Blackholes

  • We have seen how blackholes are formed in the dying stage of a star’s life cycle.
  • A dying star (leftover core) having more than three times the mass of the Sun undergoes a gravitational collapse, leading to the creation of a black hole.
  • As the star’s matter becomes compressed to enormous densities, the strength of gravity at the surface of the rapidly contracting star also increases dramatically.
  • According to the general theory of relativity, this massive contracting star has a profound effect on the spacetime immediately surrounding the star.
  • While the spacetime surrounding the black hole becomes so highly curved that it closes on itself, blackhole itself is  a place in spacetime where there is collapse of gravity.
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Behaviour of light and matter near a blackhole

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  • Light from the blockhole trying to fly outward will arc back inward stopping the light to escape from it.
  • This is true even with ordinary matter that have much lesser speeds than light.
  • They all disappear in the blackhole.
  • Anything that comes close to a blackhole gets sucked into it. How close? The boundary beyond which this happens is called the event horizon beyond which there is no return for matter or electromagnetic radiation.
  • Further while inside the blackhole gravity collapses, around the blackhole gravity is very strong as a result of which it accretes matter (sucks in) from its surroundings.
  • This matter that is about to fall into a black hole (before crossing event horizon) forms a disk around it, called an accretion disk. 

Common fallacies about blackhole

  • Fallacy 1: A blackhole is so dense that nothing not even light can escape:
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  • It is absurd to say light cannot escape because blackhole is dense. While it is true that a massive core upon contraction becomes a blackhole, it is not because of density that light cannot escape the blackhole. There are super massive blackholes which are formed out of core that are as massive as 3 billion solar mass. But density in such a super-massive blackhole is less than the density of water. This is because blackholes are places where gravity has collapsed. What happens to matter that has contracted is not known to us.
  • Fallacy 2: The gravitational field is so large that light cannot escape:
  • Again the light cannot escape blackhole because light is not able to go out of blackhole as the spacetime around blackhole is so curved that the light arcs back.  

What is a blackhole?

  • Blackholes are just one-way geometric surfaces like mirrors created by gravitational collapse due to high gravitational field. Once you go towards it there is no way you can come out of it. (Not like doors which are 2-way surfaces, you go in and come back)
  • The area surrounding blackholes are massive, high in gravitational field and violent. Inside the blackhole, nobody knows what is going on.
  • There are supermassive blackholes in middle of galaxies including milky way which has a blackhole that is formed out of core of mass equal to 3 billion solar mass.
  • A supermassive black hole is the one whose mass is millions to billion times the mass of the sun.

Event Horizon Telescope

  • In 2019 the 1st ever picture of a black hole was captured by a network of radio telescopes called the Event Horizon Telescope.  
  • This was the image of a supermassive black hole at the centre of an elliptical galaxy in the Virgo cluster.
  • In May 2022 the supermassive blackhole at the centre of Milky way galaxy was captured.

About EHT

  • Event Horizon Telescope is a network of 8 radio telescopes located in Hawaii, Arizona, Chile, Mexico and Spain, and at the South Pole.
  • It is synchronized in such a way in effect they form a radio telescope of the size of the earth itself.
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