What actually are blackholes?

Since the inception of general relativity, the solutions to general field equations lead to the prediction of black holes which dismayed even Einstein. During that time, the concept was truly mathematical based and even worried that there is such a spot in the Universe. In the mid of the first world war which broke out in 1914, a German-born physicist and astronomer Karl Schwarzschild while serving in the German army achieved in finding the exact solution to field equation in the year 1915 same year Einstein introduced his General Relativity(GR). He[Schwarzschild] solved for non-rotating black holes using Schwarzschild metrics and coordinates to find out the Schwarzschild radius which is the event horizon of the black holes. 

Fig: Karl Schwarzschild(left), Roy Kerr(right)

Image: wikipedia.org

In 1963, the mathematician Roy Kerr found a solution to Einstein's equations that precisely described space-time outside what we now call a rotating black hole. Since then, with the development of advanced radio telescopes, it is evident that black holes are real not just mathematical ideas. Most indirect evidence of black holes is found such as the gradual change in orbits of stars orbiting around the common invisible mass, and the bending of the light due to intense gravity around some point. The first visible image of blackhole [shadow] was achieved in 2019 by EHT(Event Horizon Telescope) collaboration when they released the first-ever image of the black hole at the centre of the supergiant elliptical galaxy M87, which is in the constellation Virgo. It revealed a bright ring-like structure with a dark central region — the black hole's shadow. In 2022, we finally had an image of a supermassive blackhole located at the heart of the milky way galaxy, or Sgr A* for short by the same team. It is one of the most profound achievements of humankind as this was thought to never be achieved. 

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Fig: (a) Blackhole at centre of galaxy M87(ETH)

Source: https://www.jpl.nasa.gov/edu/news/2019/4/19/how-scientists-captured-the-first-image-of-a-black-hole/

(b) Blackhole at centre of the Milky Way Galaxy(Sagittarius A*)(ETH)

Source: https://www.scientificamerican.com/article/the-first-picture-of-the-black-hole-at-the-milky-ways-heart-has-been-revealed/

Formation of Blackholes

Black holes are of three types: primordial black holes, stellar black holes and supermassive black holes. Primordial black holes are thought to have formed in the early universe, soon after the big bang. Stellar black holes form when the centre of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space. Scientists think supermassive black holes formed at the same time as the galaxy they are in. The size of the supermassive black hole is related to the size and mass of the galaxy it is in. Their size varies from a few solar masses to millions and billions of solar masses. 

Therefore, a black hole is a region in space where the pulling force of gravity is so strong that light is not able to escape. The strong gravity occurs because matter has been pressed into a tiny space. This compression can take place at the end of a star's life. Some black holes are a result of dying stars. Because no light can escape, black holes are invisible. However, space telescopes with special instruments can help find black holes. They can observe the behaviour of material and stars that are very close to black holes. According to GR, it predicts singularity at the centre of black holes which has infinite density. Depending on the mass of the black hole, there is a region around the centre of mass such that once passed has no chance of return called the event horizon. It is at this point, a human endeavour in finding what’s really inside them fails as even light can’t escape[the escape velocity is the speed of light beyond the event horizon].

Fig: Anatomy of Blackhole

Source: https://astronomy.com/magazine/news/2021/02/the-beginning-to-the-end-of-the-universe-how-black-holes-die

Blackhole characteristics

Black holes are weird spots in the universe but not always scary things to explore. The Schwarzschild radius of blackhole like our sun(2x10^{30}Kg) is nearly 3km and Earth (5.97x10^{24}Kg) is 0.88cm.  It means that if the sun is to form a black hole, it should be squeezed to 3km and Earth to 0.88cm. Therefore, at this current size of the sun 695 700Km(at the equator), the mass is insufficient to form so as the Earth(equatorial radius) (6378.1Km).

If we consider our solar system, the escape velocity of the sun at the point tangential to Earth’s orbit is 41.8Km/s, Venus is 49.8Km/s, Mercury is 67.59Km/s and at the surface of the sun is 621.2Km/s. But, if the sun is replaced by a black hole with identical mass, the escape velocity doesn’t change. It means that black holes are ordinary at a considerable distance. It begins to act weird when the distance is very close enough like the event horizon and at the centre where gravity is so strong that light can’t escape. Therefore, black holes exert ordinary gravity at astronomical distances.

Conclusion

Therefore, one should take note that black holes are good for sucking all things up creating a mess but also a fascinating substitute for stars. That is why, I always enjoy hanging on with some mind-blowing kinds of stuff. There need not be a college course or an exam to study when someone finds it interesting to learn.