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String theory
1984· Theory
SpeculativeA serious idea with no experimental proof yet.
A speculative framework in which every particle is a tiny vibrating string, and one vibration naturally behaves like gravity.
See it on the Science map →How it works
Different vibration patterns of one kind of string would look like different particles, much as one violin string plays many notes. One pattern always matches the graviton, the particle of gravity, so gravity comes built in. The math is only consistent in ten dimensions, so six must be curled up too small to see. It took off in 1984 when Green and Schwarz showed it was free of key inconsistencies.
Why it matters
It is the most developed candidate for quantum gravity and has produced powerful math tools, including the AdS/CFT link between gravity and quantum systems.
How we know
No experimental confirmation. It reproduces black-hole entropy for special cases (1996) and is mathematically consistent. The Large Hadron Collider found no supersymmetry, a feature many versions expected.
Still a mystery
It allows perhaps 10^500 possible universes, so it struggles to make unique, testable predictions. Critics question whether it is science yet.
Connections (8)
▸String theory has a "landscape" of universes, a kind of The multiverseSpeculative
The relationship
String theory's six hidden dimensions can be curled up in an enormous number of ways, perhaps 10^500. Each arrangement gives a different set of particles, forces and vacuum energy, effectively different laws of physics. Combined with eternal inflation, which keeps spawning new bubble universes, each bubble could land on a different arrangement. This collection is called the string "landscape."
How the link was found
Bousso and Polchinski (2000) and Kachru, Kallosh, Linde and Trivedi (2003) argued that many stable arrangements exist; Leonard Susskind named it the "landscape" in 2003.
Is it proven?
Not proven. The count is an estimate, the stability of these solutions is contested, and other universes cannot, as far as we know, be observed.
Open questions and mysteries
Do these solutions really exist? The "swampland" debate since 2018 questions it. Can a theory that predicts everything be tested?
▸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?
▸Quantum gravity: the missing link has a candidate in String theorySpeculative
The relationship
String theory replaces point particles with tiny strings. One vibration of a closed string behaves exactly like the graviton, so gravity appears automatically. Because strings have size, they smear out the point-like collisions that create infinities when gravity is quantized naively. The theory appears to give finite answers where the straightforward approach fails, making it the most studied quantum-gravity candidate.
How the link was found
Scherk and Schwarz, and Yoneya, in 1974 noticed strings contain a graviton; Green and Schwarz's 1984 work made the theory consistent.
Is it proven?
No experimental confirmation. Evidence is mathematical: finite calculations, the 1996 black-hole entropy match, and AdS/CFT. Its main successes are in idealized settings.
Open questions and mysteries
It is best defined for universes unlike ours, with negative vacuum energy. How to apply it to our expanding universe is open.
▸String theory requires Extra dimensionsSpeculative
The relationship
String theory's math only works if space-time has ten dimensions (eleven in M-theory). Otherwise the quantum version of a vibrating string produces impossible results, like probabilities that do not add up. Since we see three of space and one of time, the other six must be curled into shapes far too small to notice, which then set the particles and forces we observe.
How the link was found
Claud Lovelace (1971) found the original string needs 26 dimensions; superstrings need 10. Candelas, Horowitz, Strominger and Witten (1985) proposed the six curled into "Calabi–Yau" shapes.
Is it proven?
The requirement is a mathematical result. Experiments show no sign of extra dimensions: gravity behaves normally down to about 50 micrometers and colliders see no leakage.
Open questions and mysteries
Which of countless possible shapes describes our universe, and why it stays stable, are unknown.
▸String theory led to AdS/CFT: a working hologram (in the math)Well supported
The relationship
String theory contains membranes called D-branes. Maldacena studied a stack of them two ways: as particles moving on the branes, with no gravity, and as strings bending space around the branes, with gravity. Since both describe one system, he concluded the gravity theory and the gravity-free particle theory must be equivalent: AdS/CFT.
How the link was found
Joseph Polchinski identified D-branes in 1995. Juan Maldacena made the leap in late 1997. Steven Gubser, Igor Klebanov, Alexander Polyakov and Edward Witten supplied the translation rules in 1998.
Is it proven?
Thousands of calculations agree on both sides, including symmetry matches and black-hole entropy counts. Within string theory the evidence is strong, though no complete mathematical proof exists.
Open questions and mysteries
Whether the duality needs string theory at all, or is a broader fact about gravity, is open.
▸String theory counted the microstates behind Black-hole entropy: information on the surfaceWell supported
The relationship
Black-hole entropy implies a black hole has a huge number of hidden internal states. Andrew Strominger and Cumrun Vafa built a special black hole in string theory out of string-theory objects called D-branes, then counted the ways they could be arranged. The count exactly matched Bekenstein and Hawking's one quarter of the horizon area.
How the link was found
Andrew Strominger and Cumrun Vafa, 1996, using the D-branes discovered by Joseph Polchinski in 1995.
Is it proven?
The match is exact and has been repeated for many related cases, a strong sign of consistency. It works only for idealized, perfectly balanced black holes, not the spinning ones astronomers observe.
Open questions and mysteries
Can the count be done for real astrophysical black holes, and what do these microstates look like from outside?
▸Leonard Susskind co-founded String theorySpeculative
The relationship
In 1968 Gabriele Veneziano found a formula that fit how strongly interacting particles scatter. Susskind realized the formula described tiny vibrating strings rather than points. That reinterpretation turned a curve-fitting formula into a physical picture, which later grew into string theory as a candidate theory of everything.
How the link was found
Leonard Susskind, and independently Yoichiro Nambu and Holger Bech Nielsen, around 1969-70. Susskind's first paper was initially rejected by a journal.
Is it proven?
The historical link is documented in the original papers. String theory itself has never been confirmed by experiment.
Open questions and mysteries
Whether string theory describes nature remains open. It ultimately failed for the strong force but survived as a candidate theory of gravity.
▸String theory inspired the colliding-branes version of Cyclic universes (ekpyrotic and Penrose's CCC)Speculative
The relationship
In one form of string theory, M-theory, our three-dimensional space can be a membrane, or brane, floating in a higher-dimensional space beside a parallel brane. A spring-like force pulls the two together. When they collide, the energy of their motion turns into hot matter and radiation: a big bang. They then separate, and the cycle repeats.
How the link was found
Petr Hořava and Edward Witten described the two-brane setup in 1996. Khoury, Ovrut, Steinhardt and Turok built the ekpyrotic universe on it in 2001.
Is it proven?
Unproven, as is string theory itself. Renata Kallosh, Lev Kofman and Andrei Linde argued in 2001 that the model needs as much fine-tuning as it removes. Later cyclic models (Ijjas and Steinhardt, 2019) dropped the branes.
Open questions and mysteries
What happens at the moment of collision, when the extra dimension briefly shrinks to zero, is not well understood.