The second part of relativity is the theory of general relativity and lies on two empirical findings

that he elevated to the status of basic postulates. The first postulate is the relativity principle: local

physics is governed by the theory of special relativity. The second postulate is the equivalence

principle: there is no way for an observer to distinguish locally between gravity and acceleration.

Einstein discovered that there is a relationship between mass, gravity and spacetime. Mass distorts

spacetime, causing it to curve.

Gravity can be described as motion caused in curved spacetime .

Thus, the primary result from general relativity is that gravitation is a purely geometric

consequence of the properties of spacetime. Special relativity destroyed classical physics view of

absolute space and time, general relativity dismantles the idea that spacetime is described by

Euclidean or plane geometry. In this sense, general relativity is a field theory, relating Newton's

law of gravity to the field nature of spacetime, which can be curved.

Gravity in general relativity is described in terms of curved spacetime. The idea that spacetime is

distorted by motion, as in special relativity, is extended to gravity by the equivalence principle.

Gravity comes from matter, so the presence of matter causes distortions or warps in spacetime.

Matter tells spacetime how to curve, and spacetime tells matter how to move (orbits).

There were two classical test of general relativity, the first was that light should be deflected by

passing close to a massive body. The first opportunity occurred during a total eclipse of the Sun in

1919.

Measurements of stellar positions near the darkened solar limb proved Einstein was right. Direct

confirmation of gravitational lensing was obtained by the Hubble Space Telescope last year.

The second test is that general relativity predicts a time dilation in a gravitational field, so that,

relative to someone outside of the field, clocks (or atomic processes) go slowly. This was

confirmed with atomic clocks flying airplanes in the mid-1970's.

The general theory of relativity is constructed so that its results are approximately the same as those

of Newton's theories as long as the velocities of all bodies interacting with each other

gravitationally are small compared with the speed of light--i.e., as long as the gravitational fields

involved are weak. The latter requirement may be stated roughly in terms of the escape velocity. A

gravitational field is considered strong if the escape velocity approaches the speed of light, weak if

it is much smaller. All gravitational fields encountered in the solar system are weak in this sense.

Notice that at low speeds and weak gravitational fields, general and special relativity reduce to

Newtonian physics, i.e. everyday experience.