D-brane Guide, Meaning , Facts, Information and Description
In theoretical physics, D-branes are a special class of p-branes, named for the physicist Johann Dirichlet. Dirichlet boundary conditions have long been used in the study of fluids and potential theory, where they involve specifying some quantity all along a boundary. In fluid dynamics, fixing a Dirichlet boundary condition could mean assigning a known fluid velocity to all points on a surface; when studying electrostatics, one may establish Dirichlet boundary conditions by fixing the voltage to known values at particular locations, like the surfaces of conductors. In either case, the locations at which values are specified is called a D-brane. These constructions take on special importance in string theory, because open strings must have their endpoints attached to D-branes.D-branes are typically classified by their dimension, which is indicated by a number written after the D. A D0-brane is a single point, a D1-brane is a line, a D2-brane is a plane, and a D25-brane fills the highest-dimensional space considered in bosonic string theory.
| Table of contents |
|
2 See also 3 References |
Most versions of string theory involve two, closely related, types of string: open strings with endpoints and closed strings which connect upon themselves to form closed loops. Exploring the consequences of the Nambu-Goto action, it becomes clear that energy can flow along a string, slipping off the endpoint and vanishing. This poses a problem: conservation of energy dictates that energy should not be able to disappear from the system. Therefore, a consistent string theory must include places into which energy can flow once it leaves a string; these objects are termed D-branes. Any version of string theory which allows open strings must necessarily incorporate D-branes, and all open strings must have their endpoints attached to these branes. To a string theorist, D-branes are physical objects, just as "real" as strings themselves—not just mathematical surfaces where a value is specified.
All elementary particles are expected to be vibrational states of quantum strings, and it is natural to ask if D-branes are somehow "made of" strings themselves. In a sense, this turns out to be true: among the spectrum of particles which the string vibrations allow, we find a type known as a tachyon, which has some odd properties, like imaginary mass. D-branes can be thought of as large collections of tachyons, coherent in a way reminiscent of the photons in a laser beam. Many studies in string theory ignore this viewpoint, for simplicity treating the D-brane as a single object. (In thermodynamics, classroom discussions frequently involve a gas of atoms interacting with a large object, like a piston in a cylinder. Of course, physicists believe that the piston is also made of atoms, but for many problems, it is not necessary to consider all the extra complexity, and they model it as a single, macroscopic object. The case of D-branes is analogous.)
Strings that are constrained by D-branes can be studied by use of renormalizable 2-dimensional quantum field theory.
This has implications for cosmology. Because string theory implies that the Universe has more dimensions than we expect—26 for bosonic string theories and 10 for superstring theories—we have to find a reason why the extra dimensions are not apparent. One possibility would be that the visible Universe is in fact a very large D-brane extending over three spatial dimensions. Material objects, made of open strings, are bound to the D-brane, and cannot move "at right angles to reality" to explore the Universe outside the brane. This scenario is called a brane cosmology. Interestingly, the force of gravity is not due to open strings; the gravitons which carry gravitational forces are vibrational states of closed strings. Because closed strings do not have to be attached to D-branes, gravitational effects could depend upon the extra dimensions at right angles to the brane. (This is a fairly simple braneworld model. More recent innovations now under close study are more intricate, but this discussion reflects some of their spirit.)
This is an Article on D-brane. Page Contains Information, Facts Details or Explanation Guide About D-brane D-branes in string theory
Theoretical background
Braneworld cosmology
Black holes
Another important use of D-branes has been in the study of black holes. D-brane theory allows one to study the quantum states of black holes.See also
References