ABSTRACT
In 1907, two years after proposing the Special Theory of Relativity, Einstein was preparing a review of special relativity when he suddenly wondered how Newtonian gravitation would have to be modified to fit in with special relativity. At this point Einstein had a revelation, an idea that he later described as ‘the most fortunate thought of my life’. This made him conscious of the fact that an observer who is falling from the roof of a house experiences no gravity, and led him to propose his famous equivalence principle which declared acceleration to be equivalent to gravity. More precisely he stated the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame. This extends the principle of relativity to cover the case of uniformly accelerated motion of the reference frame. In simple terms Einstein told us that it is impossible to distinguish between the effects of acceleration and the effects of gravity and by doing so he succeeded in unifying all kinds of motion. It was already known that uniform motion is indistinguishable from rest, for instance, but Einstein also showed that acceleration is no different to being at rest but under the influence of some gravitational field.[14]
The unification of acceleration with gravity was a marriage with great consequences. For instance we are not accustomed to thinking of space as a thing with properties of its own, but it most certainly is. Space has three dimensions and it also has a particular geometry.[14] This tells us what happens to objects like planets, chairs and raccoons as they move through space and time.
