Encyclopedic overview

Theory of Everything

What physics means by a theory of everything, why none exists yet, and which research programmes compete for the role

Definition

A theory of everything (TOE) is a hypothetical physical and mathematical theory that would describe all four fundamental interactions — gravitational, electromagnetic, strong, and weak — within a single framework. As of 2026, no generally accepted theory of everything exists: quantum mechanics and general relativity remain fundamentally incompatible, and none of the proposed candidates has direct experimental confirmation.

Despite the name, a theory of everything is not required to explain literally everything — weather, biology, or economics. The point is a unification of foundations: the known laws of physics should follow from the single theory as limiting cases, just as Newtonian mechanics follows from relativity at low velocities. A candidate is expected to deliver at least three things: a quantum description of gravity, a reproduction of the Standard Model, and testable predictions that distinguish it from its competitors.

Looking for the 2014 film with Eddie Redmayne (The Theory of Everything)?see the film on IMDb. This page is about the physics concept.

Video overview

TL;DR: the problem in brief

  • The problem: the two pillar theories of 20th-century physics are incompatible. Quantum mechanics describes the microworld, general relativity describes gravity and the cosmos, but their mathematical languages contradict each other.

  • The status: no generally accepted theory of everything exists. The Standard Model unified the electromagnetic, weak, and strong interactions — gravity is not part of it.

  • The candidates: string theory / M-theory, loop quantum gravity, causal dynamical triangulations, asymptotic safety, causal set theory; none is experimentally confirmed.

  • The main barrier: quantum gravitational effects appear at the Planck scale (~10⁻³⁵ m), far beyond the reach of direct experiments.

  • A newer direction: several research programmes treat the observer as a fundamental element of physical description. ODTOE's stated goal is to become such a candidate: to derive the known theories as special cases of a single observer-centred metatheory.

Why is there still no theory of everything?

The short answer: the two best theories in physics are incompatible, joining them head-on fails mathematically, and the scale where the dispute would be settled is beyond experimental reach. To be clear, this does not mean physics is in crisis — both theories work flawlessly in their own domains. The problem arises where they must work together: inside black holes, in the first instants of the Big Bang, at Planck scales. Each reason deserves a closer look.

Quantum mechanics and general relativity are incompatible

Quantum mechanics and general relativity are the most precise theories in the history of science, yet they rest on incompatible foundations. Quantum mechanics describes the world probabilistically: the state of a system is a superposition of possibilities, and a measurement yields a random outcome with a computable probability. General relativity, by contrast, is deterministic and geometric: gravity is the curvature of a smooth spacetime, which is itself a dynamical object. When physicists try to apply quantum rules to spacetime itself, the familiar constructions break down: in the canonical approach, the Wheeler–DeWitt equation contains no external time parameter at all (the so-called problem of time). Each theory works superbly in its own domain, but joining them head-on produces mathematical contradictions.

Gravity is non-renormalizable

The second reason is technical but fundamental: gravity is non-renormalizable. In quantum field theory, the infinities that arise in calculations are removed by renormalization — this is how quantum electrodynamics and the whole Standard Model work. With gravity this fails: Newton's constant has negative mass dimension, so ever new types of divergences appear as calculations grow more precise. Removing them would require an infinite number of adjustable parameters, and the theory loses its predictive power. Goroff and Sagnotti demonstrated the two-loop divergence of pure gravity in 1986. This does not mean quantum gravity is impossible — it means the straightforward quantization of Einstein's theory does not work.

The Planck scale is out of reach

The third reason is experimental inaccessibility. Quantum gravitational effects become significant at the Planck length — about 1.6×10⁻³⁵ metres, corresponding to energies of order 10¹⁹ GeV. The Large Hadron Collider reaches roughly 10⁴ GeV — fifteen orders of magnitude short. A collider capable of probing the Planck scale directly by conventional means would need astronomical dimensions. Candidates for a theory of everything therefore rely on indirect tests: imprints of quantum gravity in the cosmic microwave background, possible violations of Lorentz invariance, subtle effects in light propagating from distant gamma-ray bursts. So far, no such signal has been reliably detected.

