Measuring the size of a black hole

General relativity asserts that every black hole is closed off from the outside by an event horizon whose radius is proportional to the black hole’s mass. The compact radio source Sagittarius A* at the center of the Milky Way is thought to mark a 4-million-solar-mass black hole. The corresponding event-horizon diameter, about the size of Mercury’s orbit, subtends an angle of 20 microarcseconds on the sky – the largest of any black-hole candidate, stellar or supermassive. Therefore Sgr A* has been a particularly attractive target for very-long-baseline radio interferometry. If VLBI can achieve the requisite resolution, it should see spatial effects of the event horizon on the radio source, whose emission is presumed to come from the inner edge of an accretion disk of material orbiting the black hole. Now for the first time a VLBI collaboration, led by Sheperd Doeleman of the MIT Haystack Observatory, has succeeded in detecting horizon-scale structure in Sgr A* by using a trio of millimeter-wavelength radio telescopes in Hawaii, California, and Arizona. They measure the intrinsic diameter of the Sgr A* source to be about 37 µas. So small a size largely excludes alternatives to a supermassive black hole with an event horizon. The measured source size is, in fact, somewhat smaller than one would expect from gravitational-lensing magnification of radiation from an accretion disk symmetrically centered on the black hole. More likely than a disk whose center is actually offset from the black hole is an apparent source asymmetry due to relativistic Doppler brightening and dimming if the spinning disk is being seen somewhat edge on. ( S. S. Doeleman et al. , Nature 455, 78, 2008.) — Bertram Schwarzschild