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.