Problem
A radio interferometer telescope with a baseline B between dishes, operating at wavelength l, can resolve an image with an angular size in radians as small as about l/B. Correlation of signals from many telescopes allows an image to be reconstructed. Each pair of telescopes gives information about the image structure on angular scale l/B (in radians), along the direction of their separation.
For a baseline equal to the diameter of the Earth, what wavelength is needed to resolve the event horizon of the Galaxy's black hole? This is the wavelength you would need to detect and correlate between dishes for an "event horizon telescope."