For more than 30 years, astrophysicists have believed
that black holes can swallow nearby matter and release a
tremendous amount of energy as a result. Until recently,
however, the mechanisms that bring matter close to black
holes have been poorly understood, leaving researchers
puzzled about many of the details of the process.
Now, however, computer simulations of black holes
developed by researchers, including two at Johns Hopkins,
are answering some of those questions and challenging many
commonly held assumptions about the nature of this
enigmatic phenomenon.
"Only recently have members of the research team
— John Hawley and Jean-Pierre De Villiers, both of
the University of Virginia — created a computer
program powerful enough to track all the elements of
accretion onto black holes, from turbulence and magnetic
fields to relativistic gravity," said Julian Krolik, a
professor in the Henry
A. Rowland Department of Physics and Astronomy at Johns
Hopkins and co-leader of the research team. "These programs
are opening a new window on the complicated story of how
matter falls into black holes, revealing for the first time
how tangled magnetic fields and Einsteinian gravity combine
to squeeze out a last burst of energy from matter doomed to
infinite imprisonment in a black hole."
Close to the black hole's outer edge, where the
Newtonian description of gravity breaks down, ordinary
orbits are no longer possible. At that point — or so
it has been imagined for the past three decades —
matter plunges quickly, smoothly and quietly into the black
hole. In the end, according to the prevailing picture, the
black hole, except for exerting its gravitational pull, is
a passive recipient of mass donations.
The team's first realistic calculations of matter
falling into black holes have strongly contradicted many of
these expectations. They show, for instance, that life in
the vicinity of a black hole is anything but calm and
quiet. Instead, the relativistic effects that force matter
to plunge inward magnify random motions within the fluid to
create violent disturbances in density, velocity and
magnetic field strength, driving waves of matter and
magnetic field to and fro. This violence can have
observable consequences, according to research team
co-leader Hawley.
"Just like any fluid that has been stirred into
turbulence, matter immediately outside the edge of the
black hole is heated. This extra heat makes additional
light that astronomers on Earth can see," Hawley said. "One
of the hallmarks of black holes is that their light output
varies. Although this has been known for more than 30
years, it has not been possible to study the origins of
these variations until now. The violent variations in
heating — now seen to be a natural byproduct of
magnetic forces near the black hole — offer a natural
explanation for black holes' ever-changing brightness."
One of the most striking properties of a black hole is
its ability to expel jets at close to the speed of light.
While it has long been expected that magnetic fields are
crucial to this process, the latest simulations show for
the first time how a field can be expelled from the
accreting gas to create such a jet.
Perhaps the most surprising result of the team's new
computer simulations is that the magnetic fields brought
near a rotating black hole also couple the hole's spin to
matter orbiting farther out, in the same way that a car's
transmission connects its rotating motor to the axle. Said
Krolik, "If a black hole is born spinning extremely
rapidly, its 'drive train' can be so powerful that its
capture of additional mass causes its rotation to slow
down. Accretion of mass would then act as a 'governor,'
enforcing a cosmic speed limit on black hole spins."
According to Krolik, that "governor" may have strong
implications for many of the most striking properties of
black holes. It is widely thought, for example, that the
strength of a black hole's jet is related to its spin, so a
"spin speed limit" might determine a characteristic
strength for the jets, Krolik said.
Funded by the National Science Foundation, this
research is being published in a series of four papers in
The Astrophysical Journal. The simulations were
performed at the NSF-supported San Diego Supercomputer
Center.
The research team also included Shigenobu Hirose of
Johns Hopkins.
To view video animations of the computer simulation,
as well as color stills of twisting magnetic fields, go to
www.jhu.edu/news_info/news/audio-video/blackholes.html.