For more than a decade, astronomers have been using very‐long‐baseline radio interferometry (VLBI) to investigate the structure of distant objects three orders of magnitude smaller than the resolution limit of any optical telescope. A network of seven radio telescopes, from Massachusetts to California, acting as a transcontinental interferometer, has in recent years given us radio images of quasars and galactic nuclei, showing features down to a fraction of a milli‐arcsecond. No other astronomical technique—existing or proposed—can come within two orders of magnitude of such resolving power.

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