Open conceptual questions

Finally, there remain questions that mathematics alone cannot settle. What is a measurement, and why does observation play a special role in quantum mechanics? Is spacetime fundamental, or does it emerge from something deeper? Why do the constants of nature have the values they do? Different programmes answer differently: some look for new symmetries and extra dimensions, others for a discrete structure of spacetime, still others for informational and observer-centred foundations of physics. The lack of consensus on these questions is not a weakness of science but a sign that the problem runs deeper than it seemed a century ago.

The history of the search

The idea of a unified description of nature is older than modern physics: Newton unified terrestrial and celestial mechanics, and Maxwell showed that electricity, magnetism, and light are one phenomenon. Each success of unification produced testable consequences: Maxwell's equations predicted electromagnetic waves, and electroweak theory predicted the W and Z bosons, discovered in 1983. That is why the search for a unified theory is not an aesthetic whim but the most productive strategy in the history of physics. Below are the key milestones of the 20th and 21st centuries.

1920s — 1955

Einstein and the unified field theory

After creating general relativity, Albert Einstein devoted the last thirty years of his life — from the early 1920s until his death in 1955 — to the search for a "unified field theory" that would geometrically unite gravity and electromagnetism. He tried dozens of mathematical schemes: teleparallelism, non-symmetric metrics, five-dimensional models. The programme failed for two reasons: Einstein deliberately declined to accept quantum mechanics as a fundamental description, hoping to derive it from field theory, and he lacked the full picture of the strong and weak nuclear interactions, which took shape only by mid-century. Yet the very framing of the problem — seeking a single foundation for all forces — set the agenda of theoretical physics for the century ahead.

1921 / 1926

Kaluza and Klein: the fifth dimension

In 1921 Theodor Kaluza showed that if general relativity is written in a five-dimensional spacetime, Maxwell's equations emerge from it automatically. In 1926 Oskar Klein proposed that the fifth dimension is curled into a circle of Planck size and therefore unobservable. Kaluza–Klein theory did not survive detailed scrutiny: it predicted extra fields and incorrect mass relations. But it gave physics an idea of enormous power — extra dimensions can turn different interactions into different facets of a single geometry. That idea was reborn in string theory, where the extra dimensions number six or seven.

1940s — 2012

The Standard Model: three out of four

In parallel, quantum field theory learned to unify interactions from the bottom up. In the 1940s quantum electrodynamics reached record precision. In the 1960s–70s Glashow, Weinberg, and Salam showed that the electromagnetic and weak interactions are two manifestations of a single electroweak force; quantum chromodynamics described the strong interaction at about the same time. The result was the Standard Model — a quantum theory of three of the four fundamental interactions. The discovery of the Higgs boson in 2012 completed its experimental programme. The Standard Model is the most thoroughly tested theory in history, but gravity is entirely absent from it, and its roughly two dozen free parameters are measured, not explained.

1984 / 1995

The string revolutions

In 1984 Green and Schwarz proved anomaly cancellation in superstring theory, launching the "first superstring revolution": strings came to be seen as a candidate quantum theory of all interactions, gravity included, since the string spectrum automatically contains the graviton. In 1995 Witten initiated the "second revolution" by showing that five apparently different string theories are connected by dualities and appear to be limits of a single eleven-dimensional M-theory. The price of unification is supersymmetry and 10–11 dimensions that must be compactified. The number of compactification choices turned out to be astronomical (a commonly cited estimate is ~10⁵⁰⁰), producing the "landscape problem": the theory admits too many universes to predict ours uniquely.

2026

Where things stand today

As of 2026, the picture is this: the Large Hadron Collider has found no supersymmetric particles in the accessible energy range, weakening the simplest string models though not refuting the programme as a whole. Loop quantum gravity, causal sets, causal dynamical triangulations, and asymptotic safety continue to develop as independent approaches to quantum gravity. Interest is growing in informational and observer-centred foundations of physics — from quantum information and holography to relational interpretations. There is still no generally accepted theory of everything; even the question of what form it should take — an equation, a principle, or a research programme — remains open.

Candidates for a theory of everything

None of the current candidates is experimentally confirmed — the table below reflects the scientific consensus on each approach's key ideas and testability status. A fair comparison rests on three criteria: mathematical consistency, the ability to reproduce known physics (the Standard Model and general relativity in their respective limits), and the presence of testable predictions. Note that some programmes (loop quantum gravity, for instance) address the narrower problem of quantum gravity without claiming to unify all interactions.

