists calculated how the signal delay depends on α and on the black hole's spin (a). They extended the known Laguna–Wolszczan formula, adding more subtle effects.

It turns out that different values of α can noticeably change pulse arrival times — from a few microseconds to fractions of a second. These are tiny shifts, but modern radio telescopes can detect them. All that's needed is to find a suitable pair: a pulsar and a black hole where the pulsar regularly passes behind the black hole. Then the delay becomes a natural laboratory for testing the laws of gravity.