# Nature of Gravity: General Theory Of Relativity

## What Is General Theory Of Relativity?

• The General Theory of Relativity, proposed by Einstein a decade after his Special Theory of Relativity, revolutionized our understanding of gravity by describing it as a curvature of spacetime rather than a force that pulls things.”
• It is a unique theory of gravitation that identifies gravitation with the curvature of spacetime.
• It describes how matter and energy warp spacetime, which in turn affects the motion of objects within it.
• To understand the fabric of spacetime visualize a rubber-sheet. The rubber sheet is flat when there is no mass in it, but has a dent when a heavy ball is placed curving the rubber sheet. (just an analogy and not a perfect representation)
• According to general relativity theory gravitational effects are felt only because of the curvature of the space-time.
• In short, according to general theory of relativity matter interacts with spacetime by curving it and the effect is what we experience as gravity.
• Therefore all gravitational effects can be simulated by the rubber sheet, and this is why light must bend as it moves in this curved spacetime.
• In short, Einstein focused on motion, rather than force, in thinking about gravity.
• Further Einstein gave Gravity-Acceleration Equivalence principle according to which gravity is akin to acceleration.
• Thus, what you feel as downward pull of gravity can be duplicated by an upward acceleration of the observer. In short, you can create gravity by acceleration.

### Predictions of General Theory of Relativity

• Bending of light: Because light is travelling in spacetime it should bend when it is passing through a curvature of spacetime.
• Gravitational redshift of light: Light moving from a place strong gravity to a place of weak gravity is red-shifted. (this is very important to know the presence of blockholes, dark matter etc)
• Precession of orbit of mercury: The perihelion (point closest to sun) of the orbit of mercury changes.
• Expansion of universe: General theory of relativity was consistent with the idea expanding universe. However, Einstein was not convinced with the idea of expanding universe. In his equations he introduced a Cosmological Constant as a counter to expanding universe which he called his greatest blunder. (it is relevant in our understanding of the mysterious dark energy)
• Interestingly today it is the cosmological constant that is used to explain the accelerated expansion of the universe by dark energy

### Fruits of relativity

• The theory has contributed to the understanding of black holes, gravitational waves (1050 times weaker than electromagnetic waves), and the structure of the universe.
• Accordingly in the recent times we have observed the presence of supermassive black holes at the centers of galaxies and detected gravitational waves from merging black holes and neutron stars etc.

### Life story of a star: Red giant, White dwarf, the Chandrasekhara limit, Neutron star, Pulsar, Blackhole and Quasar

• Stars are born from clouds of gas and dust that are scattered throughout the universe. These clouds begin to contract under the pull of gravity. As the internal pressure builds and the temperature rises, hydrogen fusion begins and the star is born.
• In the core of the star, hydrogen nuclear fusion leads to the formation of helium.
• When all the hydrogen in the core is consumed on fusion reaction a star evolves into a red giant.
• Hydrogen depletion and core contraction: This is because as hydrogen is depleted, the core contracts due to gravity, causing the region surrounding it to become hot to trigger fusion reactions in the outer shell.
• The heat generated in the outer shell pushes the gases outside expanding rapidly and emitting red light. This is a red giant.
• The left over inner core (where fusion has come to a halt) becomes a white dwarf. This may be regarded as death of a star.
• The above story is true for stars whose core has a mass  within the limit of 1.44 solar mass. This is called Chandrasekhara limit.
• Stars within the Chandrasekhara limit become white dwarf at death.
• A core of a star having a mass more than 1.44 times that of the Sun becomes a neutron star or black hole at death.
• Heavier stars become a super red giant which blows off outer core violently in a supernova explosion. The left over core becomes a neutron star.
• This is true for stars with a core that have 1.5-3 solar masses. They are called neutron stars as temperature become so high that the protons and electrons in the interior of these stars combine to form neutrons.  This is because strong nuclear force overcomes electromagnetic force. This leaves behind neutrons in the gaseous form.
• The collapse of the core causes the neutron star to spin rapidly. This rapidly spinning neutron star emits radiation periodically. This spinning neutron stars are called pulsars.
• On the other hand if the mass of the core is more than 3 solar masses the core collapses under its weight resulting in the formation of a blackhole.
• The spinning blackhole gives out radiation similar to neutron star. This spinning blackhole is called a quasar.