I would not even be prepared to call string theory a “theory.” Just a hunch. ~ Gerard ‘t Hooft
String theory postulates subatomic particles as infinitesimally thin strings vibrating through a holistic dimensionality (HD) that has more than 4 dimensions.
We still don’t know what string theory is. ~ American particle physicist and string theorist David Gross
The “string” in string theory seems somewhat misleading, as the significance is that particle fields have resonances at different frequencies, harmonically interacting with their brethren. Vibe theory sounds more appropriate.
String theory is 21st-century physics that accidentally found its way into the 20th century. ~ Ed Witten
String theory was presaged by Einstein’s 1907 generalization of photons: a suggestion that solids came as vibrating particles, now termed phonons. Einstein was guessing. The structure of atoms was not discovered until 1911.
Yet Einstein’s phonon serendipity hit the right note. Phonons are relevant to characterizing Bose-Einstein condensate and other exotic thermodynamic phenomena. A phonon is a quasiparticle that represents the excited state which brings electrons together into an entangled Cooper pair.
Phonons were more formally conceptualized by Russian physicist Igor Tamm in 1932 as the particle form of wave/particle fields performing a certain vibration.
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The 1st string theory was proposed in 1926, in the swirl of the quantum revolution, then lost, only to be rediscovered decades later.
Early string theories worked for bosons, leaving fermions out. Bosonic string theory posited 26d. The extra spatial dimensions were necessary to remedy inconsistencies.
In 1968, Italian physicist Gabriele Veneziano was working with the Euler Beta function: an equation used to characterize scattering amplitude. He noticed that it could explain particle reactions involving the strong nuclear force. Others then realized the equation made sense to them when they thought of subatomic particles as connected by little strings, vibrating their very little hearts out.
The concept was controversial. Shortly thereafter, the Standard Model swept aside strings as the great explainer of particle interactions.
But interest in strings abided. Variant models were devised, all constrained to bosonic strings. A major flaw of bosonic string theory was it positing the tachyon: a superluminal particle with imaginary mass.
In 1971, French physicist Pierre Ramond worked a 2d model that included fermions by generalizing the Dirac particle equation into string form. As reality is known to have more than 2 dimensions, Ramond had built a starter kit.
With André Neveu and John Schwarz in tow, Ramond’s concept was extended into a 10d supersymmetry string theory, aka superstring theory.
I think all this superstring stuff is crazy and is in the wrong direction. ~ Richard Feynman
In 1995, American particle theorist Edward Witten, who had been fiddling strings for over a decade, had a vision of unifying the variant quantum field theories. The result was Witten’s M-theory, which postulates 11 dimensions of spacetime: 10 of space and 1 of time. ‘M’ stood for membrane.
M-theory is naturally extensible in the number of dimensions. In M-theory, a single brane string may be a membrane of greater dimensions.
American string theorist Joseph Polchinski and Czech string theorist Petr Horava independently extended M-strings into higher-dimensional objects: D-branes (a Horava term). Among other things, D-brane theory attempts to characterize string endpoints.
D-branes add rich mathematical texture to M-theory, paving the way for constructing more intricate cosmological models with greater explanatory power. Numerous braneworld (brane cosmology) models have emerged.
String theory has been derided by partisans for its lack of track record.
String theory cannot give any definite explanations of existing knowledge of the real world and cannot make any definite predictions. ~ American theoretical physicist Daniel Friedan
One experiment tried to simulate the trillion-Kelvin conditions just after the supposed birth of the universe, by smashing gold ions together at 99.99% of the speed of light. Instead of the expected gaseous plasma, a hot quark soup with liquid-like behavior was produced.
Another experiment confined lithium atoms and cooled them to as cold as practically possible: within 1 x 10–8 K, barely above absolute zero. The behavior was also liquid-like.
Both these liquids at opposite extremes exhibited collective behavior: flowing with the lowest possible viscosity. String theory successfully modeled these phenomena as strongly coupled particles linked by ripples traveling extra- dimensionally. In contrast, the Standard Model cannot account for these stringy liquids.
A new truth always has to contend with many difficulties. If it were not so, it would have been discovered much sooner. ~ Max Planck