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# Black Holes 101

Updated: Oct 20, 2019

By Monish Singhal

## “Listening to a song being played on a piano with broken keys”

We have taken the first picture of a black hole” announced Sheperd S. Doeleman, director of the Event Horizon Telescope (EHT) project, on 10th April 2019. Previously, black holes had only been imagined or conceptualised through illustrations and computer-generated graphics, including the famous one in the movie ‘Interstellar’.

How was it possible to capture an actual image of a black hole? Critical to the team’s success was Katie Bouman, a Computational Imagist, who led the development of the algorithm that enabled the imaging of black holes, known as CHIRP (Continuous High-resolution Image Reconstruction using Patch priors). Bouman describes the process of her team’s work as being “like listening to a song being played on a piano with broken keys” since there were gaps in imaging between the EHT telescopes.

Bouman with the first actual image of a black hole, which she led her team to capture.

## Tracing the journey from theoretical concept to actual image

The term black hole was first derived from Albert Einstein’s theorems of relativity. Prior to Einstein’s theorising, it had been believed that heavenly bodies moved around each other as a result of the force of gravity, calculated by Sir Isaac Newton’s equation GM1M2R2. Einstein’s calculations showed us that heavenly bodies did not move around each other due to gravity. Rather, these motions are due to a bend in the space-time fabric. (Note: this did not mean that equation was incorrect.)

###### Images from Science Fiction now jostle with reality.

Space-time is the fourth dimension. It involves the three dimensions as we know, length, breadth and height, and also involves time. Here, time cannot be separated from the three dimensions of space since the observed rate at which time passes for an object depends on the object's velocity relative to the observer. General relativity, in addition, provides an explanation of how gravitational fields can slow the passage of time for an object as seen by an observer outside the field. This, as for the other three dimensions, is invisible to the human eye; however, it’s effects are experienced and its presence has been proved by several experiments, most recently by the discovery of gravitational waves, which are simply ripples in space-time.

Space-time is the dimension that controls the universe. Einstein realised that massive objects cause a distortion in space-time, such that the lesser the mass of an object, the lesser its distortion and vice versa. Thus, the sun would have a larger distortion in space-time than Earth.

Imagine it this way: there is a stretched rubber sheet over which the sun and Earth are placed. This would cause deepened depressions in gravitational waves near the sun than Earth, which is what causes the Earth to move around the Sun.

## Heavier and heavier, things get curiouser and curiouser!

Consider now placing heavier and heavier, and increasingly concentrated weights on the rubber sheet. They will depress the sheet more and more. Eventually, at a critical weight and size, they will make a bottomless hole in the sheet, that particles can fall into, but nothing can get out of. This is a black hole.

###### The concept behind a black hole.

No escape! Black holes exert such a strong force that nothing, not even light [electromagnetic waves], which can cover over 293,792,458 metres in a second, can escape it. Hence, a black hole is virtually undetectable by any sorts of electromagnetic waves, including radio waves and microwaves. Yet its effects are extremely pronounced, enabling astronomers to discover them. However, few are discovered: the nearest being about 3000 light years away.

## Gravitational time dilation and spaghettification

We know that speed = distance/ time, and that as distance increases at constant speed, time reduces, causing gravitational time dilation.

Black holes cause objects rotating around them to experience gravitational time dilation, by exerting a great amount of bend in the space-time fabric. This forces light (and any other electromagnetic waves) to take a larger path around the body in question.

## Hawking Radiation and the “No-Hair Theorem”

What is left for us to discover? Hawking Radiation theory states that a black hole constantly emits radiation in the form of energy, and that this energy degenerates the black hole. It has been proved experimentally, using a lab created model of a black hole, but the next big discovery for black holes is likely to be astronomical evidence of this. Another conundrum that has yet to be tangibly evidenced is the “No-Hair Theorem”, which postulates that static and isolated black holes can be fully characterized by just two numbers, the mass (M) and the angular momentum (J).

## Sources:

“Astronomers capture first image of a black hole”, National Science Foundation, 10/04/2019 [https://www.nsf.gov/news/news_summ.jsp?cntn_id=298276 ], Retrieved 30/05/2019

“Black Holes”, Astronomical Society of the Pacific, John Percy, [n.d], [https://astrosociety.org/edu/publications/tnl/24/24.html], Retrieved 31/05/2019

“Event Horizon Telescopes: Press Releases”, Event Horizon Telescopes, [n.d.], [https://eventhorizontelescope.org/latest], Retrieved 31/05/2019