Albert Einstein's General Theory of Relativity is published

On the recommendation of Italian mathematician Tullio Levi-Civita, Einstein began exploring the usefulness of general covariance (essentially the use of tensors) for his gravitational theory.

For a while Einstein thought that there were problems with the approach, but he later returned to it and, by late 1915, had published his general theory of relativity in the form in which it is used today.[48] This theory explains gravitation as distortion of the structure of spacetime by matter, affecting the inertial motion of other matter. During World War I, the work of Central Powers scientists was available only to Central Powers academics, for national security reasons. Some of Einstein’s work did reach the United Kingdom and the United States through the efforts of the Austrian Paul Ehrenfest and physicists in the Netherlands, especially 1902 Nobel Prize-winner Hendrik Lorentz and Willem de Sitter of Leiden University.

Eleven years after On the Electrodynamics of Moving Bodies, Einstein published his second groundbreaking work on General Relativity, which continues and expands the original theory. A preeminent feature of General Relativity is its view of gravitation. Einstein held that the forces of acceleration and gravity are equivalent. Again, the single premise that General Relativity is based on is surprisingly simple. It states that all physical laws can be formulated so as to be valid for any observer, regardless of the observer's motion. Consequently, due to the equivalence of acceleration and gravitation, in an accelerated reference frame, observations are equivalent to those in a uniform gravitational field.

This led Einstein to redefine the concept of space itself. In contrast to the Euclidean space in which Newton’s laws apply, he proposed that space itself might be curved. The curvature of space, or better spacetime, is due to massive objects in it, such as the sun, which warp space around their gravitational centre. In such a space, the motion of objects can be described in terms of geometry rather than in terms of external forces. For example, a planet orbiting the Sun can be thought of as moving along a "straight" trajectory in a curved space that is bent around the Sun.

On the following pages we will examine spacetime and other fascinating aspects of Relativity in some detail and see how Relativity leads us to new insights about the structure and the creation of the universe.

In 1907, Einstein said that when he "was sitting in a chair in the patent office at Bern when all of a sudden a thought occurred to me: 'If a person falls freely he will not feel his own weight.' I was startled. This simple thought ... impelled me toward a theory of gravitation. ..."

This was the fundamental principle for his General Theory of Relativity, which was published in 1916. Its foundation is that the laws of nature in an accelerating frame are equivalent to the laws of a gravitational field. This is known as the Equivalence Principle. In 1915, Einstein proposed a new theory of gravity, which is now called the General Theory of Relativity:

In 1666, Sir Isaac Newton had proposed a theory of gravity called Newton's Universal Law of Gravitation. Newton's Law had worked very well, but there were slight discrepancies between what was observed and what was mathematically predicted. An example is that Newton's theory cannot explain Mercury's peculiar rosette-shaped elliptical orbit. However, Einstein's General Relativity can.

General Relativity describes gravity as a warping of space itself, not as a force. Einstein pictured space as a three-dimensional version of a thin rubber sheet. If you put a heavy object on the sheet, it makes a dent, and therefore an object's path would be affected by that dent. So, planets orbit the sun because the space around the sun is curved in the 2-D equivalent of a funnel or basin.