A theory of everything, 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 theory of everything is one of the major unsolved problems in physics.
Over the past few centuries, two theoretical frameworks have been developed that, together, most closely resemble a theory of everything.
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 that describes the three non-gravitational forces — strong, weak, and electromagnetic force — as well as all observed elementary particles.
Physicists have experimentally confirmed virtually every prediction made by GR and QFT when in their appropriate domains of applicability. Nevertheless, GR and QFT are mutually incompatible – they cannot both be right. Since the usual domains of applicability of GR and QFT are so different, most situations require that only one of the two theories be used.
As it turns out, this incompatibility between GR and QFT is only an issue in regions of extremely small scale – the Planck scale – such as those that exist within a black hole or during the beginning stages of the universe (i.e., the moment immediately following the Big Bang).
To resolve the incompatibility, a theoretical framework revealing a deeper underlying reality, unifying gravity with the other three interactions, must be discovered to harmoniously integrate the realms of GR and QFT into a seamless whole: the theory of everything is a single theory that, in principle, is capable of describing all phenomena in the universe.
In pursuit of this goal, quantum gravity has become one area of active research. One example is string theory, which evolved into a candidate for the theory of everything, but not without drawbacks (most notably, its lack of currently testable predictions) and controversy.
String theory posits that at the beginning of the universe (up to 10−43 seconds after the Big Bang), the four fundamental forces were once a single fundamental force. According to string theory, every particle in the universe, at its most microscopic level (Planck length), consists of varying combinations of vibrating strings (or strands) with preferred patterns of vibration.
String theory further claims that it is through these specific oscillatory patterns of strings that a particle of unique mass and force charge is created (that is to say, the electron is a type of string that vibrates one way, while the up quark is a type of string vibrating another way, and so forth).
Antiquity to 19th century
Ancient Babylonian astronomers studied the movement pattern of heavenly bodies against a background of the celestial sky, with their interest being to relate celestial movement to human events (astrology), and the goal is to predict events by recording events against a time measure and then look for recurrent patterns.
The debate between the universe having either a beginning or eternal cycles can be traced back to ancient Babylonia.
The natural philosophy of atomism appeared in several ancient traditions. In ancient Greek philosophy, the pre-Socratic philosophers speculated that the apparent diversity of observed phenomena was due to a single type of interaction, namely the motions and collisions of atoms.
The concept of ‘atom’ proposed by Democritus was an early philosophical attempt to unify phenomena observed in nature. The concept of ‘atom’ also appeared in the Nyaya-Vaisheshika school of ancient Indian philosophy, and the Ashʿari school of early Islamic philosophy.
Archimedes was possibly the first philosopher to have described nature with axioms (or principles) and then deduce new results from them. Any “theory of everything” is similarly expected to be based on axioms and to deduce all observable phenomena from them.
The scientific method emphasizing precise observation and controlled experimentation were largely developed in the science of the Islamic world, by Arabic alchemists and particularly the Arab physicist Ibn al-Haytham, who proposed that rays of light were streams of tiny particles traveling in straight lines at a finite velocity.
Arabic alchemists proposed the theory of corpuscularianism, where unified sulfur and mercury corpuscles (particles), differing in purity, size, and relative proportions, form the basis of a much more complicated process.
Following earlier atomistic thought, the mechanical philosophy of the 17th century posited that all forces could be ultimately reduced to contact forces between the atoms, then imagined as tiny solid particles.
In the late 17th century, Isaac Newton‘s description of the long-distance force of gravity implied that not all forces in nature result from things coming into contact.
Newton’s work in his Mathematical Principles of Natural Philosophy dealt with this in a further example of unification, in this case unifying Galileo’s work on terrestrial gravity, Kepler‘s laws of planetary motion and the phenomenon of tides by explaining these apparent actions at a distance under one single law: the law of universal gravitation.
In 1814, building on these results, Laplace famously suggested that a sufficiently powerful intellect could, if it knew the position and velocity of every particle at a given time, along with the laws of nature, calculate the position of any particle at any other time:
An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes. — Essai philosophique sur les probabilités, Introduction. 1814
Laplace thus envisaged a combination of gravitation and mechanics as a theory of everything. Modern quantum mechanics implies that uncertainty is inescapable, and thus that Laplace’s vision has to be amended: a theory of everything must include gravitation and quantum mechanics.
In 1820, Hans Christian Ørsted discovered a connection between electricity and magnetism, triggering decades of work that culminated in 1865, in James Clerk Maxwell’s theory of electromagnetism. During the 19th and early 20th centuries, it gradually became apparent that many common examples of forces – contact forces, elasticity, viscosity, friction, and pressure – result from electrical interactions between the smallest particles of matter.
In his experiments of 1849–50, Michael Faraday was the first to search for a unification of gravity with electricity and magnetism. However, he found no connection.
In 1900, David Hilbert published a famous list of mathematical problems. In Hilbert’s sixth problem, he challenged researchers to find an axiomatic basis for all of physics. In this problem, he thus asked for what today would be called a theory of everything.
Early 20th century
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:
“the underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known”.
After 1915, when Albert Einstein published the theory of gravity (general relativity), the search for a unified field theory combining gravity with electromagnetism began with a renewed interest.
In Einstein’s day, the strong and the weak forces had not yet been discovered, yet, he found the potential existence of two other distinct forces -gravity and electromagnetism- far more alluring. This launched his thirty-year voyage in search of the so-called “unified field theory” that he hoped would show that these two forces are really manifestations of one grand underlying principle.
During these last few decades of his life, this quixotic quest isolated Einstein from the mainstream of physics. Understandably, the mainstream was instead far more excited about the newly emerging framework of quantum mechanics. Einstein wrote to a friend in the early 1940s:
“I have become a lonely old chap who is mainly known because he doesn’t wear socks and who is exhibited as a curiosity on special occasions.”
Prominent contributors were Gunnar Nordström, Hermann Weyl, Arthur Eddington, David Hilbert, Theodor Kaluza, Oskar Klein (see Kaluza–Klein theory), and most notably, Albert Einstein and his collaborators. Einstein intensely searched for but ultimately failed to find, a unifying theory.
More than half a century later, Einstein’s dream of discovering a unified theory has become the Holy Grail of modern physics.
Late 20th century and the nuclear interactions
In the twentieth century, the search for a unifying theory was interrupted by the discovery of the strong and weak nuclear forces (or interactions), which differ both from gravity and from electromagnetism.
A further hurdle was the acceptance that in a theory of everything, the theory of everything 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 classical 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 combined in 1967–68 by Sheldon Glashow, Steven Weinberg, and Abdus Salam into the “electroweak” force.
Electroweak unification is a broken symmetry: the electromagnetic and weak forces appear distinct at low energies because the particles carrying the weak force, the W and Z bosons, have non-zero masses of 80.4 GeV/c2 and 91.2 GeV/c2, whereas the photon, which carries the electromagnetic force, is massless. At higher energies, Ws and Zs can be created easily and the unified nature of the force becomes apparent.
While the strong and electroweak forces peacefully coexist in the Standard Model of particle physics, they remain distinct.
So far, the quest for a theory of everything is thus unsuccessful on two points: neither a unification of the strong and electroweak forces – which Laplace would have called ‘contact forces‘ – nor a unification of these forces with gravitation has been achieved.