Traditional View of Gravity from Newton

Gravity:

• an attractive force

• strength determined by distance (r) between 2 masses

• F_g = (Gm₁m₂)/r²

• Inversely proportional to the distance

Einstein’s Theory of General Relativity (GR)

• Properties not covered in Newton’s Theory of Universal Gravitation

• Acceleration & gravitation can’t be distinguished (equivalence principle)

• A: elevator accelerates upwards in 0 gravity

• B: room isn’t accelerating, but is subject to gravity

• Light ‘falls downwards’ in both situations

Takeaway: gravity affects light’s path even though photons are massless & we think of gravity as acting between masses

1st Verification of General Relativity

• Verified that bending of light by mass == bending of space

• Light just follows this curve

• Any particle w/ mass or energy is affected by gravity

• I.e massless photons w/ energy

Application of Einstein’s General Relativity

• NASA’s probe B

• Verified 2 predictions of General Relativity

Geodetic Effect: spacetime warping via gravitational field of massive object (Earth)

Frame Dragging Effect: the amount a rotating object pulls on spacetime

Analogy: “[imagine] Earth spinning in honey. As Earth rotates it drags space and time with it as it would a surrounding sea of honey.”