UPSC Express

Basics: Black holes in theory

General theory of relativity and Black holes

• Einstein completely revolutionized the idea of space and time in his general theory of relativity.
• Accordingly, space and time is not something that is static, monotonous and unchanging as imagined in Newtonian physics.
• The general theory of relativity describes gravity entirely in terms of the geometry of both space and time.
• If large amount of matter is concentrated in a small volume, general theory of relativity predicted the presence of a a black hole.
• The matter in the black hole is so greatly compressed that it violently warps space and time.

 

How are Black holes formed?

• In the last stages of its life cycle, a dying star contracts under its own gravity to become either a white dwarf or a neutron star or a black hole depending on its mass.
• If the mass of the star is less than 1.4 mass of the sun, it contracts into a white dwarf.
• If the mass of the star is more than 1.4 times the mass of the sun it contacts into a neutron star.
• If the mass of the star is more than 2 to 3 times the mass of the sun it contracts into a black hole.

 

What are Black holes?

• 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 gravity has a profound effect on the spacetime immediately surrounding the star.
• The space surrounding the black hole becomes so highly curved that it closes on itself (See figure).
• Photons (light packets) flying outward at an angle from the star’s surface arc back inward, thus light cannot escape from it.
• Ordinary matter that has much lesser speeds than light also disappears in the gravity well.
• Thu,s an object from which neither matter nor electromagnetic radiation can escape is called a black hole

 

 

 

In focus: Capturing Black holes

• Observing black holes are difficult because light cannot escape from inside the black hole.
• The only way to observe black holes are the effects of a black hole’s powerful gravity on surrounding matter and spacetime.
• One key to detecting black holes is to search for the radiation emitted by material as it falls into a hole.

 

Structure of the black hole

The structure of black hole includes

• Gravity wellThe dark hole inside which nothing can be seen.
• Event horizonEvent horizon can be thought of as a ‘surface’ of a black hole.
  • Any matter surrounding the black hole collapses within the event horizon to completely disappear in the black hole.
  • Because this surface is like a horizon beyond which we cannot see any events, it is called event horizon.
  • At the event horizon, the escape velocity for any matter equals speed of light.
  • Therefore in the picture, one can see photons orbiting the black hole.

 

 

A brief analysis of the picture

• Photon sphereThe orbiting photons in the event horizon make up for the light ring around the black hole in the picture.
• X-Ray burstsFurther the disk of matter in the event horizon which is falling into the hole emits regular bursts of X-rays every few seconds.
• Radio-burstsOccasionally the matter in the event horizon emits bursts of radio waves.
• Relativistic JetsFurther from these accretion disks also emerge hot glowing relativistic jets that are formed by strong electric and magnetic fields.
• Singularity or Gravity wellThe dark hole inside which nothing can be seen.

 

 

In brief: Event Horizon Telescope

• 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.
• The EHT has generated petabytes of data including images in the radio and X-Ray frequencies.
• These data have been processed by super computer to arrive at the image that is released.