Fuzzball (string theory)
Fuzzballs (also called stringy stars) are conjectured by some string theorists to be the true quantum description of black holes. The proposal is that string theory would resolve two major problems of classical black holes—the singularity of infinite space-time curvature, and the missing multiplicity of states to account for Black Hole Entropy—by replacing the usual black hole interior by complicated (or fuzzy) spacetimes reacting to the presence of strings.
Physical characteristics
Samir Mathur of Ohio State University, with postdoctoral researcher Oleg Lunin, proposed that the strings, which are thought to comprise all protons, neutrons, electrons, and other subatomic particles, coalesce in a black hole to form a large ball of strings with a definite volume; it is not a singularity. Whereas the event horizon of a classic black hole is thought to be very well defined and distinct, they further calculated that the event horizon of a fuzzball should be very much like a mist; fuzzy, hence the name “fuzzball.” They also found that the surface of the fuzzball would have a mean radius equal to that of the event horizon of a classic black hole; for both, the Schwarzschild radius for a median-size stellar black hole of 6.8 solar masses is 20 km.
With classical black holes, objects passing through the event horizon on their way to the singularity are thought to enter a realm of curved space‑time where the escape velocity exceeds the speed of light. It is a realm that is devoid of all structure. Further, at the singularity—the heart of a classic black hole—space‑time is thought to have infinite curvature since its mass is believed to have collapsed to zero (infinitely small) volume. Such infinite conditions are problematical for physics. With a fuzzball however, the strings comprising an object are believed to simply fall onto and absorb into the surface of the fuzzball, which is located at the threshold at which the escape velocity equals the speed of light.
Since the volume of fuzzballs tracks the Schwarzschild radius (2.95 km per solar mass), fuzzballs have a variable density that decreases as the inverse square of their mass (twice the mass is eight times the volume and one‑quarter the density). A typical, 6.8-solar‑mass fuzzball would have a density of 4.0 × 1017 kg/m3. A bit of such a fuzzball the size of a drop of water would have a mass of twenty million metric tons, which is the mass of a granite ball 240 meters in diameter (as wide as a 70‑story building is tall). Though such densities are almost unimaginably extreme, they are—mathematically speaking—infinitely far from infinite density. Although the densities of stellar-mass fuzzballs are quite great—greater than that of neutron stars—their densities are many orders of magnitude less than the Planck density, which is equivalent to 1023 solar masses squeezed into the volume of an atomic nucleus.
Due to the mass-density inverse-square rule, all fuzzballs need not have unimaginable densities. There are also supermassive black holes, which are found at the center of virtually all galaxies. Sagittarius A*, the black hole at the center of our galaxy, is 3.7 million solar masses. If it is actually a fuzzball, it has a density 70 times that of gold. At 3.9 billion solar masses, near the upper bounds for supermassive black holes, a fuzzball would have a density equal to air (1.2 kg/m3).
Information paradox
According to the black hole information paradox, classic black holes create a problem for physics; all the quantum nature (information) of the matter and energy that had ever fallen into a classic black hole is thought to entirely vanish from existence. The Ohio State University physicists’ theory states that the quantum nature of all the strings that fall into a fuzzball is preserved as it contributes to the fuzzball’s string makeup; no quantum information is squashed out of existence.
Even if fuzzballs were proven to exist, the fact that they do not destroy quantum information does not imply that any technical means could theoretically access the information; it is merely a thought experiment—the realization that the quantum information still technically exists in our universe. Moreover, Hawking radiation (black‑body thermal radiation thought to be emitted from the proximity of black holes) can reveal only the mass, angular momentum, and charge of classic black holes or fuzzballs; it can not contain internal quantum information about a body whose escape velocity equals the speed of light.