Since special relativity only dealt with inertial frames of reference, many physicists believed that there was a more general form of special relativity. So, the hunt for general relativity began. In 1907, Einstein began with a concept known as the equivalence principle, believing that there is no physical distinction between accelerating in free space and the force inflicted by gravity. For example, if an observer is in an accelerating box in space, free of gravitational fields, there is no experiment that the observer can do to prove that they are accelerating in free space or to prove if they are in the presence of a gravitational field.

Einstein proposed that space-time, where time is a pseudo-dimension of space is curved, meaning it can be represented by matrices and geometric equations. But since tensors, a mathematical expression of matrix algebra, represent spatial dimensions, the dimension-like time was introduced. This also was due to time seemingly being dependent on space, and so was considered another term of space, hence space-time. After many years, in 1915, Einstein devised the Einstein Field Equations relating the curvature of space-time with the mass, energy, and momentum within the space around it. Space-time can be expressed as a mathematical equation.

Where R𝜇𝜈 is the Ricci curvature tensor, R is the scalar curvature, g𝜇𝜈 is the metric tensor, Λ is the cosmological constant, G is Newton's gravitational constant, c is the speed of light in vacuum, T𝜇𝜈 is the stress–energy tensor.