Black holes are not what they consume. Einstein’s theory of general relativity forecasts that regardless of what a black hole gobbles up, its external properties are based only on its rotation, mass and electric charge.
A simulation of the magnetic field lines (green) surrounding a black hole (left). As the field lines break and reconnect, pockets of plasma form (center of green circles). These plasma pockets launch inward toward the black hole or outward into space, draining energy from the magnetic field. Image Credit: A. Bransgrove et al./Physical Review Letters 2021.
All other information about its diet vanishes. Astrophysicists whimsically name this the no-hair conjecture. (According to them, black holes “have no hair.”)
Possibly, there is a hairy risk to the conjecture. Black holes can originate with a powerful magnetic field or achieve one by consuming magnetized material. A field like that must rapidly fade away for the no-hair conjecture to be true.
However, real black holes do not have the ability to survive in isolation. They can be encircled by plasma — a gas so energized that electrons are emitted from their atoms —
that can sustain the magnetic field, probably proving the conjecture wrong.
Scientists from the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City, Columbia University, and Princeton University employed supercomputer simulations of a plasma-engulfed black hole to discover that the no-hair conjecture is true. They have reported the study results in the Physical Review Letters journal on July 27th, 2021.
The no-hair conjecture is a cornerstone of general relativity. If a black hole has a long-lived magnetic field, then the no-hair conjecture is violated. Luckily a solution came from plasma physics that saved the no-hair conjecture from being broken.
Bart Ripperda, Study Co-Author and Research Fellow, Flatiron Institute’s Center for Computational Astrophysics; Postdoctoral Fellow, Princeton University
The simulations of the group demonstrated that the magnetic field lines present near the black hole split and rejoin rapidly, thereby making plasma-filled pockets that launch into space or fall into the black hole’s maw. This process helps drain the magnetic field quickly and could be behind the flares observed next to supermassive black holes, report the scientists.
Theorists didn’t think of this because they usually put their black holes in a vacuum. But in real life, there’s often plasma, and plasma can sustain and bring in magnetic fields. And that has to fit with your no-hair conjecture.
Bart Ripperda, Study Co-Author and Research Fellow, Flatiron Institute’s Center for Computational Astrophysics; Postdoctoral Fellow, Princeton University
The study was co-authored by Ripperda along with Columbia graduate student Ashley Bransgrove and CCA associate research scientist Sasha Philippov, who is also a visiting research scholar at Princeton.
A study performed in 2011 on the issue indicated that the no-hair conjecture was in a predicament. Yet, that study just observed such systems at low resolution, and it considered plasma as a fluid. However, the plasma present near a black hole is so diluted that particles hardly run into one another, and, therefore, treating it as fluid is an oversimplification.
As part of the new study, the scientists performed high-resolution plasma physics simulations with a general-relativistic model of a black hole’s magnetic field. Altogether, it took nearly 10 million CPU hours to produce all the calculations.
We couldn’t have done these simulations without the Flatiron Institute’s computational resources.
Bart Ripperda, Study Co-Author and Research Fellow, Flatiron Institute’s Center for Computational Astrophysics; Postdoctoral Fellow, Princeton University
The consequent simulations illustrated the evolution of the magnetic field around a black hole. In the beginning, the field stretches in an arc from the north pole of the black hole to its south pole. The interactions that occur inside the plasma cause the field to inflate in the outward direction. This opening up makes the field divide into separate magnetic field lines that tend to radiate outward from the black hole.
The field lines get interchanged in the direction, either toward or away from the event horizon. Magnetic field lines in the vicinity get connected, forming a braided pattern of field lines merging and breaking apart. A gap exists between two connection points that are filled with plasma.
The magnetic field energizes the plasma, launching inward into the black hole or outward into space. As the process lasts, the magnetic field tends to lose its energy and ultimately shrivels away.
The process takes place very quickly. The scientists discovered that the black hole exhausts its magnetic field at a rate of 10% of the speed of light. “The fast reconnection saved the no-hair conjecture,” stated Ripperda.
The scientists suggest that the mechanism inducing the noted flares from the supermassive black hole at the heart of the Messier 87 galaxy could be described by the balding process observed in the simulations.
The researchers have stated that the initial comparisons between them seem to be encouraging, although a more robust evaluation is required. If they happen to get aligned, energetic flares induced by magnetic reconnection at black hole event horizons may be considered as an extensive phenomenon.
Journal Reference:
Bransgrove, A., et al. (2021) Magnetic Hair and Reconnection in Black Hole Magnetospheres. Physical Review Letters. doi.org/10.1103/PhysRevLett.127.055101.