Last week a museum visitor wondered, “What’s the volume of a black hole?” A question easier asked than answered.
Let’s look at black hole (“BH”) anatomy. If you’ve seen Interstellar, you saw those wonderful images of “Gargantua,” the enormous BH that plays an essential role in the plot. (If you haven’t seen the movie, do that. It is so cool.)
A BH isn’t just a blank spot in the Universe, it’s attractively ornamented by the effects of its gravity on the light passing by:
Working from the outside inward, the first decoration is a background starfield warped as though the stars beyond had moved over so they could see us past Gargantua. That’s because of gravitational lensing, the phenomenon first observed by Sir Arthur Eddington and the initial confirmation of Einstein’s Theory of General Relativity.
No star moved, of course. Each warped star’s light comes to us from an altered angle, its lightwaves bent on passing through the spatial compression Gargantua imposes on its neighborhood. (“Miles are shorter near a BH” — see Gravitational Waves Are Something Else for a diagrammatic explanation.)
Moving inward we come to the Accretion Disc, a ring of doomed particles destined to fall inward forever unless they’re jostled to smithereens or spat out along one of the BH’s two polar jets (not shown). The Disc is hot, thanks to all the jostling. Like any hot object it emits light.
Above and below the Disc we see two arcs that are actually images of the Accretion Disc, sent our way by more gravitational lensing. Very close to a BH there’s a region where passing light beams are bent so much that their photons go into orbit. The disc’s a bit further out than that so its lightwaves are only bent 90o over (arc A) and under (arc B) before they come to us.
By the way, those arcs don’t only face in our direction. Fly 360o around Gargantua’s equator and those arcs will follow you all the way. It’s as though the BH were embedded in a sphere of lensed Disclight.
Which gets us to the next layer of weirdness. Astrophysicists believe that most BHs rotate, though maybe not as fast as Gargantua’s edge-of-instability rate. Einstein’s GR equations predict a phenomenon called frame dragging — rapidly spinning massive objects must tug local space along for the ride. The deformed region is a shell called the Ergosphere.
Frame dragging is why the two arcs are asymmetrical and don’t match up. We see space as even more compressed on the right-hand side where Gargantua is spinning away from us. Because the effect is strongest at the equator, the shell should really be called the Ergospheroid, but what can you do?
Inside the Ergosphere we find the defining characteristic of a BH, its Event Horizon, the innermost bright ring around the central blackness in the diagram. Barely outside the EH there may or may not be a Firewall, a “seething maelstrom of particles” that some physicists suggest must exist to neutralize the BH Information Paradox. Last I heard, theoreticians are still fighting that battle.
The EH forms a nearly spherical boundary where gravity becomes so intense that the escape velocity exceeds the speed of light. No light or matter or information can break out. At the EH, the geometry of spacetime becomes so twisted that the direction of time is In. Inside the EH and outside of the movies it’s impossible for us to know what goes on.
Finally, the mathematical models say that at the center of the EH there’s a point, the Singularity, where spacetime’s curvature and gravity’s strength must be Infinite. As we’ve seen elsewhere, Infinity in a calculation is Nature’s was of saying, “You’ve got it wrong, make a better model.”
So we’re finally down to the volume question. We could simply measure the EH’s external diameter d and plug that into V=(πd3)/6. Unfortunately, that forthright approach misses all the spatial twisting and compression — it’s a long way in to the Singularity. Include those effects and you’ve probably got another Infinity.
Gargantua’s surface area is finite, but its volume may not be.
~~ Rich Olcott