Misplaced Pages

#100899

95-597: Its primary areas of research was astrophysics and solar system physics . A great deal of the research was concentrated on X-ray coming from astronomical objects . DRI have had X-ray equipment on board the Russian Satellite Granat and the European EURECA satellite. The now concluded Danish Small Satellite Program , which resulted in the aforementioned Oersted ( Ørsted ) satellite, was headed by DRI. The DRI space missions were supervised by

190-401: A brane is a physical object that generalizes the notion of a point particle to higher dimensions. For instance, a point particle can be viewed as a brane of dimension zero, while a string can be viewed as a brane of dimension one. It is also possible to consider higher-dimensional branes. In dimension p , these are called p -branes. The word brane comes from the word "membrane" which refers to

285-453: A quantum field theory . One of the challenges of string theory is that the full theory does not have a satisfactory definition in all circumstances. Another issue is that the theory is thought to describe an enormous landscape of possible universes , which has complicated efforts to develop theories of particle physics based on string theory. These issues have led some in the community to criticize these approaches to physics, and to question

380-613: A D-brane. The letter "D" in D-brane refers to a certain mathematical condition on the system known as the Dirichlet boundary condition . The study of D-branes in string theory has led to important results such as the AdS/CFT correspondence, which has shed light on many problems in quantum field theory. Branes are frequently studied from a purely mathematical point of view, and they are described as objects of certain categories , such as

475-448: A collection of weakly interacting particles in a completely different theory. Roughly speaking, a collection of particles is said to be strongly interacting if they combine and decay often and weakly interacting if they do so infrequently. Type I string theory turns out to be equivalent by S-duality to the SO (32) heterotic string theory. Similarly, type IIB string theory is related to itself in

570-529: A connection called supersymmetry between bosons and the class of particles called fermions . Five consistent versions of superstring theory were developed before it was conjectured in the mid-1990s that they were all different limiting cases of a single theory in eleven dimensions known as M-theory . In late 1997, theorists discovered an important relationship called the anti-de Sitter/conformal field theory correspondence (AdS/CFT correspondence), which relates string theory to another type of physical theory called

665-963: A four-dimensional (4D) spacetime . In this framework, the phenomenon of gravity is viewed as a consequence of the geometry of spacetime. In spite of the fact that the Universe is well described by 4D spacetime, there are several reasons why physicists consider theories in other dimensions. In some cases, by modeling spacetime in a different number of dimensions, a theory becomes more mathematically tractable, and one can perform calculations and gain general insights more easily. There are also situations where theories in two or three spacetime dimensions are useful for describing phenomena in condensed matter physics. Finally, there exist scenarios in which there could actually be more than 4D of spacetime which have nonetheless managed to escape detection. String theories require extra dimensions of spacetime for their mathematical consistency. In bosonic string theory, spacetime

760-416: A given version of string theory, there is only one kind of string, which may look like a small loop or segment of ordinary string, and it can vibrate in different ways. On distance scales larger than the string scale, a string will look just like an ordinary particle consistent with non-string models of elementary particles, with its mass , charge , and other properties determined by the vibrational state of

855-625: A group of ten associate editors from Europe and the United States, established The Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics . It was intended that the journal would fill the gap between journals in astronomy and physics, providing a venue for publication of articles on astronomical applications of the spectroscope; on laboratory research closely allied to astronomical physics, including wavelength determinations of metallic and gaseous spectra and experiments on radiation and absorption; on theories of

950-508: A guide to understanding of other stars. The topic of how stars change, or stellar evolution, is often modeled by placing the varieties of star types in their respective positions on the Hertzsprung–Russell diagram , which can be viewed as representing the state of a stellar object, from birth to destruction. Theoretical astrophysicists use a wide variety of tools which include analytical models (for example, polytropes to approximate

1045-749: A link between matrix models and M-theory on the one hand, and noncommutative geometry on the other hand. It quickly led to the discovery of other important links between noncommutative geometry and various physical theories. In general relativity, a black hole is defined as a region of spacetime in which the gravitational field is so strong that no particle or radiation can escape. In the currently accepted models of stellar evolution, black holes are thought to arise when massive stars undergo gravitational collapse , and many galaxies are thought to contain supermassive black holes at their centers. Black holes are also important for theoretical reasons, as they present profound challenges for theorists attempting to understand

SECTION 10

#1732779987101

1140-463: A millisecond timescale ( millisecond pulsars ) or combine years of data ( pulsar deceleration studies). The information obtained from these different timescales is very different. The study of the Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Understanding the Sun serves as

1235-561: A model or help in choosing between several alternate or conflicting models. Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model. Topics studied by theoretical astrophysicists include stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in

1330-410: A mystery why there was not just one consistent formulation. However, as physicists began to examine string theory more closely, they realized that these theories are related in intricate and nontrivial ways. They found that a system of strongly interacting strings can, in some cases, be viewed as a system of weakly interacting strings. This phenomenon is known as S-duality. It was studied by Ashoke Sen in

1425-457: A natural explanation for the weakness of gravity compared to the other fundamental forces. A notable fact about string theory is that the different versions of the theory all turn out to be related in highly nontrivial ways. One of the relationships that can exist between different string theories is called S-duality . This is a relationship that says that a collection of strongly interacting particles in one theory can, in some cases, be viewed as

1520-422: A nontrivial way by S-duality. Another relationship between different string theories is T-duality . Here one considers strings propagating around a circular extra dimension. T-duality states that a string propagating around a circle of radius R is equivalent to a string propagating around a circle of radius 1/ R in the sense that all observable quantities in one description are identified with quantities in

