Three antennas of a radio telescope listen to the same distant galaxy. The signal from each antenna jitters due to the atmosphere, but if you add the phases of the three signals, the jitter disappears. What remains is the closure phase — a compass showing whether the source is symmetric. Zero means symmetric; nonzero means asymmetric.
Previously, that's where they stopped. But sometimes the closure phase jumps sharply by π radians. These jumps were considered noise. A new theory shows: a jump occurs when the signal passes through a visibility null — a point where the interference pattern completely extinguishes. Around such a null, the phase swirls like water around a drain. This is a vortex with a topological charge.
The benefit is direct: now, from phase jumps, you can extract details of the source's shape without reconstructing the entire image. For the black hole M87*, this has already worked: its glowing ring's asymmetry is described by five numbers.
The phase jump of π when crossing a visibility zero is analogous to the sign change of a wavefunction when encircling a singularity in quantum mechanics. In interferometry, such a jump is unavoidable and indicates a true zero, not a calibration artifact.
The idea of using phase singularities to extract information echoes the concept of 'phase defects' in science fiction, for example in Stanislaw Lem's novel 'Solaris', where the ocean creates structures inaccessible to direct observation but manifesting through interference effects.