Notwithstanding all the intuition defying phenomenon in the universe, the following assertion by Brian Greene doesn’t seem quite right: “The flip side of it is, we also typically have an intuition that black holes are really dense, right? That's usually the way we think about them. But if you make something sufficiently large, regardless of how low its density is, it will also become a black hole. So you can make a black hole out of air by just having enough air. If you have enough air, sufficiently large sphere of air, it would become a black hole too with the density of air. So all the intuitions that we typically have about black holes, that they have to be dense, and they have to be gargantuan, not right.” It certainly makes sense that a sufficiently large volume of air could/would become a black hole. But wouldn’t that be the case only after the volume of air has gravitationally collapsed and exceeded a certain density threshold? Isn’t density the most fundamental property of a black hole? If so, then how does Brian Greene’s assertion that “all the intuitions that we typically have about black holes, that they have to be dense and they have to be gargantuan, not right” make any sense?
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