One of the problems with trying to sort reality from theory is that mathematically a lot of things are possible that aren’t actually real. For instance, we could turn the Earth into a black hole that could politely sit in the palm of your hand, at least for a moment, before it completely consumed you with its gravity. The issue is that there aren’t any situations in which the Earth is going to find itself suddenly compressed. Even if some advanced alien civilization did exist with the desire to compress the Earth into a black hole, they wouldn’t because the required energy is just… a lot.
Basically, black holes could exist of absolutely any size. All that is required is for something to get compressed down so tiny that the escape velocity for that something – the speed that you have to go to escape its gravity – that speed just has to equal the speed of light, and boom – a black hole. With sufficient energy, anyone out there could be turned into a black hole. You wouldn’t survive the process, but, the math says it can happen.
When astronomers go looking for black holes, what we find is objects with masses between roughly 50-120 solar masses just aren’t out there. This is because the most massive stars don’t form black holes when they die; they simply explode themselves completely apart and leave behind an empty supernova remnant. We see black holes from around three solar masses and up, but there is a naturally occurring gap starting around fifty solar masses.
The gap could have another edge if black holes form during the death of even larger objects that are too heavy to blow themselves apart. These objects… we’re still looking for them, but in a new paper in The Astrophysical Journal Letters by Jose María Ezquiaga and Daniel E. Holz, we learn that if they exist (and we have no idea if stars survive with enough mass to form them) they could be detected during the next LIGO / Virgo run as the most massive black hole binaries merging. They can also potentially be seen in the future LISA mission.
Maybe. But it is neat to think stars could form black holes with this weird distribution that starts around three Solar Masses, builds up to fifty solar masses, and then just skips ahead to 120 solar masses. We don’t know if it’s real, but it’s a neat idea worth thinking about.
Jumping the Gap to Probe Large Black Holes (AAS Nova)
“Jumping the Gap: Searching for LIGO’s Biggest Black Holes,” Jose María Ezquiaga and Daniel E. Holz, 2021 March 15, The Astrophysical Journal Letters