I love black holes.  They are big, puzzling, so distant and without mercy.  They are also very simple to describe as far as exterior goes (think just how many elements you have in house or our planet and how time would take to describe against pure and elegant black hole which only has spin, mass and charge - simple, but also most puzzling objects out there).





I assume just like a child that loves dinosaurs, I love black holes - there is something attractive in them (literally that would be their mass and gravitational pull).  While you won't find #1, #2 and #3 titled as such, I did write about black holes earlier here:


When I was a kid I used to imagine black holes as vortices. The idea, for kid's mind, is quite obvious - both tend to suck up the things which get near them. In real life, they are quite different, operate completely different - actually they nothing in common. Or do they? Let's see. Black holes are regions of spacetime in which gravity is strong enough to prevent anything escaping, even light. As many things, they were first discovered in the early 20th century as mathematical solutions to the equations of general relativity and it was much later that astronomers began to gather observational evidence of their existence. One of the curious features of general relativity is that the same mathematics crops up in various other situations. In recent years, for example, physicists have worked out how to create invisibility cloaks by steering light around objects using metamaterials. Black holes steer light in the same way by bending space-time. In fact, the mathematics that describe both systems are formally equivalent. Because of that, it should come as no surprise that engineers have used metamaterials to create analogues of black holes that prevent light escaping.


Back to vortices. I was a bit surprised to see new paper which seems to put these two next to each other. George Haller at the Swiss Federal Institute of Technology in Zürich and Francisco Beron-Vera at the University of Miami in Florida have found another analogue of a black hole, this time in the world of turbulence. The vortices that can form in turbulent water are a familiar sight.




They can be thought of as coherent islands in an incoherent flow. As such, they are essentially independent of their environment, surrounded by a seemingly impenetrable boundary and with little, if any, of the fluid inside them leaking out. If you’re thinking that this description has a passing resemblance to a black hole, you'd be right. Haller and Beron-Vera put this similarity on a formal footing by describing the behavior of vortices in turbulent fluids using the same mathematics that describes black holes. They show that each vortex boundary in a turbulent fluid contains a singularity, just like an astrophysical black hole. That has important implications for the study of fluids and the identification of vortices, which are otherwise tricky to define and spot. In this case, it is simply question of looking for the singularity and the boundary that surrounds it. And that’s exactly what Haller and Beron-Vera have done in the pattern of currents in the south west Indian Ocean and the South Atlantic. A well-known phenomenon in this part of the world is called the Agulhas leakage which comes from the Agulhas current in the Indian Ocean.  At the end of its southward flow, this boundary current turns back on itself, creating a loop that occasionally pinches off and releases eddies (Agulhas rings) into the South Atlantic.




Researchers used satellite images of the South Atlantic Ocean from between November 2006 and February 2007 to look for vortices using a set of simple computational steps that spots black hole analogues. In this three-month period they found eight candidates, two of which turned out to be black hole analogues containing photon spheres. That’s an interesting result that could have significant implications for our understanding of the way ocean currents transport material. Since anything that gets into these black holes cannot get out, this should trap any garbage, oil or indeed water itself, moving it coherently over vast distances. The work also raises the possibility that black hole analogues will occur in other situations, such as in hurricanes and not just on Earth. By this way of thinking, the Great Red Spot on Jupiter might well be the most famous black hole in the Solar System.




I recently read The Baffling Simplicity of Black Holes - Out There whee Corey S. Powell says:


For an ordinary sphere - a bowling ball, for example - the mass increases as the cube of the radius. If one bowling ball is twice the diameter of another it will weigh eight times (2 cubed) as much. The rule breaks down a bit for large objects like planets, but in a very straightforward way. Their incredible bulk compresses their insides, so as planets get more massive their interiors tend to get more dense, assuming you are making an apples-to-apples comparison of the same type of planet. Some planets around other stars have masses several times that of Jupiter, but they are similar in size because of this gravitational squishing.


Black holes do something completely different, however. Their radius increases in direct proportion to the mass. Double the mass of a black hole, and its diameter doubles as well.


Diameter above is black hole horizon - point of no return.  If you think about it, you easily come to conclusion that there is certain limit to density so anything you throw in black hole, black hole needs to expand. Wrong!  It's not compressed inside - it's as if density increases. So what that means is that bigger the black hole, less the density is. Interesting, huh? However, based on observations as such, we can easily figure out mass of black hole.  For example, black hole of our sun's mass would be 6km in diameter.  How big is the black hole in center of our Milky Way? Based on star movements, we believe it is mass of 3.6 million suns.  This will give you diameter of 21.6 million km. The galaxy M87 contains a monster black hole that astronomers have measured as having the mass of 6.6 billion suns. Its density is about 1/3000th the density of water which is similar to the density of the air you are breathing. However, here is the part which makes you wonder:


If you keep going to higher masses, the radius of the black hole keeps growing and the density keeps shrinking. Let’s examine the most extreme case: What is the radius of a black hole with the mass of the entire visible universe? Turns out that its radius is…the same as the radius of the visible universe. Almost as if the entire universe is just one huge black hole.


You didn't see that coming, did you? That's why I love fractals.  You care more here.


Quasars are active black holes - primarily from the early universe. Using a special method where you observe light that has been bent by gravity on its way through the universe, a group of physics students from the Niels Bohr Institute have observed a quasar whose light has been deflected and reflected in six separate images. This is the first time a quasar has been observed with so many light reflections. It looks as following:




The light observed came from a quasar, which is an active, supermassive black hole at the center of a distant galaxy. Such active, supermassive black holes swallow gas from its surroundings. Due to the tremendous gravitational pull, the gases are pulled from the surrounding region into the black hole with incredible speed and gases near the black hole are heated to millions of degrees. This extremely hot gas emits radiation, which then heats the enormous dense clouds of dust and gas that circulate at a slightly greater distance from the black hole. The heat causes the gas to light up with incredibly powerful emission of light - stronger than the light from many galaxies. Quasars are thus extremely luminous and can be observed across the entire universe. But, as shown in video above, light does not always move in a straight line. Light is affected by the gravity of objects it encounters in its path.


Recently we found a star, named PSR J1745-2900, which lives next to black hole in center of our galaxy. Now, when a massive star dies, it can collapse in on itself, resulting in a smaller star made almost entirely of neutrons. When spinning neutron stars emit beams of radiation from their poles, they’re called pulsars as the one discussed above (short for pulsating stars). And certain pulsars have extremely strong magnetic fields, about 100000 billion times stronger than Earth’s field; these are called magnetars. PSR J1745-2900 is a neutron star, and a pulsar, and a magnetar. And, it is also just within a light-year of Sagittarius A* (Milky Way's supermassive black hole). Finding magnetar so close our black hole is always of the value as due to what we know and new observations, we can learn new tricks. And we observed something - how strong, large scale magnetic field infuses the area around Sgr A*. That magnetic field affects the dynamics and flow patterns governing how the black hole accretes its meals of hot gas. The strong magnetic field could be slowing down the accretion process, meaning less hot gas for the hungry black hole. This, in turn, could answer some of the questions  astronomers have about radiation coming from Sgr A*, as well as explain why the black hole appears to be less active than other galaxies’ central black holes.






Credits: arXiv, Bill Andrews, Corey S. Powell