1. Principle of Equivalence: Einstein 1907
Box stationary in gravity field
Box
accelerates
in empty space
Box
falling freely
Box moves through
space at constant velocity

2. Aberration of Light
Moral: direction of light beam is relative

3. Gravitational deflection of Light
Now assume boxes are accelerating
Light path is curved

4. Gravity attracts light!
Light ray curved in accelerating frame
Principle of Equivalence (acceleration = gravity)
Gravity attracts light!

5. Paradox: How can gravity attract light
if light has no mass?

6. MASS-ENERGY EQUIVALENCE
Gravity extracts energy from escaping matter
Gravity extracts energy from escaping light
Gravitational redshift, time dilation
Other points of view same result:
accelerating frames of reference - apply special relativity
spacetime is curved

7. CURVATURE OF SPACETIME
How to tell difference between accelerating frame and gravity?
tidal forces curvature
Eliminates Newton’s “action at a distance”
Freely moving bodies follow “shortest path”
not necessarily a straight line

8. GENERAL RELATIVITY: EINSTEIN 1915
Matter + energy determine curvature of spacetime
Curvature of spacetime determines motion of matter + energy