The UniverseExplainer
Is There a Theory of Everything?
Physicists have merged forces of nature before. Gravity is the holdout, and no candidate theory of everything has yet passed a decisive test.
THE QUESTION LIBRARY THE UNIVERSE
What is a theory of everything, and will we ever have one?
Physics already explains three of nature’s four forces with stunning accuracy, so here is what is missing, who is trying to fill the gap, and why some great minds doubt a final theory exists.
THE SHORT ANSWER
Nobody knows yet. A theory of everything would be one set of rules for all four forces of nature and all particles. Today physics has two great theories that each work very well: the Standard Model for particles, and general relativity for gravity. Low-energy quantum calculations work; a complete, tested theory remains missing. The leading attempts to join them, such as string theory and loop quantum gravity, have no confirmed test yet. Some thinkers, including Stephen Hawking, even wondered if a final theory can ever be written down.
- Nature has four known forces: gravity, electromagnetism, the strong force and the weak force.
- The Standard Model covers three of those forces and has explained almost all experimental results, but it leaves out gravity.
- Some forces have already been joined: electricity and magnetism in the 1860s, and electromagnetism with the weak force in a theory honored by the 1979 Nobel Prize.
- Most ideas about quantum gravity predict new effects only at energies far beyond any machine we can build.
- In 2002, Stephen Hawking said he had come to doubt that a final theory could be written as a finite set of rules.
About this picture
The Standard Model’s particles: twelve matter particles in three generations, the carriers of three forces, and the Higgs boson. Gravity, and its hoped-for carrier the graviton, is not on the chart, which is the gap a theory of everything would fill.
WHERE THE EVIDENCE STANDS
How sure are we?
- The Standard Model describes three of the four forces and nearly all particle experiments with great accuracy.CERN says the Standard Model has explained almost all experimental results and is a well-tested theory; the Higgs boson it needed was found at CERN in 2012 and honored with the 2013 Nobel Prize.
- A complete, experimentally tested theory of quantum gravity remains missing.Low-energy calculations work; their extension to extreme conditions remains unresolved.
- String theory or loop quantum gravity is the correct theory of quantum gravity.The Stanford Encyclopedia of Philosophy reports no real consensus, because concrete predictions are very hard to make and there are no experiments in quantum gravity yet.
- A final theory of everything exists, and people will one day find it.Hawking’s 2002 lecture argued a final theory might not fit in a finite number of statements; that is his reasoned view, not a proof, and it leans on an analogy with Gödel’s theorem, which is a result about mathematics.
WHAT WE DON’T KNOW YET
Nobody knows which, if any, of today’s candidate theories is right, or how gravity behaves at the very smallest scales. Nobody knows whether a final theory exists, or whether every deeper theory will open new questions.
WHAT WOULD CHANGE THIS ANSWER
A candidate theory that predicted a new, surprising measurement, which experiments then found, would change this answer quickly. A clear lab result showing how gravity behaves at the quantum level would also narrow the field.
Nobody knows yet. A “theory of everything” is the dream of one set of rules that explains all the forces of nature and all the basic particles at once. Physics is closer to that dream than ever, but it is not there, and it may never get there.
Here is where things stand. Nature has four known forces. Gravity is the weakest, but it reaches across the whole universe. Electromagnetism, carried by particles of light called photons, also reaches far and is much stronger. The strong and weak forces work only over tiny distances, at the scale of the particles inside atoms. Three of these four are described by the Standard Model, a theory built in the early 1970s. According to CERN, Europe’s particle physics lab, it has explained almost all experimental results in its field.
Gravity is the odd one out. It is described by Einstein’s general relativity, which also works beautifully. It predicted the gravitational waves that detectors first caught in 2015. The trouble is that the two theories speak different languages. Quantum calculations of gravity work at low energies; a complete theory remains unresolved.
People ask about a theory of everything for many reasons. Some want to know why the world is the way it is. Some have heard that string theory already did the job. Some wonder whether a final answer is even possible. These are fair questions, and the honest answer is that the search is still open.
The leading candidates, string theory and loop quantum gravity, are serious math. But they are very hard to test, because the effects they predict are expected to show up only at energies far beyond any machine on Earth. And some thinkers, including Stephen Hawking, came to suspect that there may be no final theory at all. We label that question Open question.
THE LONG ANSWER
THE LONG ANSWER · ABOUT 9 MINUTES
You’ve read your 3 free long answers this month.
The short answer, how sure we are, what we don’t know, what would change our mind and every source stay free on every page, for everyone.
All Access members read the long answer: the evidence piece by piece, the strongest objections, and how to check it yourself.
Join All Access · $1 a week Or $6 a month
Billed $52 once a year. Cancel anytime in your account.
Until December 4, 11:59 pm Pacific: the Founding rate is $39 a year, and it stays $39 while you stay a member.
Create a free account: 6 long answers a month, email only.
Paying never buys a different answer. Our Paywall Promise
WHERE THE AUTHOR’S RESEARCH TOUCHES THIS
Author’s hypothesis. Ricardo Maldonado, author of GOD PLAYS DICE, runs a research program on this question, which the site’s Research Notebook describes on its Unification page. It asks whether one set of ideas could link the laws of the very large with the laws of the very small. Its current public record works on one narrow piece: neutrinos, very light particles, inside a standard textbook setup called the Type-I seesaw. It keeps exact formulas for that setup, and it publishes a self-correction: a proposed cancellation in a harder quantum calculation is now kept only as “conditional algebra.”
