One of the most interesting and awe-inspiring concepts of astrophysics that has intrigued physicists and enthusiasts alike is the mysterious black hole – an invisible object warping space-time around it and exerting gravitational forces so intense that not even light can escape from around it .
In 1967, American astronomer John Wheeler first coined the term “black hole”. Long before it was discovered in 1971, Albert Einstein first predicted its existence in 1916, in his “General Theory of Relativity” in order to explain the fabric of space-time and has intrigued us ever since. However, it was considered only a theoretical concept, one whose existence was debatable at that time . One of his contemporaries, Karl Schwarzschild found out its importance and presented solutions to Einstein’s unfinished work before it was first discovered in 1971 .
SPACE-TIME AND GEODETIC EFFECT
One of the most important theories of general relativity that served as a corner stone in understanding black holes is its new way of describing gravity. Unlike Newton’s laws that describes it, as a pulling force, Einstein explained it as a distortion of space-time(space and time considered as a fourth dimensional entity) -known as geodetic effect. Geodetic effect is a phenomenon that actually happens in 4-d space-time but can be understood with the help of a two dimensional analogy.
Geodetic effect is a lot like placing a heavy-ball on a stretched fabric that causes it to dimple and sag the point of contact, and regions around it. Unlike the heavy-ball, however, the entire thing happens in fourth dimensional space-time which is difficult to picture due to our dimensional limitations.
Irrespective of whether we are familiar with the equations of gravity and space-time warping, we all witness its presence in our everyday lives. An apple after separating itself from the branch, falls straight towards the earth’s surface, instead of floating around of moving in an arbitrary direction. Gravity exerts a force on a body of mass m with a magnitude mg,
A very important concept used by astronauts and space research organizations is the concept of escape-velocity which is the lowest/threshold velocity away from a planet that would allow an object around a planet’s vicinity to escape its gravitation pull. Escape velocity of earth is about 11.2 km/s and for moon, it is about 2.4 km/s. The equation of escape velocity is as follows
where gravitational constant ,
and M and R are the planet’s mass and radius respectively. From the above equation, we can infer that escape velocity depends on the square root of a bodies, mass to size ratio, and not just one of it.
Thus, if an enormously large mass could be packed inside an extremely small volume, the results can be as fascinating and different from what we are familiar with. This, is what exactly happens during the formation of a black hole. The gravitational pull of a black hole is so large that even light cannot escape its pull, and thus, is sucked in.
WHAT DOES A BLACK HOLE LOOK LIKE?
True to its name, a black hole appears as pitch-black emptiness in the universe spiraling and warping light around it into a bottomless pit of darkness. Its true shape is unknown, because light doesn’t escape from it in order to reach the lenses of a camera or our eyes for that matter. However, a very interesting observation, is that while viewing a black hole, some objects that are directly behind it( that is supposed to remain blocked from our field of vision) could be seen as a spiral-distorted image around the dark region.
This is because, some light rays travel at distances, close enough to be influenced by its gravitational field, yet not close enough to be pulled into it. These rays bends before traveling further(which is in contrast to the theory of rectilinear propagation of light). A black hole, alternately is termed as singularity due to the its point nature – an entity with zero volume yet with mass. The spherical boundary around a black hole, beyond which nothing can escape its pull is known as “event horizon”. The radius of this event horizon is named after the German physicist Karl Schwarzschild himself and is known as the Schwarzchild radius. It’s calculation is not limited only to existing singularities and can be calculated for any object/planet for that matter. For e.g., if the entire mass of the sun were to be compressed into a single point in space, its Schwarzchild radius would be roughly 3 km. The same for our planet, would be a little shy of one-third of an inch!
FORMATION OF A BLACK HOLE
Black holes do not exist perpetually. Like all other events in history, it begins somewhere and in a very interesting manner. A star in the sky, that appears as a ball of fire, is held together by two opposing forces – emitted radiation and gases that stretch it outward and gravity that attempts to pull it into itself. The phenomenon responsible for counteracting against its own gravitational pull is fusion process (exothermic in nature) that burns stockpiles of elements starting with hydrogen, and as a consequence exerts a force radially outward . This hydrogen, through fusion, gets converted to helium, and then to oxygen. For stars as large as the sun, the fusion reactions doesn’t proceed beyond the formation of oxygen and cools down to what is known as a dwarf star. But for stars, five times larger, this chain of reaction continues through silicon, aluminium, potassium all the way to iron. No reaction proceeds beyond iron, and the fuel for fusion gets depleted in fractions of a second. As a result of which a massive dis-balance in force gets created and gravitational force overpowers causing a star to implode.
During the formation of white dwarf or a neutron star, which are the two stable destinies of a star, the final force barrier that counteracts gravitational collapse is the fermionic repulsion pressure(also known as electron degeneracy pressure) – which is simply, a pressure that prevents matter from getting compressed into smaller volumes of itself . It serves as a minimum bound for an object’s volume. During formation of a black hole, even this pressure is not enough to stop the ultimate gravitational collapse from happening.
TRAVELLING INSIDE A BLACK HOLE
A better way of understand the nature of gravitational forces in a black hole and its intensity, is to observe the way an astronaut, traveling inside a black hole would perceive space and time. Contrary to what our intuition might suggest, the astronaut doesn’t get sucked into its orbit rapidly. Instead, its speed of approach will be seen as getting slower and slower, till it finally gets red shifted and then fades away. The observer wont observe the astronaut to actually enter the black hole beyond a certain point. From the astronaut’s perspective, as he enters the black hole, its blackness will engulf his field of vision. The rapid change of gravitational forces increases rapidly after the event horizon. At this point, the part of his body, near the hole will get stretched rapidly or spaghettified, while the part away will experience a delay before adapting to the rapidly acting forces of gravity(inertia of rest and motion).
A solution of Einstein’s field equation is the formation of a wormhole which is a moving black-hole that is thought to connect two points in space-time. The distances between two points in space can be from a few metres to billions of light years away from one another, or even different time.
This interesting, but theoretical concept of two points as space-time portal has been used in several science fiction films such as Star-Trek(2009), Interstellar(2014) – just to name a few.