1968
The first formulaGabriele Veneziano writes an amplitude for colliding hadrons that nobody can derive from particles. It is the first string formula, found before anyone knew it was about strings.
INSPIREString theory · The most-travelled road to door seven, quantum gravity
Everything is a vibrating string.
One idea, pushed as far as it will go: the smallest things are not points but tiny loops and strands of energy, and every particle is one note that such a string can play. Pull on the idea and gravity falls out of it uninvited, the world needs ten dimensions, and five different theories turn out to be one. Fifty-eight years after the first equation, nobody has measured a string. This room holds what the theory says, what it has delivered, what it has not, and where to learn it at no cost. The links go to the original papers.
The one picture A string, its notes, and why a note is a particle
A guitar string of fixed length can only vibrate in whole numbers of half-waves: one hump, two, three. Each pattern is a note. String theory says a fundamental string is the same, except that each of its notes shows up to us as a different particle, with a different mass and spin. Press a mode below and watch the string change its note. The drawing is a cartoon: the real notes are quantum states of a string in ten dimensions, but the logic is the same. Mode 2 is the one that matters most: in every version of the theory a closed string vibrating this way has exactly the properties of the graviton, the particle that would carry gravity. That is why the theory is on this site at all; it is the only reason anyone kept working on it after 1974 (Scherk and Schwarz, 1974; Yoneya, 1974).
One hump. The string’s lowest note. In the simplest bosonic version this note comes out with imaginary mass, the tachyon, which is one reason the real theory needs supersymmetry.
How small From you to the string, on one ruler
Each step on this ruler is ten times smaller than the last. A string is thought to sit near the Planck length, 1.616 × 10−35 metres (CODATA, NIST). A proton, the smallest thing most people have heard of, has a charge radius of 8.4 × 10−16 metres (CODATA, NIST). The gap between those two numbers, nineteen steps of ten, is the whole problem: no machine reaches anywhere near it, which is why the Large Hadron Collider was never expected to see a string directly.
Five theories that were one The 1995 map
By 1985 there were five consistent superstring theories and no way to choose. In 1994 Hull and Townsend argued the five were linked by dualities, exact dictionaries between theories that look nothing alike (arXiv hep-th/9410167). In 1995 Witten showed they were five corners of one eleven-dimensional theory he called M-theory, whose low-energy limit is eleven-dimensional supergravity: Type IIA at strong coupling grows an eleventh dimension, the E₈×E₈ heterotic string does the same on an interval, and Type IIB is its own strong-coupling dual (arXiv hep-th/9503124; see also Hořava and Witten). The same year Polchinski found that the theory contains higher-dimensional objects, D-branes, that strings can end on (arXiv hep-th/9510017). Nobody has written down what M-theory is in full; the map below is the shape of what is known.
The record Fifty-eight years, in the papers themselves
1969
It is a stringNambu, Nielsen and Susskind each see that Veneziano’s formula is what you get from a vibrating one-dimensional object.
Nambu · Koba and Nielsen · Susskind1971
Fermions, and superstringsRamond puts fermions on the string; Neveu and Schwarz build the other half. Supersymmetry is born inside string theory, years before anyone used it for particles.
Ramond · Neveu and Schwarz1974
Gravity falls outThe theory had failed as a theory of hadrons. Scherk, Schwarz and, independently, Yoneya notice that its massless spin-2 note is a graviton, and propose it as a theory of everything instead.
Scherk and Schwarz · Yoneya1976
The GSO projectionGliozzi, Scherk and Olive remove the tachyon and make the spectrum supersymmetric. The superstring becomes consistent.
INSPIRE1984
The first revolutionGreen and Schwarz show the anomalies cancel for gauge group SO(32). The rush into string theory begins.
INSPIRE1985
Heterotic strings and Calabi–YauGross, Harvey, Martinec and Rohm build the heterotic string; Candelas, Horowitz, Strominger and Witten curl the six extra dimensions into Calabi–Yau shapes to get something like our world.
Heterotic · Vacuum configurations1995
The second revolutionWitten unifies the five theories as M-theory; Polchinski finds D-branes.
Witten · Polchinski · Hořava and Witten1996
Black hole entropy, countedStrominger and Vafa reproduce the Bekenstein–Hawking entropy of a black hole by counting string states. The first quantitative success of any quantum theory of gravity.
arXiv1997
HolographyMaldacena’s AdS/CFT: a string theory with gravity in a curved space equals a quantum field theory without gravity on its boundary. On INSPIRE it is the most cited theory paper in high-energy physics, with more than 22,000 citations.
Maldacena · GKP · Witten2000
Too many universesBousso and Polchinski count the ways to thread fluxes through the extra dimensions and find a vast number of possible vacua. The landscape begins.
arXiv2003
KKLT and the landscapeKachru, Kallosh, Linde and Trivedi construct vacua with positive energy like ours; Susskind names the landscape and argues for anthropic reasoning. The field splits over it.
KKLT · Susskind2005
The swamplandVafa asks which low-energy theories cannot come from string theory at all. The answer, if it can be made sharp, would be a prediction.
Vafa · Palti’s review2018
Trouble with de SitterObied, Ooguri, Spodyneiko and Vafa conjecture that stable universes with positive energy, like the one we measure, may not exist in string theory. Still contested.
arXiv2026
Where it standsQuanta asks working theorists whether strings are still the best hope for a theory of everything, and gets every answer.
QuantaWhat it has delivered, and what it has not Both columns are true at once
Delivered
- A quantum theory in which gravity is not optional: the graviton is a vibration mode, and the theory is finite order by order where general relativity alone is not (1974).
- The first microscopic count of black-hole entropy that matches Hawking’s formula (Strominger and Vafa).
- Holography: gravity in a volume equals a quantum theory on its surface (Maldacena), now a working tool far outside string theory.
- The unification of five theories and supergravity into one framework (Witten), and D-branes (Polchinski).
- Supersymmetry itself, which began on the string in 1971 (Ramond).
Not delivered
- A single experimental test. No measurement has ever required a string to explain it.
- Supersymmetric particles at the LHC. The Particle Data Group’s 2025 review records the absence of any experimental evidence for supersymmetry so far (PDG). Low-energy supersymmetry is not required by string theory, but its absence removed the most hoped-for signal.
- A unique prediction. The landscape of vacua is enormous (Bousso and Polchinski; Susskind), and whether our universe’s positive energy fits in it at all is disputed (de Sitter swampland).
- A definition of M-theory. The central node on the map above is a name for a theory nobody has written down.
- The Yang–Mills mass gap, door four on this site, which the Clay Institute still lists as open (Clay); string dualities inspired approaches but not a proof.
Learn it, free From a first lecture to the full course
Susskind, Stanford: lecture 1 of 10Play
Brian Greene, TED: 19 minutesPlay
David Tong’s lecture notes, CambridgeThe free textbook most graduate students actually read first; the full PDF is also on arXiv.
MIT 8.251, String Theory for UndergraduatesBarton Zwiebach’s course on OpenCourseWare: syllabus, notes, problem sets, free.
The state of the field, 2026Quanta Magazine’s long report, with the critics and the defenders in their own words.
Door seven on this siteQuantum gravity among the thirteen locked doors, with its facts, stories and the newest papers.