ApproachKey ideaWhat it unifiesTestability status
String theory / M-theoryFundamental objects are one-dimensional strings and higher-dimensional branes in 10/11 dimensions; requires supersymmetry.All four interactions: the graviton arises as a vibrational mode of the string.No experimental confirmation; no superpartners found at the LHC; the landscape of ~10⁵⁰⁰ vacua obstructs unique predictions.
Loop quantum gravitySpacetime itself is quantized: geometry is described by spin networks, with discrete spectra of areas and volumes.Gravity with quantum mechanics; unifying all interactions is not the programme's goal.Mathematically developed; predicts discrete geometry at the Planck scale; no direct experimental tests.
Causal Dynamical Triangulations (CDT)Spacetime is assembled from elementary simplices with a fixed causal structure; the dynamics is explored numerically.A quantum theory of gravity; other interactions must be added externally.Simulations reproduce a four-dimensional de Sitter-like universe; no experimental tests.
Asymptotic safetyGravity remains a consistent quantum field theory thanks to a non-trivial ultraviolet fixed point of the renormalization group.Gravity with the framework of quantum field theory; extensions to Standard Model matter are studied.Evidence from the functional renormalization group is computational; no experimental confirmation.
Causal set theorySpacetime is fundamentally discrete: a locally finite, partially ordered set of events.The causal structure of general relativity with quantum discreteness.Sorkin's early estimate anticipated the order of magnitude of the cosmological constant; no systematic experimental tests.
ODTOEThe observer is a fundamental primitive; reality is a configuration actualized by the act of observation: R = Ô(Ψ).Aims to unify all interactions through a single observation operator: the quantum and classical regimes are limits of one description governed by the coherence parameter S; known theories derive as special cases of the metatheory.A research programme built precisely as a theory-of-everything candidate: the formalism is published, the empirical programme is in development.

Data reflects the scientific consensus as of 2026: none of the candidates has direct experimental confirmation.

The observer: a missing element?

The programmes above share a common trait: they seek unification in structure — new symmetries, extra dimensions, discrete geometry. Yet a century of quantum mechanics points to another possible entry point: the act of observation. The measurement problem — why and how a single definite outcome emerges from a superposition of possibilities — remains unsolved. The Copenhagen interpretation, Everett's many-worlds interpretation, QBism, and Rovelli's relational quantum mechanics differ precisely in the role they assign to the observer.

John Archibald Wheeler, one of the great physicists of the 20th century, distilled this intuition into his "it from bit" programme: every element of physical reality derives, in the end, from acts of posing questions and registering binary answers — that is, from observer participation. His delayed-choice thought experiment, later confirmed in the laboratory, showed that what a quantum object "was" in the past depends on the question asked of it in the present. Wheeler spoke of a participatory universe, in which observers are not spectators but participants in the becoming of physical reality.

In the 21st century this line of thought stopped being marginal. Quantum information showed that informational notions — entanglement, the bit, the channel — work as building material for physical theories; the holographic principle tied the geometry of spacetime to information on its boundary; relational quantum mechanics and QBism made the system–observer relation the basic unit of description. Extended Wigner's-friend scenarios, tested experimentally in 2019, sharpened the question of whether facts recorded by different observers can be reconciled. The observer has returned to the foundations of physics — this time as a subject of rigorous analysis.

If this line of reasoning is correct, the difficulties of the classical unification programmes may stem not from a lack of mathematics but from the exclusion of the observer from the description. ODTOE (Observer-Dependent Theory of Everything) is a research programme by Anton Pankratov that makes the opposite move: the observer is introduced as a fundamental primitive rather than a derived object. Its central formula, R = Ô(Ψ), reads: actual reality R is the result of applying the observation operator Ô to the field of potentialities Ψ. The quantum and classical regimes are described as limits of one equation governed by the coherence parameter S.

The positioning matters: ODTOE does not claim that the unification problem is solved. It is one of several contemporary research approaches — with a published formalism (an axiom, six postulates, an operator apparatus), a corpus of ~97 articles, and an empirical programme in development. It should be judged the way any research programme is judged: by internal consistency, explanatory economy, and, ultimately, testable consequences.

