No single telescope can resolve a black hole's shadow. The Event Horizon Telescope did it by combining radio dishes around the world into a virtual telescope the size of Earth. That technique is interferometry. It links multiple telescopes together and combines their signals to achieve the resolution of a much larger instrument.

The principle is wave interference. When two telescopes observe the same source, the light waves arrive at slightly different times. Combining those waves produces an interference pattern that encodes fine detail. The spacing between telescopes determines the resolution. The wider the spacing, the finer the detail.

Interferometry works across the electromagnetic spectrum. Radio interferometry is the most mature. Arrays like the Very Large Array in New Mexico and ALMA in Chile produce images far sharper than any single dish. Optical interferometry is harder because light waves must be combined with extreme precision. But it has been done, and it is improving.

The technique is not just for imaging. It is used to measure the diameters of stars, the orbits of binary systems, and the positions of radio sources with milliarcsecond accuracy. Interferometry turns separate telescopes into one giant eye.