Alpha Beta What is a Black Hole?: The black hole is a location of spacetime where gravity is so much stronger than nothing. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. It has an enormous effect on the fate and circumstances of an object crossing through it, according to general relativity it has no locally detectable features.
In many ways, the black hole acts like an ideal dark body, because it reflects no light. The quantum field theory in curved spacetime predicts that the event horizons emit Hawking radiation, with the same spectrum as a dark body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe directly.
What is a black hole?
Most of what you may think of as “gravity” is really a result of the interaction of matter with gravity. But to understand what happens at the very heart of the matter that is beneath a black hole, imagine taking the surface of a sphere, and slowly pulling it to a sphere whose centre is the event horizon of the black hole — the boundary that exists in every object that falls into the event horizon and never comes back.
Such a region is an area in which the total curvature is zero, giving rise to no attraction between any matter falling on or close to the horizon and the gravity of the black hole. Black holes are regions of spacetime with a uniform gravitational field. Their spatial dimension is 4, so they are a special kind of manifold with a four-dimensional Euclidean geometry.
Black hole facts
In fact, a general discussion of black holes will include many subtle points that appear completely unrelated to their physical nature. Consider, for example, what happens if you find a particle in a region beyond a black hole’s event horizon.
The initial situation is thus like the one with a tennis ball bouncing off a wall — an infinite wall made of infinitely dense material. Where does the ball go? Nobody can answer this question, because it is so far outside of our physical understanding that it does not exist, and it would be almost as incomprehensible as that ball itself.
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Types of black holes
A stellar-mass black hole will weigh on the order of 10 times the Sun, while supermassive black holes can weigh on the order of 100,000,000,000,000 times the mass of the Sun. Either will become a supernova, with its energy going away in a very short time and not emitting any radiation as it does so. The accretion disks around supermassive black holes, however, are very efficient at expelling black holes of various sizes, depending on how hot their mass is.
About 60% of such black holes will eventually be kicked into high-energy, very luminous quasars, while the rest will either fade into supernova remnants or fall into the galaxy, glowing brightly but never reaching temperatures high enough to emit Hawking radiation.
The origin of black holes
The new observations that black holes have exotic properties are only one aspect of the very long list of scientific discoveries about black holes that have been made by American astronomers in the last 20 years. During that period, astronomers from the University of Chicago, Harvard, MIT, Caltech, Princeton, Yale, University of Michigan, Cornell, Penn State, and other institutions have shown that black holes seem to behave quite differently than was previously believed.
But despite a plethora of information, the complete story about black holes is still shrouded in mystery. This becomes apparent when the quest for further explanation of the properties of black holes is discussed with researchers from the University of Chicago and MIT.
How do black holes form?
In the same way that a ring makes the sun look bigger than the moon, a rotating black hole makes its accretion disk look larger than it is. In our universe, galaxies are found to be tidally locked with respect to their centers of gravity, rotating around their centers of mass in the same direction that their stars move. And the stars move along the rotational axes at roughly the same rate.
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(You can see this in the image, taken from a simulation of this very phenomenon.) This is due to the vast gravitational fields that have a strong effect on the motion of the stars, causing them to bump into one another and collide. Because of this, the denser areas of the disk near the black hole are stretched out. This means that the black hole does not weigh in the same proportion as the galaxy.
Supermassive black holes are at the center of most galaxies.
Type II Supermassive Black Holes
Supermassive black holes are those that are just too large for a normal black hole, which is only a few times the mass of the Sun. One such supermassive black hole is the Sagittarius A* (Sgr A*), the supermassive black hole at the center of our own galaxy. Astronomers estimate that it contains about 4 million times the mass of the Sun.
In fact, we know of about half a dozen other black holes that are about this large, and some of these may even be more massive than Sgr A*! Given their relative rarity, their exact mass is unknown. But given their size, the amount of material they receive from their host galaxies every year must be equivalent to the mass of the Sun 1,000 times over.
Conclusion of What is a Black Hole?
What is a Black Hole? Astronomers have found evidence of other black holes, but they have not been able to definitively identify them. While there is some debate about the veracity of the discovery, in all likelihood it is the remains of a star that collapsed under its gravity, leaving behind an accretion disk of matter in orbit around the central black hole.
