A
theory of everything (
ToE) or
final theory,
ultimate theory, or
master theory is a hypothetical single, all-encompassing, coherent
theoretical framework of physics that fully explains and links together all physical aspects of the
universe. Finding a ToE is one of the major
unsolved problems in physics. Over the past few centuries, two theoretical frameworks have been developed that, as a whole, most closely resemble a ToE. These two theories upon which all modern physics rests are
general relativity (GR) and
quantum field theory (QFT). GR is a theoretical framework that only focuses on
gravity for understanding the universe in regions of both large-scale and high-mass: stars, galaxies, clusters of galaxies, etc. On the other hand, QFT is a theoretical framework that only focuses on three non-gravitational forces for understanding the universe in regions of both small scale and low mass: sub-atomic particles, atoms, molecules, etc. QFT successfully implemented the
Standard Model and unified the interactions (so-called
Grand Unified Theory) between the three non-gravitational forces:
weak,
strong, and
electromagnetic force.