Througout the field of cosmology, black holes has stood out to be among the most fascinating things ever studied about our universe. The idea that a super massive and dense object exist in space where gravity pulls so strong that it crushes and sucks anything across is path has continued to marvel astronomers and astrophysicists over the past years. Most famously, the winners for the 2020 noble prize award in the physics categories were all involved in various discoveries about black holes.
So one may ask, what's really a black hole and how does it work?
Definition of a black hole
A black hole is a region in space where the force of gravity pulls so strong that it prevent the escape of even the fastest of particles. Not even electromagnetic radiation such as light can escape it.
A black hole's gravitational pull is so intense that no matter of any form can escape it once inside a certain region, called the event horizon.
So why are they called "black holes" anyway?
Ironically, black holes are not inherently black, neither are they conical spinning holes in the universe as some images may depict. The facts remains that the gravitational pull being experience on a black holes is so strong that it ends up crushing and sucking(like a hole), any matter found near it surrounding, including any rays of light. When all light rays directed at a black hole are all sucked up to the last photon, what will be left is total darkness at the center. That is why black holes are often depicted as black looking monster tonadoes in space.
So a clear answer to why black holes are so named is simply because:
"on a black hole, gravity pulls so extreme that it ends up sucking(like a hole) any matter found near it territory, including any traces of light rays, making it appear as black at the center"
Image depicting a star being pulled by a black hole's instense gravity
So what makes a black hole's gravity pulls so strong?
Being called a "hole" doesn't mean it's an empty space. It in fact a huge volume of matter squeezed and packed into a very small radius. A perfect analogy taken from the NASA website says it will be like squeezing a star ten times more massive than the sun into a sphere approximately the size of New York City. The result would turn out to produce an extremely dense structure in space, one in which it mass will cause an infinite curvature in space-time and it gravity, so strong that no matter can escape it immerse pull.
When the statement "sucks any matter found near in vicinity" is often made anywhere when talking about black holes, that shouldn't make one try to visualize black holes as cosmic vacuum cleaners that swallow up any matter close to it. Although it seems quite easier to try to view from this perspective, but the truth remains that, black holes doesn't actually swallow matters to itself (as would vacuum cleaners), their gravity just smear and pulls objects towards it center. These objects in turn, due to being acted upon by series of chaotic forces will be spewed out in some way. Thus, it will be more accurate to think of a giant tonadoe when thinking of a black hole, but this time with a drawing force caused by gravity, not by rotating wind.
The event horizon
The center(or the core) of a black hole is where the extreme gravitational pull takes place, and this region is associated with an event horizon. On a black hole, an event horizon is the boundary where the speed needed to escape a black hole's extreme gravitational pull will exceed the speed of light. That means that an object needs to travel faster than light to escape a black hole's event horizon.
But according to Einstein's special theory relativity, nothing can travel faster through space than the speed of light. This means a black hole's event horizon is essentially the point from which absolutely nothing can
return.
Formation of black holes
The idea behind the formation of black holes were first predicted by Einstein's field equations for the general theory of relativity. The prediction was that if an object is sufficiently massive or dense,(like a neutron star) there will become a point that it will collapse in upon itself and then become even more denser that it creates an infinite distortion in space-time, resulting in an object with a very strong gravitational influence on nearby objects. The object having such property were later termed as a black hole.
In accordance to that, it was thus observed that when a neutron star(a very dense, heavy type of star) finally becomes too massive and its gravity becomes too much to handle, the star collapses on itself and explodes leaving behind a black hole.
Black holes were never seen or observed by scientists with ordinary space telescopes that were used to detect light and other space radiations. Even after years of studying, research, argument, etc. It continued to remain a hypothetical prediction.
Astronomers and astrophysicists however we're only able to deduce the presence of a black hole on space by detecting it gravitational effect on nearby stars and objects. For example, when a star or other interstellar objects crosses a black hole, they will be teared apart and drawn inward Not long after the invention of the event horizon telescope that astronomers were now able to photograph the first image of a black hole.
Image showing the first actual photo of a black hole and it shadow, taken by the event horizon telescope at the center of galaxy M87.
Historical prediction of black holes
In 1915, Einstein published his general theory of relativity in which he showed that gravity is not an attractive force existing between two masses as Newton said, but gravity is a function of space-time curvature. He included series of equations along with his theories to justify his claims. These equations are popularly known as "Einstein's field equations"(EFE). These equations related how the geometry of space-time affected other parameters like mass, energy, momentum, and stress. In short terms, the equations summaries the relationship between matter and the geometry of space-time. Due to the non-linear nature of Einstein's equations, Einstein himself assume that they were unsolvable. Lots of effort were made by other mathematicians and physicists in solving the field equations and understanding it solutions.
Not long after, in January 1916 Karl Schwarzschild a German physicist and astronomer produced the first exact solution to the field equations. He used Einstein's field equations to calculate the outcome of the gravitational effect of a single spherical body, such as a star.
The first case was that : If a mass of a star is neither very large or highly concentrated(made to be very dense), the gravitational influence of the star on other nearby matter will just be the same as that given by Newton's theory of gravity. That is to say that the gravitational force of attraction of the star will be given by F=GMm/R². Thus, Newton's theory was not incorrect, rather it justifies general relativity under certain conditions.
For a weak gravitational field, the result of general relativity does not differ significantly from Newton laws of gravitation. But for intense gravitational field, (i.e one cause by a large or extremely dense mass generating instense curvature in space time) the result diverge, and general relativity has been proven to predict the correct result.
The second case was the most important one, as it predicted an extraordinary object in space. The second case was that : If a body of mass were concentrated or confined within a vanishingly small volume, the resulting calculation shows that there will come to occur a point of infinite curvature and density - a singularity- in at the center, where gravity will become so strong that nothing pulled into the surrounding region can ever leave. Even light cannot escape.
Yes, black holes were first predicted by Einstein's general law of relativity. But as technologies advanced, we were able to observe our first actual black hole back in 2016. It was thus proving Einstein's prediction to be correct.
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