String theory
String theory explanation including many subtopics around the subject
Introduction
This paper provides an in-depth explanation of string theory and my current understanding of how it works. I will introduce the Standard Model for the fundamental particles, the fundamental forces and their interactions with particles, the principle of String Theory, the role of further dimensions, supersymmetry, the differing types of string theory (M-Theory), and applications of String Theory.
This is a conceptual introduction with limited mathematical detail. Links to further reading and sources appear at the end; it does not present an experimental dataset.
The Standard Model of Particle Physics
The Standard Model is the theoretical framework which governs fundamental particles that make up the universe and the interactions between them. This standard model is a combination of quantum mechanics and Einstein’s theory of special relativity, providing an accurate model of small scale nature. The standard model has been since updated many times and added to.
Standard Model particles are classified as fermions or bosons. Quarks and leptons are fermions. The photon, gluons, and W and Z bosons mediate the Standard Model interactions; the Higgs boson is a scalar excitation of the Higgs field.
The standard model describes three of the four fundamental interactions (forces): the electromagnetic force, the weak nuclear force, and the strong nuclear force. These interactions are mediated by gauge bosons; photons, W/Z bosons, and gluons. Gravity is experimentally well established, but it is not included in the Standard Model. The graviton, a hypothetical quantum particle associated with gravity, has not been detected. LIGO’s detection of gravitational waves is evidence for gravitational phenomena, not a detection of individual gravitons.
Although the standard model remains one of the most successful scientific theories ever developed, it is yet to be completed. These gaps include; dark matter, dark energy, neutrino mass, and matter, anti-matter imbalance throughout the observable universe. Another large gap would be gravity as a fundamental interaction. Despite these limitations, the standard model provides an essential framework which allows us to better understand Higgs Boson and Higgs field.
The Principle of String Theory
Origins and Particle Replacement
String Theory was thought up by the theoretical physicist Gabriele Veneziano. While trying to describe the strong nuclear force using mathematical proofs he unexpectedly discovered a formula which matched the behaviour of strongly interacting particles, called the Veneziano amplitude (Veneziano formula). This formula proved to describe the behaviour one dimensional ‘strings’, rather than particles. I have personally questioned whether this idea could link to the wave particle duality nature of light. These proposed strings have a characteristic length set by the string scale. This is often discussed alongside the Planck length, ℓₚ, approximately 1.6 × 10⁻³⁵ m, although the two scales need not be equal. Planck’s constant, h, is a different quantity, measured in joule-seconds.
Vibrational Modes
In String Theory, strings can vibrate in many different ways, with different quantum states corresponding to different particle types. This is a proposed description of fundamental particles and has not been experimentally confirmed.
Interactions
In String Theory, interactions are described through strings joining and splitting. At low energies, their behaviour can be described by quantum field theories, including a description of gravity. String theory therefore does not simply contradict quantum field theory.
Unification in Quantum Gravity
One of the aims of String Theory is to describe all four fundamental interactions within a single quantum framework. Gravity appears naturally because the theory contains a massless spin-2 state with the properties expected of a graviton. This theoretical state is not an experimentally detected particle.
String Theory provides a framework for studying quantum gravity and the relationship between quantum mechanics and general relativity. It is valuable for investigating unresolved questions, although it has not been experimentally established as the correct description of our universe.
Supersymmetry
Supersymmetry (SUSY) is a proposed symmetry relating fermions and bosons. In common extensions of the Standard Model, particles have superpartners whose spins differ by ½; it is not a claim that every superpartner has spin ±0.5.
Supersymmetry is central to superstring theories. Some string constructions contain tachyonic states, which signal an instability of the chosen background rather than observed particles travelling faster than light. Suitable supersymmetric constructions can avoid these instabilities; supersymmetry itself remains unconfirmed experimentally.
Supersymmetry also provides additional groundwork, such as the hierarchy problem, which talks about why gravity is so much weaker than the other three Fundamental forces, and why the Higgs Boson has a mass so much less than the highest energy predicted by quantum physics.
Searches at the Large Hadron Collider have not established supersymmetric particles. Null results constrain particular models and parameter ranges; they do not demonstrate that all possible forms of supersymmetry are excluded.
The Differing Types of String Theory (M-Theory)
As String Theory developed, physicists discovered that there were five consistent versions. Although each one describes the behaviour of one-dimensional strings, they also differ in the types if strings they contain, their symmetries, and the number of super symmetries they need to work.
Type I String Theory
Type I String Theory consists of both open strings and closed strings. Open strings have two distinct endpoints, whereas closed strings form a loop. Type I String theory predicts a gauge symmetry and naturally links gravity through the existence of closed strings. Type I comes under the SO(32) Gauge group.
Type IIA and IIB String Theory
unlike Type I String Theory, these types contain only closed strings meaning the differ in mathematical properties of their supersymmetry. Type IIA is referred to as a non-chiral theory and Type IIB is referred to as chiral. Both theories exist in ten dimensions and include gravity as a fundamental interaction.
Heterotic SO(32) and HE
These two types of String Theory ‘combine mathematical descriptions for the left-moving and right-moving vibrations of a string’.
Research Uses and Experimental Status
String theory has important uses in theoretical research. These should not be confused with demonstrated everyday technologies, or treated as experimental evidence that the particles in our universe are strings.
One example is the counting of microscopic states for certain theoretical black holes, reproducing their expected entropy under specified assumptions. Another is gauge/gravity duality, which relates particular quantum field theories to gravitational theories. These are research frameworks with defined domains, not universal recipes for quantum computing, medical imaging, energy storage or financial prediction.
Primary research: https://arxiv.org/abs/hep-th/9601029 (opens in a new tab) and https://arxiv.org/abs/hep-th/9711200 (opens in a new tab)
Links/References
Theory of Everything (ToE)
https://physics.stackexchange.com/questions/77663/equation-of-everything (opens in a new tab)
Original background reading on speculative applications (not evidence of established everyday uses)
M-Theory/General Information
https://www.ctc.cam.ac.uk/outreach/origins/quantum_cosmology_four.php (opens in a new tab)
Supersymmetry
https://home.cern/science/physics/supersymmetry/ (opens in a new tab)
M-Theory/General Information
Sources for the scientific clarifications
Standard Model particles and interactions: https://home.cern/science/physics/standard-model/ (opens in a new tab)
Gravitational-wave observation: https://ligo.org/detections/gw150914/ (opens in a new tab)
Planck length and Planck constant (2022 CODATA): https://physics.nist.gov/cuu/Constants/Table/allascii.txt (opens in a new tab)
String-theory framework and particle spectrum: https://www.damtp.cam.ac.uk/user/tong/string.html (opens in a new tab)
Supersymmetry: https://home.cern/science/physics/supersymmetry/ (opens in a new tab)
Was this paper useful?
Loading votes…