What Does The Inside Of A Black Hole Look Like?

What is a Black Hole?

A region of space having a gravitational field so intense that no matter or electromagnetic radiation can escape  such as light.


How did Black Holes form? 

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, super-massive black holes of millions of solar masses (M_☉) may form. There is general consensus that super-massive black holes exist in the centers of most galaxies.

Who was the first person known to propose the existence of Black Holes?

John Michell (25 December 1724 – 29 April 1793) was an English clergyman and natural philosopher who provided pioneering insights in a wide range of scientific fields, including astronomy, geology, optics, and gravitation. Considered "one of the greatest unsung scientists of all time", he was the first person known to propose the existence of black holes in publication.The idea of a body so massive that even light could not escape was briefly proposed by astronomical pioneer John Michell in a letter published in 1783-4. Michell's simplistic calculations assumed that such a body might have the same density as the Sun, and concluded that such a body would form when a star's diameter exceeds the Sun's by a factor of 500, and the surface escape velocity exceeds the usual speed of light. Michell correctly noted that such super-massive but non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies.

General relativity

When Einstein developed relativity theory, it took him about ten years to work out the math using a daunting form of mathematics called tensor calculus. He was only able to approximate the solutions to his own equations and the math still perplexes even the best scientific brains. However, the challenge did nothing to deter one of Einstein's contemporary astronomers- a theoretical physicist named Karl Schwarzschild. Schwarzschild was a practical individual by nature. He pioneered new methods of studying spectra, for example. But he excelled in his abilities to deal with theoretical concepts and when Einstein's articles on general relativity were published in 1915, Schwarzschild was one of the first to recognize their importance. 

Schwarzschild was also a German patriot, so he set aside his astronomical studies when World War 1 erupted and enlisted in the army. By the time he had read Einstein's papers, he had already seen action in Belgium, France and on the Russian front. Nonetheless, he was attracted to the essentialness of general relativity and began to seek exact answers for its equations. Two months after contracting a life threatening disease and being sent home to recuperate, Schwarzschild was finally able to concentrate on completing his calculations. Shortly before his death in 1916, Schwarzschild completed his work and it was published later the same year. Titled On the Field of Gravity of a Point Mass in the Theory of Einstein, it became one of the pillars of modern relativistic studies and in it Schwarzschild presented his solutions to Einstein's unfinished equations. 

Significantly, it provided support for a, then, seemingly implausible situation about the effects of severely compressed matter on gravity and energy. 

Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light. Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, and the gravitational time delay. The predictions of general relativity have been confirmed in all observations and experiments to date. Although general relativity is not the only relativistic theory of gravity, it is the simplest theory that is consistent with experimental data. However, unanswered questions remain, the most fundamental being how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity.

When Einstein wrote his general theory of relativity, he found a new way to describe gravity. It was not a force, as Sir Isaac Newton had proposed, but a consequence of a distortion in space and time, conceived together in his theory as 'space-time'. According to Einstein, matter and energy exist on a background of space and time. There are three spatial dimensions (backwards-forwards, left-right and up-down) and one time dimension (which flows at one second per second). Objects distort the fabric of space-time based on their mass- more massive objects have a greater effect. 

Just as a bowling ball placed on a trampoline stretches the fabric and causes it to dimple or sag, so planets and stars warp space-time - a phenomenon known as the 'geodetic effect'. A marble rolling across the trampoline will be inexorably drawn towards the bowling ball. Thus the planets orbiting the Sun are not being pulled by the Sun; they are following the curved space-time deformation caused by the Sun. The reason the planets never fall into the Sun is due to the speed at which they are traveling. The astrophysicist, John Archibald Wheeler, who coined the name "Black Hole", said it succinctly: "matter tells Space-Time how to curve, and Space-Time tells matter how to move." 

Schwarzschild realized the escape velocity from the surface of an object depends on both its mass and radius. For example, the escape velocity of the Earth is about 11.2 kilometers per second- this is the speed a rocket must attain before it can depart the Earth on a journey to the Moon or more distant planets. The Moon's escape velocity, however, is only 2.4 kilometers per second because the Moon is one fourth the size of our planet and possesses only slightly more than 1% of its mass. But, if nature can make the radius of a given mass small enough, the escape velocity will increase until it reaches the speed of light, or 300,000 kilometres (186,000 miles) per second. At that point, neither matter nor radiation can escape from the object's surface. Additionally, atomic or subatomic forces become incapable of holding the object up against its own weight. Therefore, the object collapses into an infinitesimal point- the original object disappears from view and only its gravity remains to mark its presence. As a result, it creates a bottomless pit in the fabric of space-time. 

In 1931, Subrahmanyan Chandrasekhar calculated, using special relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at 1.4 M☉) has no stable solutions. His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse. They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star, which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately 3 M☉ (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars", because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius.

What Does The Inside Of A Black Hole Look Like?

A black hole has a boundary, called the event horizon. It is where gravity is just strong enough to drag light back, and prevent it escaping. Because nothing can travel faster than light, everything else will get dragged back also. Falling through the event horizon, is a bit like going over Niagra Falls in a canoe.