ODTOE's goal: to become a theory-of-everything candidate

ODTOE is being built precisely as a candidate for the theory of everything and for the unification of physics. The programme constructs a single formal metatheory from which the known theories — quantum mechanics, general relativity, the Standard Model — are to be derived as special cases. What sets it apart from the classical programmes: unification is sought not in a new symmetry or extra dimensions, but in introducing the observer as a fundamental primitive of physics.

  1. Unify the foundations

    The quantum and classical regimes are described as limits of one equation governed by the coherence parameter S, and gravity and quantum mechanics share a single operator language, R = Ô(Ψ). The incompatibility of the two pillar theories is resolved at the level of a common foundation rather than patched at the boundary.

  2. Derive the constants from first principles

    Instead of measured-but-unexplained parameters — a derivation of fundamental constants from the geometry of self-observation: derivations of the proton-to-electron mass ratio μ ≈ 1836 and the fine-structure constant α⁻¹ ≈ 137 are published, with no adjustable parameters.

  3. Deliver testable consequences

    An empirical programme in development: comparison of derived cosmological fractions with Planck data, predictions for the constants as CODATA values are refined, and an open corpus of ~97 articles exposed to criticism. The programme is prepared to be judged by the same three criteria as any candidate in the table above.

Status as of 2026: a research programme. The formalism is published; the claim to unification is a matter of open verification, not an accomplished fact.

Explore ODTOE

Frequently asked questions about the theory of everything

What is the theory of everything in simple terms?

It is a hypothetical "formula of nature" — a single theory from which all four fundamental interactions would follow: gravity, electromagnetism, and the strong and weak nuclear forces. Physics today uses two incompatible descriptions — quantum mechanics for the microworld and general relativity for gravity. A theory of everything would replace them with one consistent description. It has not been built yet.

Why did Einstein fail to create a theory of everything?

From the 1920s until his death in 1955, Einstein searched for a unified field theory joining gravity and electromagnetism. Two things were missing: he did not accept quantum mechanics as a fundamental description, and he lacked the full picture of the strong and weak interactions, whose physics took shape later. The problem turned out to be broader than the geometric unification of two fields.

How does a theory of everything differ from a unified field theory?

"Unified field theory" is a historical term from Einstein's era: a classical (non-quantum) unification of gravity and electromagnetism through geometry. A theory of everything is a broader goal: a quantum-consistent description of all four interactions, including the strong and weak forces, and ideally an explanation of the constants of nature. Any theory of everything would contain a unified field theory as a special case.

What candidates for a theory of everything exist today?

The main programmes are string theory / M-theory (the most developed), loop quantum gravity, causal dynamical triangulations, asymptotic safety, causal set theory, and observer-centred approaches, including ODTOE. None has direct experimental confirmation; they differ in their starting principles and degree of mathematical maturity.

What is ODTOE?

ODTOE (Observer-Dependent Theory of Everything) is a research programme by Anton Pankratov built as a theory-of-everything candidate: its goal is to derive the known physical theories as special cases of a single observer-centred metatheory. Its central formula is R = Ô(Ψ): reality R arises as the result of applying the observation operator Ô to the field of potentialities Ψ. The formalism is published (~97 articles at odtoe.org); the empirical programme is in development.

Is string theory testable?

Not so far — and that is its central problem. Characteristic string effects appear near the Planck energy (~10¹⁹ GeV), unreachable by accelerators. Indirect hopes — supersymmetric particles, cosmological imprints, microscopic black holes — have not materialized: the LHC found no superpartners. The string programme remains mathematically productive but empirically unconfirmed.

What is the film "The Theory of Everything" about?

It is a 2014 biographical drama about the physicist Stephen Hawking and his first wife Jane, with Eddie Redmayne in the lead role (Academy Award). The film relates only indirectly to the physics concept itself — this page is devoted to the scientific problem of a theory of everything.

Will a theory of everything ever be built?

Unknown. Optimists point to the history of successful unifications — from Maxwell to the Standard Model. Sceptics point to the inaccessibility of the Planck scale and decades without experimental confirmation of any candidate. A revision of foundations may be required: the status of spacetime, the role of information and the observer. The answer will come not from convictions but from consistent mathematics and testable predictions.