Special relativity and E=mc2 led to the most powerful unification of physical concepts since the

time of Newton. The previously separate ideas of space, time, energy and mass were linked by

special relativity, although without a clear understanding of how they were linked.

The how and why remained to the domain of what is called general relativity, a complete theory of

gravity using the geometry of spacetime. The origin of general relativity lies in Einstein's attempt to

apply special relativity in accelerated frames of reference. Remember that the conclusions of

relativity were founded for inertial frames, i.e. ones that move only at a uniform velocity. Adding

acceleration was a complication that took Einstein 10 years to formulate.

Equivalence Principle:

The equivalence principle was Einstein's `Newton's apple' insight to gravitation. His thought

experiment was the following, imagine two elevators, one at rest of the Earth's surface, one

accelerating in space. To an observer inside the elevator (no windows) there is no physical

experiment that he/she could perform to differentiate between the two scenarios.

An immediate consequence of the equivalence principle is that gravity bends light. To visualize

why this is true imagine a photon crossing the elevator accelerating into space. As the photon

crosses the elevator, the floor is accelerated upward and the photon appears to fall downward. The

same must be true in a gravitational field by the equivalence principle.

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