It is believed that the centre of essentially every galaxy, including our own, plays host to a supermassive black hole. In a small fraction of galaxies, large quantities of gas rain down into these giant black holes, causing the black hole to grow while releasing enough energy within the central few light hours of the galaxy to outshine all of the galaxy's stars thousands of times over. This is more than a mere cosmic firework show; the energy released as the black hole grows can shape or even shut off the processes by which the galaxy itself forms. In other words, supermassive black holes may well be the safety valve that regulates galaxy formation, preventing galaxies from growing too big too fast. But although they are rapidly becoming a standard part of our model of how galaxies form and evolve, it is important to step back and ask just how strong is the case that these monster black holes actually exist.

The Galactic Centre. This radio image, obtained with the Very Large Array of telescopes, shows the central region of our Milky Way galaxy. The bright object at the centre is Sagittarius A*, the enigmatic source of radio waves that has long been suspected of harbouring a supermassive black hole. Credit: KASSIM, LAROSA, LAZIO & HYMAN/NAVAL RESEARCH LAB.

On page 78 of this issue, Doeleman et al.1 report new observations of Sagittarius A* (Sgr A*), the enigmatic source of radio waves at the centre of our Galaxy2 that has long been suspected as signposting our very own supermassive black hole. These new data have allowed the authors, for the first time, to detect structure in the radio emissions from Sgr A* on scales as small as 50 million kilometres. The diameter of our Galaxy's black hole (which has a mass 4 million times that of the Sun) is expected to be approximately 12 million to 24 million kilometres. But the strong bending of light rays within the gravitational field of the black hole will double the apparent size of the event horizon, the boundary of the area around the black hole from which nothing, not even light, can escape. Thus Doeleman and colleagues' observations have finally brought us to the threshold of imaging horizon-scale structures — a holy grail of radio astronomy.

With the new data, the authors have attained a resolution of about 40 microarcseconds (about one-hundred millionth of a degree), five times better than the best previous measurement3. This advance has been made possible by extending the technique of very long baseline interferometry (VLBI) to shorter radio wavelengths — indeed, into the microwave region of the electromagnetic spectrum. In VLBI, data from radio telescopes spread across the globe are combined to produce vastly superior image resolution than can be achieved by any one telescope; but this process requires keeping track of the precise phase of the incoming waves. This technological feat becomes increasingly challenging as the wavelength of the waves is decreased in the search for superior resolving power. The observation reported by Doeleman et al.1, made with telescopes in Arizona, California and Hawaii, is one of the first to exploit VLBI with 1.3-mm waves.

Black holes are truly bizarre objects. Einstein's theory of general relativity tells us that they are objects in which gravity has run amok, cutting off a region of space (inside the event horizon) from the outside Universe. Inside the event horizon, theory predicts the existence of regions in which densities and temperatures climb to such extreme values that all currently understood laws of physics break down. These new results1 put us a step closer to confirming that nature really is this anarchistic. Assuming that the central object must be smaller than the surrounding 'cloud' of radio-emitting gas that we see, the case for a black hole looks compelling. Even a 4-million-solar-mass boson star, an exotic hypothetical object sometimes discussed as an alternative to black holes4, will be much larger in extent than the 50-million-kilometre limit implied by Doeleman and colleagues' data. Given these data, only gross deviations in the behaviour of gravity itself from the behaviour predicted by general relativity can invalidate the case for black holes.

Efforts to improve the sensitivity and imaging ability of millimetre-wavelength VLBI promise further dramatic advances in our understanding of Sgr A*. For example, future studies will reveal effects related to the spin of the black hole. Although still the subject of intense research, the complex gas flows close to a black hole can be strongly affected by the tornado-like motion of space-time close to a spinning black hole5, as can the appearance of the 'shadow' of the event horizon6. Characterizing these phenomena will allow us to determine the spin rate of the black hole, offering a window into its long cosmic history. Did it grow through the successive mergers of smaller black holes as galaxies came crashing together? Or did it grow through the accretion of gas and, if so, did it snack on gas hundreds of times7 or feast just once or twice? The spin of the black hole encodes, albeit crudely, this history and may be one of our best handles for understanding the evolution of this, and other, supermassive black holes8.

We have entered a new era, one in which we can now directly image structure at the event horizon of a black hole. As the VLBI array capable of millimetre resolution is expanded and its sensitivity increased, the distorted world at the edge of the black hole will literally come into focus.