Black holes are what happens when matter is packed so densely that it compresses into a singularity... but some of them are still as insubstantial as air. If you think that's weird, it gets weirder. You make can make these ultra-light black holes even less dense by adding mass.
Black holes are so inescapably dense that we shouldn't even try to imagine what goes on inside them. Once anything passes the event horizon, it's inevitably headed for a grouping of matter so dense it compresses into a point. Although, of course, there's no way to add density to something that's infinitely dense — an infinite amount of mass in an infinitely small space — throwing matter over the event horizon has to increase a black hole's density, doesn't it?
You'd think so, but no. Adding mass to a black hole actually decreases its density. The more mass gets tossed in, the less dense a black hole gets. This happens because a black hole doesn't increase the way a regular ball of matter does. Two earths smashing into one another would not form a ball with twice the radius of the Earth. The volume may be doubled, but volume is determined by the cube of the radius, not the radius itself. So the radius of the new Double-Earth will only increase by the cube root of two.
When scientists worked out the radius of two black holes smashed together, though, they discovered that their radius does double when the mass doubles. The volume of a sphere is determined by the radius cubed, and thus cubing the number 2 shows us that the volume is now eight times what it was before the mass doubled. The average density of a black hole, or anything else, is the mass divided by the volume. The mass of the black hole just doubled, but the volume is now eightfold what it was before. Two divided by eight is one-fourth. So by doubling the mass, we decreased the average density of the black hole to one-fourth of what it was before.
This leads us in strange directions. A black hole with the mass of 387 million of our suns would have the average density of water. And, according to some, a black hole with the mass of our universe would have the average density of our universe — meaning that getting swallowed up by a black hole isn't quite as bad as it sounds.
Image: NASA, ESA.