Theory of Gravitation

• Albert Einstein's happiest thought of his life


  Envisioning a person falling from the roof of a building and becoming weightless in free fall, which refers to Einstein's Principle of Equivalence
• Einstein's Principle of Equivalence states that gravity is equivalent to a uniformly accelerated reference frame and served as the backbone of his General Theory of Relativity
• General Relativity


  Deals with non-inertial or accelerating frames of reference, where Einstein did not consider gravity as a force but a product of the warping or curving of space-time
• Space-time


  A four-dimensional continuum composed of the three dimensional space and one dimension of time
• Warping or curving of space-time


  1. Can be simulated with the use of a fabric

  2. Rolling a ball across the fabric makes it move in a straight line

  3. Placing a heavier ball or object at the center distorts the fabric

  4. Rolling a ball again will make it follow a curved path around the heavier ball at the middle
• The fabric analogy helps us visualize the warping of space-time, where space-time is like a fabric that can be deformed
• General Relativity explains the warping of space-time quantitatively, defining how the mass and energy of a cosmic body determines the shape of the space-time
• Perihelion


  A point in a planet's orbit closest to the sun
• Aphelion


  The point farthest from the sun
• According to Newtonian Gravitation, Mercury's perihelion advances by about 531 seconds of arc (arcsec) per century due to the motion of other planets
• However, in 19th century the actual advancement is 574 arcsec per century.
• This discrepancy led to Einstein's explanation of perihelion shift as a result of the warping of space-time, with Mercury being the most affected by the sun's warping of space-time
• Gravitational lensing


  Light emitted by distant stars or galaxies bends following the curvature of space-time created by a massive object in the universe
• Arthur S. Eddington, a British astronomer, conducted two expeditions in 1919 to measure the gravitational deflection of light passing near the sun, following Einstein's suggestion during a solar eclipse
• The expeditions were successful, revealing the stars near the sun were indeed displaced
• Black hole


  An extremely dense collapsed star that not even light can escape from its gravitational field
• German astronomer, Karl Schwarzschild, was the first to use the general relativity in predicting the existence of black holes
• A dense neutron star has an escape velocity equal to the speed of light, meaning not even light can escape its gravitational pull