A theory of everything (TOE) is a hypothetical theory of theoretical physics that fully explains and links together all known physical phenomena. Initially, the term was used with an ironic connotation to refer to various overgeneralized theories.

There have been numerous theories of everything proposed by theoretical physicists over the last century, but as yet none has been able to stand up to experimental scrutiny, there being tremendous difficulty in getting the theories to produce experimentally testable results. The primary problem in producing a TOE is that the accepted theories of quantum mechanics and general relativity propose radically different descriptions of the universe: straightforward ways of combining the two lead quickly to the renormalization problem, in which the theory does not give finite results for experimentally testable quantities. Lastly, a number of physicists do not expect a TOE to be discovered.

November 2025 - Physicists are still chasing the dream of Albert Einstein and Stephen Hawking to capture the workings of the entire universe in a single equation. The Theory of Everything is an overarching hypothetical framework that would explain the physics of the entire universe in a single equation. But unifying theories that define the large-scale cosmological structure of the universe with those that describe the minuscule quantum world of the subatomic particles has been a challenge for over a century.

Although equations that describe the universe on the largest and smallest scales have become more precise over the decades, they still don't unite to provide a complete picture of the physical world. Hawking himself had given up on the search for a Theory of Everything before his death in 2018.

String theory: Scientists are trying new ways to verify the idea that could unite all of physics   PhysOrg - November 13, 2025

String theory unifies all the forces of nature. Forces that seem very different, such as gravity and electricity, are deeply related to one another. The forces are linked by so-called dualities: the same underlying phenomena can be described in different ways.

Although this is usually cited as a statement of determinism, a "single formula" may still exist even if physics is fundamentally probabilistic, as taught by modern quantum mechanics.

Since ancient greek times, philosophers have speculated that the apparent diversity of appearances conceals an underlying unity, and thus that the list of forces might be short, indeed might contain only a single entry. For example, the mechanical philosophy of the 17th Century posited that all forces could be ultimately reduced to contact forces between tiny solid particles.

This was abandoned after the acceptance of Newton's long-distance force of gravity; but at the same time Newton's work in his Principia provided the first dramatic empirical evidence for the unification of apparently distinct forces: Galileo's work on terrestrial gravity, Kepler's laws of planetary motion, and the phenomenon of tides were all quantitatively explained by a single law of universal gravitation.

In 1820 Hans Christian Oersted discovered a connection between electricity and magnetism, triggering decades of work that culminated in James Clark Maxwell's theory of electromagnetism.

Also during the 19th and early 20th Centuries it gradually became apparent that many common examples of forces - contact forces, elasticity, viscosity, friction, pressure - resulted from electrical interactions between the smallest particles of matter.

In the late 1920s the new quantum mechanics showed that the chemical bonds between atoms were examples of (quantum) electrical forces, justifying Dirac's boast that "[t]he underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known".

Attempts to unify gravity with electromagnetism date back at least to Michael Faraday's experiments of 1849Ð50. After Einstein's theory of gravity (general relativity) was published in 1915, the search for a unified field theory combining gravity with electromagnetism began in earnest. At the time it seemed plausible that no other fundamental forces existed.

Prominent contributors were Gunnar Nordstrom, Hermann Weyl, Arthur Eddington, Theodor Kaluza, Oskar Klein, and most notably, many attempts by Einstein and his collaborators. None of these proposals were successful.

The search was interrupted by the discovery of the strong and weak nuclear forces, which could not be subsumed into either gravity or electromagnetism. A further hurdle was the acceptance that quantum mechanics had to be incorporated from the start, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped.

Gravity and electromagnetism could always peacefully coexist as entries in a list of Newtonian forces, but for many years it seemed that gravity could not even be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason work on unification for much of the twentieth century focused on understanding the three "quantum" forces: electromagnetism and the weak and strong forces. The first two were unified in 1967-1968 by Sheldon Glashow, Steven Weinberg, and Abdus Salam.

The strong and electroweak forces peacefully coexist in the standard model of particle physics, but remain distinct. Several Grand Unified Theories (GUTs) have been proposed to unify them. Although the simplest GUTs have been experimentally ruled out, the general idea, especially when linked with supersymmetry, remains strongly favored by the theoretical physics community.

In current mainstream physics, a Theory of Everything would unify all the fundamental interactions of nature, which are usually considered to be four in number: gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force. Because the weak force can transform elementary particles from one kind into another, the TOE should yield a deep understanding of the various different kinds of particles as well as the different forces. The expected pattern of theories is: