LIKE a giant pale blue eye, the Earth stares at the centre of our galaxy. Through the glare and the fog it is trying to catch a glimpse of an indistinct something 30,000 light years away. Over there, within the sparkling starscape of the galaxy's core... no, not those giant suns or those colliding gas clouds; not the gamma-ray glow of annihilating antimatter. No, right there in the very centre, inside that swirling nebula of doomed matter, could that be just a hint of a shadow?
The shadow we're straining to see is that of a monstrous black hole, a place where gravity rules supreme, swallowing light and stretching the fabric of space to breaking point. Black holes are perhaps the most outrageous prediction of science, and even though we can paint fine theoretical pictures of them and point to evidence for many objects that seem to be black hole-ish, nobody has ever actually seen one.
All that could change in the next few months. Astronomers are working to tie together a network of microwave telescopes across the planet to make a single instrument with the most acute vision yet. They will turn this giant eye towards what they believe is a supermassive black hole at the centre of our galaxy, code name Sagittarius A
Even part-built, the microwave eye has already produced a hazy picture of Sagittarius A*. Last September, a team led by Shep Doeleman of the Massachusetts Institute of Technology's Haystack Observatory in Westford published results that are almost good enough to show the reputed black hole (Nature, vol 455, p 78).
Soon, Doeleman and his team hope to see the hole's dark silhouette. Then they want to watch matter falling into it in order to trace out the twisted space-time around the black hole. That could tell us how it formed and grew.
These observations will also be the sternest test yet of Einstein's general theory of relativity, which predicts the existence of black holes. If relativity breaks down, Doeleman and his team might not see a black hole at all, but something even stranger.
What we do know for sure is that something big lurks at the centre of our galaxy - because its powerful gravity affects the motion of nearby stars and gas. That something is about 4.5 million times the mass of the sun and crammed into an area the size of the inner solar system. There are few obvious ways to pack stuff in so tightly. Four million suns would be a dead giveaway, for instance. A swarm of neutron stars or small black holes would be highly unstable. So our best bet is one massive black hole.
We know for sure that something big lurks at the centre of the galaxy. Seeing it is not easy, though
A supermassive black hole is thought to sit at the centre of most large galaxies. In some so-called active galaxies, enormous quantities of gas are swirling into the black hole, forming a disc of hot matter around it that often outshines the billions of surrounding stars.
Our own galactic monster is less well fed, surviving on only a thin gruel of gas streaming out from nearby stars. As this gas falls towards the hole it also heats up and shines, though more faintly than the disc in an active galaxy. All kinds of electromagnetic radiation are emitted, ranging from radio to X-rays.
Of course, the black hole itself does not shine since it actually swallows light. That is how we hope to be able to see it: light from gas swirling round the hole will be devoured, so the hole should show up as a shadow or silhouette against the background of hot, shining gas.
Seeing this shadow is not easy. It won't have sharp edges because we will still see light and other radiation from gas in front of the hole. It will also look very small. According to relativity, a black hole of 4.5 million solar masses should be 27 million kilometres across, and even though its gravity warps nearby light rays, making it appear about twice that size (see diagram) it will still seem very small. From our distant viewpoint halfway across the galaxy, that would cover an angle of only about 50 micro-arcseconds - the size a football would appear on the moon, or a small bacterium held at arm's length.
No ordinary telescope could see such a small dark smudge. Instead, Doeleman is using a well-tested technique called very long baseline interferometry or VLBI. By combining the observations from widely separated dishes across the planet, radio astronomers can effectively reconstruct what would be seen by one enormous dish - even one as large as the Earth. Because small dishes collect less light, a VLBI image is less bright than the image from a real planet-sized dish would be, but it can reveal just as much detail.
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