Regarding recent posts about black holes, here’s another YouTube video by The Science Asylum on the topic (published February 16, 2019): “Black Holes can SPIN?!?”
Most sources talk about Schwarzschild black holes, but those don’t spin. Most are what we call Kerr black holes, or rotating black holes, surrounded by a region called an ergosphere. The spacetime around them is not only stretched, but also twisted, leading to some strange phenomena.
I’ve previously highlighted The Science Asylum as a science communicator. This latest video is an interesting overview of black holes: non-spinning vs. spinning, the math vs. reality inside and outside event horizon(s), the ergosphere, frame dragging.
And here’s a YouTube video on the ehtelescope channel “Zoom into M87 Explaining Formation of the Supermassive Black Hole Image” (published on Apr 15, 2019) which visualizes a black hole which has been much in the news lately.
This artist’s impression based on real black hole accretion simulations delves into the surroundings of a black hole, showing an accretion disc of superheated plasma and a relativistic jet. It also shows the paths of photons in the vicinity of a black hole, as they curve close to the event horizon due to strong gravity. The gravitational bending and capture of light by the event horizon is the cause of the black hole shadow captured by the Event Horizon Telescope in the galaxy M87. Credit: Nicolle R. Fuller / NSF Scientific Advisory, Simulations, Final Image: EHT Collaboration
Astronomers orchestrated radio dish telescopes across the world into an Earth-size virtual camera for a bold new experiment attempting to deliver the first-ever image of a black hole. The telescope collaboration is set to make a big announcement of results this week [week of April 8, 2019], and members also described their research approach at a talk in March.
The astronomers’ idea is to photograph the circular opaque silhouette of a black hole cast on a bright background. The shadow’s edge is the event horizon, a black hole’s point of no return.
“The event horizon is the ultimate prison wall — one can get in but never get out,” Avi Loeb, chair of astronomy at Harvard University, told Space.com.
“The event horizon protects us from the unknown physics near a singularity,” Loeb said.
The size of an event horizon depends on the black hole’s mass. If Earth were compressed until it became a black hole, it would have a diameter of about 0.69 inches (17.4 millimeters), a little smaller than a dime; if the sun were converted to a black hole, it would be about 3.62 miles (5.84 kilometers) wide, about the size of a village or town. The supermassive black holes that the Event Horizon Telescope is observing are far larger; Sagittarius A*, at the center of the Milky Way, is about 4.3 million times the mass of our sun and has a diameter of about 7.9 million miles (12.7 million km), while M87 at the heart of the Virgo A galaxy is about 6 billion solar masses and 11 billion miles (17.7 billion km) wide.
Good recap of non-rotating vs. rotating black holes.
The content of this article appeared on a number of science news feeds last week.
Caltech > News > “Black Hole Bends Light Back on Itself” by Whitney Clavin (April 8, 2020) – New study proves a theory first predicted more than 40 years ago.
You may have heard that nothing escapes the gravitational grasp of a black hole, not even light. This is true in the immediate vicinity of a black hole, but a bit farther out—in disks of material that swirl around some black holes—light can escape. In fact, this is the reason actively growing black holes shine with brilliant X-rays.
Now, a new [collaborative] study accepted for publication in The Astrophysical Journal offers evidence that, in fact, not all of the light streaming from a black hole’s surrounding disk easily escapes. Some of it gives in to the monstrous pull of the black hole, turns back, and then ultimately bounces off the disk and escapes.
The new findings were made possible by combing through archival observations from NASA’s now-defunct Rossi X-ray Timing Explorer (RXTE) mission, which came to an end in 2012. The researchers specifically looked at a black hole that is orbited by a sun-like star; together, the pair is called XTE J1550-564. The black hole “feeds” off this star, pulling material onto a flat structure around it called an accretion disk. By looking closely at the X-ray light coming from the disk as the light spirals toward the black hole, the team found imprints indicating that the light had been bent back toward the disk and reflected off.
The new study, titled, “Evidence for Returning Disk Radiation in the Black Hole X-ray Binary XTEJ1550-564,” was funded by NASA, the Alexander von Humboldt Foundation, and the Margarete von Wrangell Fellowship.
The ergosphere is related to the event horizon of a black hole. This Space.com article discusses the upcoming reveal of the first-ever photo of a black hole (one of many teasers for this event): “The Event Horizon Telescope Is Trying to Take the First-Ever Photo of a Black Hole.”
Article contains good overview video.
Wiki: Event Horizon Telescope
And today (April 9, 2019) Space.com published another article on black holes” “What Exactly Is a Black Hole Event Horizon (and What Happens There)?”
Good recap of non-rotating vs. rotating black holes.
The content of this article appeared on a number of science news feeds last week.
Caltech > News > “Black Hole Bends Light Back on Itself” by Whitney Clavin (April 8, 2020) – New study proves a theory first predicted more than 40 years ago.