- Each one of us carry our own coordinates and our own time. There is no absolute space or absolute time.
- My time is a mixture of your spacetime, your spacetime is a mixture of my space and vice-versa.
- Why did Einstein feel the need to come up with GR?
- The two important principles for deriving GR — equivalence of acceleration and gravity, and; link between space and time.
- Happiest thought of my life: Deriving equivalence between acceleration and gravity.
- Proving that space is warped.
- Proving that time is warped.
- Understanding gravity using an example.
- Einstein’s equations for GR
- Experimental evidence for Einstein’s GR.
- Applications of GR
I. Why did Einstein feel the need to come up with General Relativity (GR)?
II. Two important principles for deriving GR
- Equivalence of acceleration and gravitation: It means that effects observed in accelerated reference frames are applicable in same fashion to reference frames with gravity and vice-versa.
- Link between space and time: As per SR, space and time are interlinked. It means that if you prove that space is warped, then it also means that time is warped and vice-versa.
III. Equivalence of acceleration and gravitation
IV. Space is warped
Clocks tick slower at places with greater gravity.
Gravity warps space.
Gravity warps time.
Gravity hence warps the spacetime.
VI. Understanding gravity warping spacetime, using an example.
VIII. Experimental evidence for GR
- Orbit of Mercury: Orbit of Mercury was slightly off the path calculated using Newtons laws. People thought that probably it’s due to gravity of another mass which we haven’t discovered yet. But Einstein’s equations calculate it precisely.
- Shift in apparent position of star: Light of stars passing from around Sun bend because Sun’s gravity bends the path in which the light travels. Newton also says that light can bend due to attraction but Einstein’s equations show that it bends due to curved spacetime and they bend twice the amount Newton predicted. Experiments proved that the bending of light is in agreement with Einstein’s equations.
- Gravitational waves: Einstein’s GR says that rotating bodies in spacetime send out gravitational waves. We recently detected gravitational waves. The waves detected match with the template of waves constructed using Einstein’s equations for two black bodies.
- Whirlwind in space: When a body rotates in spacetime, it grabs the spacetime around it and twists it, causing the surrounding spacetime to rotate. Gravity Probe B detected such effect.