Einstein postulated that all mass bends space–time, and that enough mass placed in a sufficiently small space will break it.
The bending of space–time is spectacularly demonstrated in gravitational lenses (Fig. 1), where a massive object such as a galaxy lies between us and a distant bright object, such as another galaxy. It is as if a glass of water has been placed in front of us, and we observe a number of magnified images of the room beyond. Gravitational lensing is the universe’s own natural telescope, and astronomers have long used the phenomenon to explore the details of both lensed and lensing objects.
The breaking point of Einstein’s law, along with all the other laws of physics, occurs when so much mass is accumulated in such a tiny region that nothing can prevent its gravitational collapse into an infinitely small point. This is a black hole, or a singularity in space–time: a point of infinite curvature. Here, the laws of physics are bent to their breaking point.
As a result, black holes remain a fascinating mystery. While we now have good evidence that extremely massive black holes exist at the centres of all galaxies, we cannot observe them directly: we can only observe their gravitational effects on the surrounding region.
Over the past decade astronomers have watched the centre of our own galaxy, the Milky Way, as the...
