General relativity, Albert Einstein's grand theory of gravity, has stood the test of time more than a century after it was first proposed. Our understanding of gravity has changed as a result of general relativity, which shows gravity as a result of how space and time curve in the presence of mass and energy rather than as an attracting force between enormous objects as was previously believed. From 1919 measurements that proved light bends in the gravitational field of the sun to 2019 observations that showed the silhouette of a black hole, the theory has gained astounding victories. The fact that general relativity is still being developed may surprise some people.

The theory does not provide a straightforward or uniform method of calculating an object's mass, despite the fact that the equations Einstein proposed in 1915 deal with the curvature caused by large objects. Even more difficult to define is the idea of angular momentum, which is a gauge of an object's rotating motion in space-time.

General relativity has a feedback loop, which contributes to some of the problems. The space-time continuum is curved by matter and energy, yet this bending generates energy on its own and can lead to more bending, a phenomena known as the "gravity of gravity." Furthermore, it is impossible to distinguish between an object's inherent mass and the additional energy brought on by this nonlinear effect. Furthermore, without a good understanding of mass, it is impossible to define momentum or angular momentum.

Although he acknowledged the difficulties in defining and measuring mass, Einstein never went into great detail. The first precise definition wasn't put forward until the late 1950s and early 1960s. The mass of an isolated object, such as a black hole, as seen from a nearly infinite distance, when space-time is almost flat and the object's gravitational pull is essentially nil, was defined by physicists Richard Arnowitt, Stanley Deser, and Charles Misner.

This method of determining mass, called "ADM mass" after its creators, has been beneficial, however it does not enable scientists to measure the mass inside a limited space. Imagine, for example, that they are investigating the merger of two black holes and would like to know the mass of each black hole separately rather than the mass of the system as a whole. "Quasilocal mass" refers to the mass contained inside any specific location as measured from that region's surface, where gravity and space-time curvature may be quite high.