It is difficult to put one’s finger on what a hole is. Roberto Casati and Achille C. Varzi propose in their book Holes and Other Superficialities that our easiest way out is to argue that, by holes, we actually mean ‘holed objects’, the material that the holes are in (p. 2). ‘Indeed, the fact that holes are not made of anything seems to be a major hindrance to giving adequate identity to holes’, they write (ibid.), a challenge that they venture to overcome.
However, black holes are not in anything; on the contrary they are surrounded by void sprinkled with matter. Even more, they themselves are made of something, and not just anything, but highly condensed matter – condensed enough that a black hole of the mass of the earth would only have 18 mm in diameter. But most black holes are formed by collapsing stars, retaining their masses, which is, on average, ten times more than that of the sun. Black holes are, therefore, the other way around: rather than lacks within an object, they are immense amounts of matter amongst nothing.
As Casati and Varzi’s book shows, thinking holes is thinking reality itself, questioning the relation between something and nothing which we usually take for granted. Black holes take this one step further, pushing us to challenge our own words and, quite literally, our own place in the universe. They are, in one respect, similar to the holes around us: we associate holes with falling, and maybe not being able to get out, just as black holes are famous for having such a powerful gravitational pull that nothing, not even light, can escape it. Their immense mass means that the speed needed to escape a black hole’s gravity (the ‘escape velocity’) is higher than the speed of light. Since no light can come back to us, we cannot see black holes, yet what we do observe, the swirls matter forms when ‘falling’ into them, does not look much like the holes we are used to. Thinking beyond the immediately visible changes that. Within general relativity, gravity is a curvature of spacetime: imagining spacetime as a flexible surface, an object with mass creates a dent in that surface, and nearby objects fall within that dent. With immense concentrated mass, black holes create steep, deep dents from which nothing can escape – a funnelling which physicist John Wheeler had in mind when coining the name.
Black holes, therefore, are not holes in substance – they are holes in spacetime itself. In order to understand our own (very commonly used) name for them, we need to stop thinking about what we see and try to envision something different. We use the same word for a pothole in front of our house, and for an immense, unobservable cosmic event millions of light-years away. Different as they may seem, they satisfy, in the end, Casati and Varzi’s promulgation that ‘To be a hole is to be a hollow, a tunnel, or a cavity’ (p. 6). One is a hollow in asphalt, the other in spacetime itself. A connection between this local, immediate lack within our perceivable reality, and something much bigger than us and our senses happens through only one word. Wildly different levels of existence are united by the way holes work within our imagination, by the connection we make between this elusive concept and the world.
Andrei Belibou (Department of Politics and International Studies, University of Warwick)