Fuzzball (string theory)
Fuzzballs are hypothetical objects in superstring theory, intended to provide a fully quantum description of the black holes predicted by general relativity.
General relativity predicts that at the singularity, the curvature of spacetime becomes infinite, and it cannot determine the fate of matter and energy that falls into it.
[1][3] As no direct experimental evidence supports either string theory in general or fuzzballs in particular, both are products purely of calculations and theoretical research.
The two theories diverge only at the quantum level; that is, classic black holes and fuzzballs differ only in their internal composition and how they affect virtual particles that form close to their event horizons (see § Information paradox, below).
A bit of such a non-spinning fuzzball the size of a drop of water would, on average, have a mass of twenty million metric tons, which is equivalent to that of a granite ball 243 meters in diameter.
The fuzzball model predicts that a non-spinning supermassive black hole with the same mass as Sagittarius A* has a mean density "only" 51 times that of gold.
Moreover, at 3.9 billion M☉ (a rather large super-massive black hole), a non-spinning fuzzball would have a radius of 77 astronomical units—about the same size as the termination shock of the Solar System's heliosphere—and a mean density equal to that of the Earth's atmosphere at sea level (1.2 kg/m3).
In the fuzzball model, the hadrons in its core (neutrons and perhaps a smattering of protons and mesons) decompose into what could be regarded as the final stage of degenerate matter: a ball of strings, which the fuzzball model predicts is the true quantum description of not only black holes but theorized quark stars composed of quark matter.
A black hole that fed primarily on the stellar atmosphere (protons, neutrons, and electrons) of a nearby companion star should, if it obeyed the known laws of quantum mechanics, grow to have a quantum composition different from another black hole that fed only on light (photons) from neighboring stars and the cosmic microwave background.
Moreover, even if quantum information was not extinguished at singularities, it could not climb against infinite gravitational intensity and reach up to and beyond the event horizon where it could reveal itself in normal spacetime.
[1] Fuzzball theory's proposed solution to the black hole information paradox resolves a significant incompatibility between quantum mechanics and general relativity.
[14] All bosons (e.g., photons) and the boson-like sfermions will readily overlap each other when crowded, whereas fermions and the fermion-like gauginos possessing mass (such as electrons, protons, and quarks) will not; this is one reason why superpartners—if they exist—have properties that are exceedingly different from their Standard Model counterparts.
Conversely, the electron (spin 1/2) is an example of a mass-carrying fermion where its superpartner is the spin-0 selectron, which is a massless boson but is not considered to be a primary force carrier.
[13] This underlies why theoretically perfectly quiescent black holes (ones in a universe containing no matter or other types of electromagnetic radiation to absorb) evaporate so slowly as they lose energy (and equivalent amounts of mass) via Hawking radiation; even a modest 4.9 M☉ black hole would require 1059 times the current age of the Universe to vanish.
[9] Such an infinitesimal transmitted power is to one watt as 1/3000th of a drop of water (about one-quarter the volume of a typical grain of table salt) is to all the Earth's oceans.
Even if such a black hole was only 100 lightyears away, the odds of just one of its Hawking radiation photons landing anywhere on Earth—let alone being captured by an antenna—while a human is watching are astronomically improbable.
For instance, M87* which is an unremarkable supermassive black hole, emits Hawking radiation at a near-nonexistent radiant power of at most 13 photons per century and does so with a wavelength so great that a receiving antenna possessing even a modest degree of absorption efficiency would be larger than the Solar System.
However, an Italian team of scientists that ran computer simulations suggested in 2021 that existing gravitational-wave observatories are capable of discerning fuzzball-theory-supporting evidence in the signals from merging binary black holes (and the resultant effects on ringdowns) by virtue of the nontrivial unique attributes of fuzzballs, which are extended objects with a physical structure.
The team's simulations predicted slower-than-expected decay rates for certain vibration modes that would also be dominated by "echoes" from earlier ring oscillations.
[5] Moreover, a separate Italian team a year earlier posited that future gravitational-wave detectors, such as the proposed Laser Interferometer Space Antenna (LISA), which is intended to have the ability to observe high-mass binary mergers at frequencies far below the limits of current observatories, would improve the ability to confirm aspects of fuzzball theory by orders of magnitude.
[20] In §3, 'Angular Momentum and Charge,' which is 2½ pages starting on p. 213, Hawking began the formula-rich section with the following: Superradiance was proposed theoretically by Robert H. Dicke in 1954, and in 1973 was experimentally observed in hydrogen fluoride atoms by N. Skribanowitz et al.
