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A theory of everything
· Open question
Open questionNobody knows the answer.
A hoped-for single theory describing all four forces and all particles, including gravity, in one quantum framework.
See it on the Science map →How it works
Today physics uses two rulebooks: quantum field theory for particles and three forces, and general relativity for gravity. Each works superbly in its domain, but combined they give nonsense inside black holes and at the Big Bang. Leading candidates are string theory, where particles are tiny vibrating strings, and loop quantum gravity, where space itself is made of discrete pieces. Einstein spent his last 30 years searching without success.
Why it matters
It would explain black hole interiors, the Big Bang's first instant, and possibly why the constants have the values they do.
How we know
None yet. String theory and rivals have produced deep mathematics but no confirmed prediction. Testing them directly may need energies about a million billion times beyond the Large Hadron Collider.
Still a mystery
Whether such a theory exists, whether it is unique, and whether humans could ever test it are all open.
Connections (4)
▸The four forces of nature uniting all four is the goal of A theory of everythingOpen question
The relationship
A theory of everything would show the four forces are faces of one underlying law. There is a precedent: electricity and magnetism were united in the 1860s, and electromagnetism with the weak force in the 1960s. Joining in the strong force, and finally gravity, would complete the pattern. Gravity is the hardest step.
How the link was found
Einstein pursued a unified theory from the 1920s until his death in 1955. Grand unified theories began with Howard Georgi and Sheldon Glashow in 1974.
Is it proven?
Not achieved. Grand unified theories predict protons slowly decay; Japan's Super-Kamiokande detector has found none, showing protons last over 10^34 years.
Open questions and mysteries
Do the forces really merge at high energy? Is there one theory, or many possible ones?
▸The Standard Model leaves out gravity, so it is not A theory of everythingOpen question
The relationship
The Standard Model covers three forces and all known particles, but it has no place for gravity. Adding gravity using the same methods produces infinite answers that cannot be removed. It also leaves out dark matter and dark energy, and has about 19 numbers it cannot explain. A theory of everything would need to contain it as a special case.
How the link was found
The gravity problem was shown in 1974 by 't Hooft and Veltman and confirmed by Marc Goroff and Augusto Sagnotti in 1986.
Is it proven?
Its gaps are clear: it misses gravity and dark matter. No proposed successor has been confirmed.
Open questions and mysteries
Whether the Standard Model is a low-energy approximation of string theory or something else, nobody knows.
▸String theory is a candidate for A theory of everythingSpeculative
The relationship
String theory describes all particles and forces as vibrations of one kind of object. It automatically includes a graviton, the carrier of gravity, alongside patterns resembling the other forces and matter. That makes it a candidate for a single framework covering everything. But it does not yet single out our universe's particular particles and constants from countless alternatives.
How the link was found
Joël Scherk and John Schwarz (1974), and independently Tamiaki Yoneya, saw that strings contain gravity. The 1984 Green–Schwarz result launched it as a unified theory; Edward Witten linked versions into "M-theory" in 1995.
Is it proven?
Unproven. It is mathematically rich and self-consistent in key ways, but has made no confirmed prediction, and no experiment has detected strings or extra dimensions.
Open questions and mysteries
Is there one string theory or many? Can it ever produce a prediction an experiment could check?
▸Gödel's incompleteness theorems may set limits on A theory of everythingPhilosophy
The relationship
Some argue that if a theory of everything is a mathematical system rich enough for arithmetic, Gödel's theorems imply it would contain true statements it cannot prove. Stephen Hawking explored this in a 2002 lecture. Others reply that physics only needs to predict measurements, which may never hit Gödel's statements. This is a philosophical argument, not established physics.
How the link was found
Raised by physicist-priest Stanley Jaki in 1966 and by Stephen Hawking in his 2002 lecture 'Gödel and the End of Physics'.
Is it proven?
Not proven. Gödel's theorems are certain, but whether they bind physics is unsettled. A 2015 result showed one question about quantum materials (the 'spectral gap') is undecidable in general.
Open questions and mysteries
Would undecidable statements ever affect a measurable prediction? Could a final theory be complete in practice even if not in principle?