Stellar mass black holes, like elementary particles, are remarkably simple objects. They have three primary observable properties: mass, spin, and electric charge. The similarities with elementary particles, like the proton, doesn’t stop there, as stellar mass black holes in binary systems can also form bound and unbound states due to interaction of orbital clouds (from boson condensates), uncannily analogous to the behavior and properties of atoms.
The spin of stellar mass black holes is a particularly significant property, as black holes have rapid rotations that generate a region of space called the ergosphere around the event horizon, where the torque on spacetime is so great that an object would have to travel at a velocity exceeding the speed of light just to stay in a stationary orbit. Analysis of this region has resulted in some interesting physics predictions, one being the phenomenon of superradiance. When a wave (whether of electromagnetic radiation or matter) enters the ergosphere with a specific trajectory, it can exit the black hole environment with a larger amplitude than the one with which it came in— this amplification process is called black hole superradiance. It was an effect first described by Roger Penrose nearly 50 years ago and describes how work can be extracted from the ergosphere of a black hole [1].
While the effect sounds exotic and mysterious, and one might naively attribute it to the non-linear gravitational effects within the ergosphere, it is actually a purely kinematic effect, and black hole superradiance is just another manifestation of the superradiance phenomenon that occur in a variety of systems. For instance Cherenkov radiation that occurs in nuclear reactors where emitted charged particles that travel faster than the speed of light in a water medium (where the speed of light is less than c) results in spontaneous emission of radiation—what is known as inertial motion superradiance.
Another interesting prediction that comes from the analysis of the ergosphere region is the effect of superradiance on generating black hole lasing, whereby massive bosonic waves form bound states with the black hole and can grow exponentially from the amplitude magnification (an effect that can lead to the hypothetical “black hole bomb“). Superradiant instabilities may create clouds of ultralight bosons, forming a state that is remarkably analogous to an atom. Astrophysicists studying this effect refer to such black holes as “gravitational atoms”, in which the resulting bosonic clouds have specific allowed orbital levels, similar to electron orbitals around a hydrogen nucleus. Now, a recent study has shown that these ultralight boson clouds can undergo a process similar to ionization, inducing binding within binary gravitational atoms (black holes) analogous to the photoelectric effect in atomic physics [2].
Figure 1. Schematic diagram of a gravitational atom in an equatorial binary inspiral. The position of the companion with mass M∗ can be described by the distance between the two black holes, R∗, and the polar angle ϕ∗. Figure description and image credit, see reference [3].
The study led by Daniel Baumann of the Gravitation Astroparticle Physics Amsterdam (GRAPPA) at the university of Amsterdam and in collaboration with John Stout of Harvard University also describe how the gravitational atom ionization effect can be observed by analyzing specific signatures within gravitational waves (in gravitational wave astronomy) emitted by inspiralling binaries [3].
A particularly well-studied example of new physics, accessible with future GW observations, are ultralight bosons. Such bosons can be generated by superradiance… forming long-lived condensates (“clouds”) around rotating black holes. Due to their strong similarity with the hydrogen atom, such systems have been called “gravitational atoms.” For isolated gravitational atoms, there are essentially two ways of inferring the presence of these boson clouds. First, rotating clouds will emit gravitational waves that can be looked for in continuous-wave searches. Second, the clouds extract spin from their parent black holes and this spin-down can be inferred statistically in a population of rotating black holes. The existence of rapidly spinning black holes would then rule out ultralight bosons in a certain mass range. Unfortunately, neither of these effects is very distinctive, so it is hard to use them as a way of unambiguously discovering gravitational atoms in the sky. –D. Baumann, G. Bertone, J. Stout, and G. M. Tomaselli, “Ionization of Gravitational Atoms,” Phys. Rev. D, vol. 105, no. 11, p. 115036, Jun. 2022.
The researchers determine that under their model the ionization and resulting gravitational atom binding of the binary system would result in dissipation of energy and accelerate the inspiral and merging of the two black holes. This would result in what the researchers describe as distinctive “kinks” in the evolution of the emitted gravitational wave frequency, a sharp feature that should be detectable in analysis of gravitational waves from some merging binary systems. Such a discovery would offer us an important insights about ultralight boson particles that currently are only theorized to exist, and yet could solve myriad puzzles in physics, such as the strong charge-parity problem (the strong CP problem has to do with the seeming remarkably strong preservation of CP-symmetry— see the article The ‘Strong CP Problem’ Is The Most Underrated Puzzle In All Of Physics).
Highlights:
If it is only logical to consider stellar mass black holes as gravitational atoms—and it may soon be possible to observe their ionization and binding in binary systems by analysis of the resulting gravitational wave signature—it is not that great a stretch of the imagination to consider atoms as quantum black holes. Indeed, the similarities in properties and behaviors can be enumerated, and the idea of the strong binding force as a gravitational interaction has been considered previously in physics [4]. The common objection is that the gravitational interaction is supposedly too weak to account for nuclear confinement, however as physicist Nassim Haramein shows in his latest research based in part on gravitoelectromagnetic calculations, there is a screening effect on the strong gravitational force of quantum black holes (protons), such that it follows a Yukawa potential, differing from an inverse-square law.
The relationship between the constants through our computed scaling factors derived from the fundamental holographic ratio Φ is further confirmed by the precise computation of the mass of the electron, the radius of the electron, the Bohr magnetron and the mass of the proton, all known to 10-10 by measurements. As a result, we can demonstrate a clear relationship between the classical scale of the gravitational constant and the quantum energy spectra of the atomic scale from first principle of theoretical tenets alone. Haramein & Alirol, Scale Invariant Unification of Forces, Fields & Particles in a Quantum Vacuum Plasma, pre-print abstract 2020.
While the unification of particles, forces, and constants in Haramein’s application is based on first principles—the mechanics can be fully delineated and described, unlike in other quantum gravitational models—the approach is not so dissimilar to those found in string theory that recognizes important duality between gravity-like and QCD-like theories, such as in the anti- de Sitter / Quantum Chromodynamic correspondence. Such AdS/QCD theories, for instance, show that quantum entanglement is analogous to multiply-connected spacetime geometry in General Relativity, such that two black holes connected by a wormhole have the same properties and behaviors as two particles that are quantum entangled. As more similarities are drawn the distinctions between a gravitational atom and a quantum black hole will lead to a better understanding of just how far the “analogy” goes.
References
[1] R. Penrose, Riv.Nuovo Cim., 1, 252 (1969).
[2] D. Baumann, G. Bertone, J. Stout, and G. M. Tomaselli, “Ionization of Gravitational Atoms,” Phys. Rev. D, vol. 105, no. 11, p. 115036, Jun. 2022, doi: 10.1103/PhysRevD.105.115036
[3] D. Baumann, G. Bertone, J. Stout, and G. M. Tomaselli, “Sharp Signals of Boson Clouds in Black Hole Binary Inspirals,” Phys. Rev. Lett., vol. 128, no. 22, p. 221102, Jun. 2022, doi: 10.1103/PhysRevLett.128.221102
[4] C. Sivaram and K. P. Sinha, “Strong gravity, black holes, and hadrons,” Phys. Rev. D, vol. 16, no. 6, pp. 1975–1978, Sep. 1977, doi: 10.1103/PhysRevD.16.1975
