Black holes are, by far, the most mysterious objects in the universe. They are objects in the cosmos where all of our knowledge of physics completely breaks down. And yet, despite their apparent impossibility, they exist. But what if these gravitational monsters aren't black holes at all, but rather the cosmic equivalent of fuzzy, vibrating balls of string?
The problem of black holes Black holes appear in Einstein's theory of general relativity, and by all rights they simply shouldn't exist. In that theory, if a clump of matter crunches down into a tiny enough volume, then gravity can become overwhelmingly strong. This insane gravitational compression can out-compete any of the other four fundamental forces of nature like the strong nuclear force that holds that clump of matter together. Once a certain critical threshold is reached, the clump of matter just squeezes and squeezes, compressing down into an infinitely tiny point. That infinitely tiny point is known as the singularity, and it's encircled by a surface known as the event horizon the place where the inward pull of gravity exceeds the speed of light.
Of course, there's no such thing as an infinitely tiny point, so this picture seems wrong. But in the mid-20th century astronomers began to find objects that looked like black holes, acted like black holes and probably smelled like black holes too. Despite their impossibility, there they were, floating around the universe.
Unraveling the yarn In string theory, black holes are neither black nor holes. Instead, the best metaphor to explain what a fuzzball is to look at another compact-and-weird object in the universe: neutron stars. Neutron stars are what happens when an object doesn't quite have enough gravity to compress into what we call a black hole. Inside a neutron star, matter is compressed into its highest density state possible. Neutrons are one of the fundamental constituents of atoms, but they usually play along with other particles such as protons and electrons. But in a neutron star, that kind of atomic camaraderie breaks down and dissolves, leaving behind just neutrons crammed together as tightly as possible.
With fuzzballs, the fundamental strings stop working together and simply crowd together, becoming a large, well, ball of strings. A fuzzball. Fuzzballs aren't fully fleshed out, even in theory, because as cool as string theory sounds, nobody has ever been able to come up with a complete mathematical solution for it and so fuzzballs aren't just fuzzy in physical reality, but also fuzzy in mathematical possibility. Still, we might be able to find fuzzballs with upcoming surveys, as described in a review article published Oct. 27 in the preprint journal arXiv. We are just now beginning to move past proving the existence of black holes and toward probing the details of how they behave, and our best way to do it is through gravitational waves.