The record itself says it does not claim a complete theory of everything or a new prediction confirmed by experiment. The Research Notebook’s status is: math checkpoint with a published self-correction; earlier dated forecasts (January and February 2026) for rare muon events await data. Muons are particles in the same family as electrons, but about 207 times heavier. Those forecasts came from an earlier part of the program, a pattern idea about particle masses, not from the current neutrino record. An idea earns trust when it predicts a new measurement and the prediction holds up. These dated forecasts are that kind of test. Even if experiments see such events, that alone would not single out this idea, because many proposed theories predict them. You can follow the forecasts, and what would count against them, on the Test Ledger.
Keep asking
Did time begin at the Big Bang?
Nobody knows yet.
New here?
God Plays Dice answers big questions in plain words, with sources and one of six honest labels.
Words worth knowing (6)
- Standard Model
- The theory of the basic particles and three of the four forces (electromagnetic, strong and weak). It leaves out gravity.
- General relativity
- Einstein’s theory of gravity, in which mass and energy bend space and time.
- Quantum gravity
- A hoped-for theory that would describe gravity using the rules of quantum physics. No one has a finished, tested version yet.
- String theory
- A candidate theory in which the basic objects are tiny vibrating strings instead of points. One vibration acts like the graviton.
- Loop quantum gravity
- A candidate theory that builds a quantum version of general relativity directly and suggests space comes in tiny separate pieces.
- Symmetry
- In physics, a change that leaves the laws the same. Symmetries, and the ways they can break, guide the search for unified theories.
Sources & further reading
(13) · Checked Sep 30, 2026
- The Standard Model ↗CERNFour fundamental forces and their ranges and strengths; developed in the early 1970s; explained almost all experimental results; well-tested; gravity omitted; no one has made quantum theory and general relativity mathematically compatible; open questions (dark matter, three generations); 4 July 2012 ATLAS and CMS particle; 2013 Nobel to Englert and Higgs; “still incomplete”.
- Unified forces ↗CERNMaxwell developed a single electromagnetic force in the 1860s; links between electromagnetism and the weak force a century later; the weak force normally hides within the atomic nucleus; the two act on equal terms only at high collision energies; a unified force would have ruled the first instants of the universe before its components separated as it cooled; grand unification cannot yet be tested directly.
- The Nobel Prize in Physics 1979 (summary) ↗NobelPrize.orgGlashow, Salam and Weinberg, “for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles”.
- The Nobel Prize in Physics 2008 (summary) ↗NobelPrize.orgNambu, “for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics”; Kobayashi and Maskawa for the origin of the broken symmetry predicting at least three families of quarks.
- Press release: The Nobel Prize in Physics 2017 ↗NobelPrize.orgGravitational waves observed on 14 September 2015 from two colliding black holes, predicted by Einstein a hundred years ago; prize to Weiss, Barish and Thorne for LIGO.
- Supersymmetry ↗CERNSupersymmetry predicts a partner for each Standard Model particle; with it, the three forces’ strengths could match at very high energies; a grand unified theory was a dream of physicists including Einstein.
- Quantum Gravity (Steven Weinstein and Dean Rickles) ↗Stanford Encyclopedia of Philosophy (substantive revision 26 Feb 2024)Quantum gravity still under construction after over 100 years; Planck scale remote from experiment; string theory and loop quantum gravity the two most popular approaches; string theories with fermions need nine space dimensions; discreteness of area and volume often called a prediction of loop quantum gravity but unclear how to observe; landscape of vacua; no real consensus; no experiments in quantum gravity; 10^19 GeV vs 10^3 GeV at Fermilab; galaxy-sized accelerator; coupling 10^-2 vs 10^-22; black hole entropy results (string theory for near-extremal black holes, loop quantum gravity up to an overall constant); Hawking radiation never observed. Also: Dawid (2014), String Theory and the Scientific Method, defends a no-alternatives argument for string theory.
- Gödel and the End of Physics (2002 lecture) ↗Stephen Hawking EstateStandard Model depends on 24 numbers fitted to observation and omits gravity; in general relativity gravity is space and time; early-universe quantum effects seen in the microwave background; Hawking once suggested the ultimate theory might be found soon, then doubted it could be stated in a finite number of statements; comparison with Gödel’s theorem; now glad the search will never end.
- Gödel’s Incompleteness Theorems (Panu Raatikainen) ↗Stanford Encyclopedia of Philosophy (substantive revision 8 Oct 2025)First incompleteness theorem: in any consistent formal system within which a certain amount of arithmetic can be carried out, there are statements that can neither be proved nor disproved; published January 1931; attempted applications in other areas of philosophy are much more controversial.
- Scientific method: Defend the integrity of physics (George Ellis and Joe Silk) ↗Nature 516, 321–323 (16 December 2014)Comment arguing that attempts to exempt speculative theories of the universe from experimental verification undermine science. Only the summary line was read; the full text is paywalled.
- General relativity as an effective field theory: The leading quantum corrections ↗John F. Donoghue; Physical Review D 50 (1994)Low-energy quantum calculations; limitations at high energy.
- Detecting single gravitons with quantum sensing ↗Germain Tobar and colleagues; Nature Communications 15 (2024)Proposed energy-exchange measurement; its quantization-proof limitation.
- Observation of Gravitational Waves from a Binary Black Hole Merger ↗LIGO Scientific Collaboration and Virgo Collaboration; Physical Review Letters 116 (2016)Original gravitational-wave observation.
See a mistake? Report a problem. Corrections are made openly.
CONTINUE THE THREAD
Where this question leads.
The UniverseAnswer
Can a Planet Orbit Two Stars?
Two stars can circle each other while a planet travels around them both. Astronomers confirmed it by watching the moving stars dim.
The UniverseAnswer
What Is the Higgs Field, and Why Call Its Particle the “God Particle”?
An everywhere-present field, a fleeting particle and a famous nickname. What did CERN’s experiments actually measure?