A stone drops into a pond, and ripples spread across the water. If a protruding branch blocks the ripples' path, part of the wave reflects, and part bends around the obstacle. Gravitational waves, ripples in spacetime itself, behave similarly when passing by galaxy clusters or black holes. Their path curves, and the detector receives not just one wave but a whole echo.

Usually, this process is described as a play of light and shadow, but with waves instead of rays. This approach works well as long as the wave remains short compared to the obstacle's size. But when the wave becomes very long, and gravitational waves can be kilometers long or more, the old optics breaks down. The authors of the study showed that in some theories of gravity, different from Einstein's, an additional interaction kicks in at such low frequencies: the wave doesn't just bend around the massive body; it starts 'talking' to it through the very curvature of spacetime.

As a result, the wave's amplification, how much louder it becomes after the lens, does not approach 1 at ultra-low frequencies, as standard theory predicts. It can be quite different. To capture this, a new mathematical framework is needed: instead of conventional wavefronts, the scientists proposed treating the wave as if it puts on a 'coat' of curved spacetime and interacts with the lens in that form. The residual lensing effect then reduces to a phase shift — the wave arrives slightly earlier or later.

If future detectors, such as the space observatory LISA, notice such unusual behavior in long gravitational waves, it would be direct evidence that gravity is more complex than we thought. And this could happen even if, in the familiar 'geometric' rays, everything looks perfectly Einsteinian. Thus, ripples on water, when they encounter an invisible barrier, can reveal its shape.