At rest in a gravitational field. Notice the inertial frame falling by.
Accelerating in empty space. Notice the inertial frame just sitting there.
The equivalence principal predicts just what Newton would have
predicted, thinking of light as little bullets moving at m/s.
In a thousandth of a second, light travels 300 kilometers and falls a distance
d = (1/2)(10m/s)(0.001s) = 0.000005m
The gravitational slowing of clocks --- usually called the gravitational red-shift --- means that electromagnetic
waves transmitted from the bottom of a tower and received at the top will have a lower frequency when
they are received than when they are sent.
- In 1922, Baron Lorand von Eotvos showed that a variety of different substances all fall toward the Earth with the same acceleration to within a few parts in a billion.
- The acceleration of different substances toward the Sun was checked by Robert H. Dicke and his co-workers in 1964 and found to vary by less than one part in a hundred billion.
- The assertion that inertial frames are actually falling with the same acceleration as freely falling objects was checked in 1960 by Pound and Rebka who showed that gamma rays are subject to the appropriate gravitational redshift (to within one part in a hundred) when they travel up and down a tall tower.