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String (physics)
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| String theory |
|---|
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| Fundamental objects |
| Perturbative theory |
| Non-perturbative results |
| Phenomenology |
| Mathematics |
In physics, a string is a physical entity postulated in string theory and related subjects. Unlike elementary particles, which are zero-dimensional or point-like by definition, strings are one-dimensional extended entities.[1] Researchers often have an interest in string theories because theories in which the fundamental entities are strings rather than point particles automatically have many properties that some physicists expect to hold in a fundamental theory of physics. Most notably, a theory of strings that evolve and interact according to the rules of quantum mechanics will automatically describe quantum gravity.[citation needed]
Overview
[edit] | This article needs additional citations for verification. (April 2024) |
In string theory, the strings may be open (forming a segment with two endpoints) or closed (forming a loop like a circle) and may have other special properties.[2] Prior to 1995, there were five known versions of string theory incorporating the idea of supersymmetry (these five are known as superstring theories) and two versions without supersymmetry known as bosonic string theories, which differed in the type of strings and in other aspects. Today, these different superstring theories are thought to arise as different limiting cases of a single theory called M-theory.
In string theories of particle physics, the strings are very tiny; much smaller than can be observed in today's particle accelerators. The characteristic length scale of strings is typically on the order of the Planck length, about 10−35 meter, the scale at which the effects of quantum gravity are believed to become significant. Therefore on much larger length scales, such as the scales visible in physics laboratories, such entities would appear to be zero-dimensional point particles. Strings are able to vibrate as harmonic oscillators, and different vibrational states of the same string are interpreted as different types of particles. In string theories, strings vibrating at different frequencies constitute the multiple fundamental particles found in the current Standard Model of particle physics. Strings are also sometimes studied in nuclear physics where they are used to model flux tubes.
As the string propagates through spacetime, a string sweeps out a two-dimensional surface called its worldsheet. This is analogous to the one-dimensional worldline traced out by a point particle. The physics of a string is described by means of a two-dimensional conformal field theory associated with the worldsheet. The formalism of two-dimensional conformal field theory also has many applications outside of string theory, for example in condensed matter physics and parts of pure mathematics.
Types of strings
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Closed and open strings
[edit]Strings can be either open or closed. A closed string is a string that has no end-points, and therefore is topologically equivalent to a circle. An open string, on the other hand, has two end-points and is topologically equivalent to a line interval. Not all string theories contain open strings, but every theory must contain closed strings, as interactions between open strings can always result in closed strings.
The oldest superstring theory containing open strings was type I string theory. However, the developments in string theory in the 1990s have shown that the open strings should always be thought of as ending on a new physical degree of freedom called D-branes, and the spectrum of possibilities for open strings has significantly increased.
Open and closed strings are generally associated with characteristic vibrational modes. One of the vibration modes of a closed string can be identified as the graviton. In certain string theories, the lowest-energy vibration of an open string is a tachyon and can undergo tachyon condensation. Other vibrational modes of open strings exhibit the properties of photons and gluons.
Orientation
[edit]Strings can also possess an orientation, which can be thought of as an internal "arrow" that distinguishes the string from one with the opposite orientation. By contrast, an unoriented string is one with no such arrow on it.
See also
[edit]References
[edit]- ^ Becker, Katrin; Schwarz, John H.; Becker, Melanie (2007). String theory and M-theory: a modern introduction. Cambridge New York: Cambridge University Press. p. 6. ISBN 978-0-511-25653-0.
- ^ Polchinski, Joseph Gerard (2004). String theory. Vol. 1: An introduction to the bosonic string / Joseph Polchinski (Reprint ed.). Cambridge: Cambridge Univ. Press. ISBN 978-0521633031.
- Schwarz, John (2000). "Introduction to Superstring Theory". Retrieved Dec. 12, 2005.
- "NOVA's strings homepage"
String (physics)
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In physics, string theory is a theoretical framework that models the fundamental constituents of the universe as one-dimensional "strings" rather than zero-dimensional point particles, with these strings vibrating in multiple spatial dimensions to produce the diverse particles and forces observed in nature.[1][2] These strings, typically on the order of the Planck length (~10^{-33} cm), can be open (with endpoints) or closed (loop-like), and their quantized vibrational modes correspond to different particle properties such as mass, spin, and charge, naturally incorporating both bosons and fermions through supersymmetric extensions.[1][3]
The primary motivation for string theory arises from the need to unify quantum mechanics and general relativity, resolving inconsistencies like the non-renormalizable infinities in perturbative quantum gravity at the Planck scale (~10^{-35} m).[4][1] Unlike the Standard Model of particle physics, which requires 23 free parameters and fails to include gravity, string theory reduces this to essentially one fundamental parameter—the string tension or length scale—while automatically incorporating gravity via a massless spin-2 particle (the graviton) emerging from closed string vibrations.[3][5] Developed initially in the late 1960s as a model for strong interactions via the Veneziano amplitude for hadron scattering, it evolved in the 1970s into a candidate for quantum gravity after the discovery of superstrings, which eliminate problematic tachyons (faster-than-light particles) present in the original bosonic formulation.[2][1]
Key features of string theory include its requirement for extra spatial dimensions beyond the familiar four spacetime dimensions: 26 for the bosonic theory (which lacks fermions) and 10 for the five consistent supersymmetric superstring theories (Type I, Type IIA, Type IIB, and two heterotic variants with gauge groups SO(32) and E₈×E₈).[1][2] These extra dimensions are postulated to be compactified—curled up at tiny scales invisible to current experiments—allowing the theory to reproduce four-dimensional physics at low energies, including the Standard Model's gauge interactions and the Einstein field equations for gravity.[4][1] Non-perturbative aspects, revealed in the 1990s through dualities (such as T-duality, relating theories of different compactification radii, and S-duality, relating strong and weak coupling regimes), demonstrate that the five superstring theories are interconnected facets of a single underlying 11-dimensional framework known as M-theory, which includes extended objects called branes.[2][1] Additionally, the AdS/CFT correspondence (proposed in 1997) provides a holographic duality between string theory in anti-de Sitter space and conformal field theories without gravity, offering insights into quantum gravity and strongly coupled systems like quark-gluon plasmas.[2]
Despite its elegance, string theory faces challenges, including the "landscape" of ~10^{500} possible vacuum states from different compactifications, complicating predictions for our universe, and the lack of direct experimental verification due to the high energy scales involved (far beyond current accelerators like the LHC).[2] Recent advances, such as bootstrap methods confirming consistency in specific limits (as of December 2024) and 2025 developments including calculations suggesting string theory's inevitability as a unified theory and observational evidence from the DESI survey linking it to evolving dark energy models, continue to bolster its mathematical and potential empirical validity, positioning it as a leading candidate for a "theory of everything."[6][7][8] Applications extend to cosmology, where string theory models early universe inflation and black hole microstates (e.g., matching Bekenstein-Hawking entropy via D-brane counting), and to condensed matter physics through dualities describing exotic phases.[2][1]