The Principle of Equivalence
The Principle of Equivalence expresses the correspondence between two seemingly different situations, represented schematically in the two systems sketched above.
System A sits in a gravitational field (set up, say, by the earth).
An observer in A will see the particle fall and (provided he can look outside and reassure himself that the earth is ‘there’) will be happy to attribute this motion to the gravitational force it experiences.
System B is accelerating (‘upward’) in free space, far from any sources of gravitational forces.
An observer resident in B will also see the particle fall. If he knows where he is (he can look outside and see nothing at all!) he will be forced to assume that his frame is accelerating, and attribute the behaviour to the associated fictitious force.
If the two observers are denied access to information about the world ‘outside’ their laboratories, they would not be able to tell whether their experience should be attributed (as in A) to a ‘real’ gravitational force or (as in B) to a ‘fictitious’ force, arising from their acceleration.
The Principle of Equivalence asserts that this equivalence (of observation and experience) extends beyond mechanical phenomena, to all the phenomena of physics.
It is then just a step to the bending of light in a gravitational field.