Few astronomers question the existence of black holes. The Universe appears to be brimming with them. And yet the evidence is decidedly circumstantial, inferred from the behaviour of other objects, such as nearby stars and clouds.

That’s hardly surprising, given the nature of black holes: regions of space from which nothing can escape.

So astronomers would like to find a way to get a direct measurement of a black hole. In the last couple of years, for example, they have used very long baseline interferometry to create extraordinary images of the supermassive black hole at the centre of our galaxy. These images and the ones that will be possible in the coming years should be able to resolve detail on the scale of the black hole’s event horizon.

So what will they see? The race is on to predict what black holes will look like and how the images can be used to test our understanding of them.

One important idea about black holes is called the “no hair theorem” and states that the properties of a black hole can be completely described by its mass and its spin.

Naturally, the no hair theorem places strict limits on the influence a black hole can have on its surroundings. Images of a black hole (actually the shadow it casts on surrounding gas and dust) could disprove the theorem if they show that the black hole, or the flow of matter into it, is distorted in some way.

That’s an exciting possibility but will require images that can resolve details in the flow of matter into the hole that are far smaller than the black hole itself.

However, Tim Johannsen and Dimitrios Psaltis at the University of Arizona in Tucson say there is an easier way to test the theorem. Today, they calculate that black holes ought to be surrounded by a ring of light. This light comes from photons that have become trapped in a circular orbit about the black hole, just outside the event horizon, which are then scattered by gas and dust falling into the hole.

This ring has some interesting properties. It should be much brighter than the surrounding gas and dust. It should have a diameter that is some ten times the size of the black hole, meaning that it should be visible in images that will soon be available and will provide a direct measure of the black hole’s mass.

And most importantly, its shape depends on the properties of the black hole, not on the structure of the gas and dust falling into the hole. That means the shape of the ring is measure of the properties of the black hole and any asymmetry in the ring will be a direct violation of the no hair theorem, say Johannsen and Psaltis.

Astronomers needn’t look far for such a ring. “The black hole in the center of the Milky Way, is the ideal candidate for a test of the no-hair theorem due to its high brightness, large angular size, and relatively unimpeded observational accessibility,” say Johannsen and Psaltis.

With a few extra wipes with a lens cloth, today’s telescopes should provide such images sooner rather than later.

Ref: arxiv.org/abs/1005.1931: Testing The No-Hair Theorem With Observations In The Electromagnetic Spectrum: II. Black-Hole Images