Before the term “black hole” became embedded in both the scientific and popular consciousness – sometime in the 1960s – the event horizon was often referred to as the “Schwarzschild throat”. Personally, I love this name, it has a robustness to it and feels rather more evocative than black hole. A ‘throat’ suggests passage to something, potentially to the kind of place you’d rather not be, like the stomach of a mighty cosmic whale.
But by that same measure, the idea of a throat leaves some ambiguity about the kind of solution to Einstein’s field equations we’re talking about. A throat could suggest a wormhole rather than the one-way street in time that leads to the Schwarzschild central singularity. Equally, it’s not quite the right term for a black hole that is spinning. In that case the shape and properties of spacetime are a bit different. Thanks to the brilliant work of the New Zealand physicist Roy Kerr back in 1965 we not only know that a solution to the field equations exists for a spinning body, but that the spinning distorts the shape of the event horizon – making it an oblate spheroid – and creates an external region called the ‘ergosphere’, where it is literally impossible for anything to stand still as the fabric of spacetime is dragged around the hole.
The remarkable image of the supermassive blackhole in the galaxy M87, produced by the Event Horizon Telescope (EHT) project, begins to reveal a little of this strange environment. But it pays to be careful about understanding what we can and can’t yet see.
The EHT team has referred to the central dark zone in the M87 image as the ‘shadow’ of the black hole, and that’s a good way to discuss it. What we’re seeing is not actually the horizon (or throat) of the mass, but rather the zone that keeps photons from our line of sight.
More specifically, if we gaze at a simple, non-rotating black hole from a long way away we will find that there is a photon-capture radius for the hole. This is the apparent zone around the hole where any light that would have otherwise carried on to be seen by us (from background stars or matter close to the hole) is instead diverted by the hole’s warping of spacetime – falling in and across the horizon or even being captured into an unstable orbit.
That region has a size that is about 5.2 times the size of the event horizon, and for a spinning hole the photon-capture zone is also slightly distorted from a circular shape, depending on the orientation of the hole’s spin with respect to our viewing direction.
The light escaping from material that might be orbiting the hole – in a literal death-spiral towards the horizon as matter is tidally pulled and jostled – is also going to be lensed by the strongly curved spacetime around the mass. The upshot being that what we expect to see of the hole is a distorted, ring-like image of the orbiting, heated matter with a thin inner ring of light corresponding to the photon-capture radius. And all of that is further modified if the hole is spinning – causing a relativistic enhancing and dimming of emission from matter moving towards or away from us.
In other words, what we see in EHT’s image of M87 is indeed a ‘shadow’. Strictly speaking the horizon of the hole, the throat, lurks still closer to the center of the picture.
But clearly this is just the beginning. Other holes, with different masses, spins, and orientations are potential targets for future observation. And as further technological refinements are made to the EHT’s capabilities we stand a good chance of producing even finer dissections of the outer anatomy of these absurd, beautiful, and terrifying places.