1615-410: A number of these dualities between different versions of string theory, and this has led to the conjecture that all consistent versions of string theory are subsumed in a single framework known as M-theory . Studies of string theory have also yielded a number of results on the nature of black holes and the gravitational interaction. There are certain paradoxes that arise when one attempts to understand

1710-404: A particular compactification of eleven-dimensional supergravity with one of the dimensions curled up into a circle. In this setting, one can imagine the membrane wrapping around the circular dimension. If the radius of the circle is sufficiently small, then this membrane looks just like a string in ten-dimensional spacetime. Duff and his collaborators showed that this construction reproduces exactly

1805-404: A precise definition of entropy as the natural logarithm of the number of different states of the molecules (also called microstates ) that give rise to the same macroscopic features. In the twentieth century, physicists began to apply the same concepts to black holes. In most systems such as gases, the entropy scales with the volume. In the 1970s, the physicist Jacob Bekenstein suggested that

1900-499: A small group of physicists were examining the possible applications of higher dimensional objects. In 1987, Eric Bergshoeff, Ergin Sezgin, and Paul Townsend showed that eleven-dimensional supergravity includes two-dimensional branes. Intuitively, these objects look like sheets or membranes propagating through the eleven-dimensional spacetime. Shortly after this discovery, Michael Duff , Paul Howe, Takeo Inami, and Kellogg Stelle considered

1995-542: A substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter , dark energy , black holes , and other celestial bodies ; and the origin and ultimate fate of the universe . Topics also studied by theoretical astrophysicists include Solar System formation and evolution ; stellar dynamics and evolution ; galaxy formation and evolution ; magnetohydrodynamics ; large-scale structure of matter in

SECTION 20

#1732779987101

2090-460: A theoretical idea called supersymmetry . In theories with supersymmetry, each boson has a counterpart which is a fermion, and vice versa. There are several versions of superstring theory: type I , type IIA , type IIB , and two flavors of heterotic string theory ( SO (32) and E 8 × E 8 ). The different theories allow different types of strings, and the particles that arise at low energies exhibit different symmetries . For example,

2185-598: A two-dimensional brane. Branes are dynamical objects which can propagate through spacetime according to the rules of quantum mechanics. They have mass and can have other attributes such as charge. A p -brane sweeps out a ( p +1)-dimensional volume in spacetime called its worldvolume . Physicists often study fields analogous to the electromagnetic field which live on the worldvolume of a brane. In string theory, D-branes are an important class of branes that arise when one considers open strings. As an open string propagates through spacetime, its endpoints are required to lie on

2280-415: A unified description of gravity and particle physics, it is a candidate for a theory of everything , a self-contained mathematical model that describes all fundamental forces and forms of matter . Despite much work on these problems, it is not known to what extent string theory describes the real world or how much freedom the theory allows in the choice of its details. String theory was first studied in

2375-458: Is 26-dimensional, while in superstring theory it is 10-dimensional, and in M-theory it is 11-dimensional. In order to describe real physical phenomena using string theory, one must therefore imagine scenarios in which these extra dimensions would not be observed in experiments. Compactification is one way of modifying the number of dimensions in a physical theory. In compactification, some of

2470-457: Is a broad and varied subject that attempts to address a number of deep questions of fundamental physics . String theory has contributed a number of advances to mathematical physics , which have been applied to a variety of problems in black hole physics, early universe cosmology , nuclear physics , and condensed matter physics , and it has stimulated a number of major developments in pure mathematics . Because string theory potentially provides

2565-514: Is a generalization of ordinary geometry in which mathematicians define new geometric notions using tools from noncommutative algebra . In a paper from 1998, Alain Connes , Michael R. Douglas , and Albert Schwarz showed that some aspects of matrix models and M-theory are described by a noncommutative quantum field theory , a special kind of physical theory in which spacetime is described mathematically using noncommutative geometry. This established

2660-465: Is an example of a duality that relates string theory to a quantum field theory. If two theories are related by a duality, it means that one theory can be transformed in some way so that it ends up looking just like the other theory. The two theories are then said to be dual to one another under the transformation. Put differently, the two theories are mathematically different descriptions of the same phenomena. In string theory and other related theories,

2755-455: Is assumed to be on the order of the Planck length , or 10 meters, the scale at which the effects of quantum gravity are believed to become significant. On much larger length scales, such as the scales visible in physics laboratories, such objects would be indistinguishable from zero-dimensional point particles, and the vibrational state of the string would determine the type of particle. One of

2850-561: Is equal to a given mass and charge for the resulting black hole. Their calculation reproduced the Bekenstein–Hawking formula exactly, including the factor of 1/4 . Subsequent work by Strominger, Vafa, and others refined the original calculations and gave the precise values of the "quantum corrections" needed to describe very small black holes. The black holes that Strominger and Vafa considered in their original work were quite different from real astrophysical black holes. One difference

2945-488: Is equivalent to type IIB string theory via T-duality, and the two versions of heterotic string theory are also related by T-duality. In general, the term duality refers to a situation where two seemingly different physical systems turn out to be equivalent in a nontrivial way. Two theories related by a duality need not be string theories. For example, Montonen–Olive duality is an example of an S-duality relationship between quantum field theories. The AdS/CFT correspondence

Danish Space Research Institute - Misplaced Pages Continue

3040-408: Is formulated within the framework of classical physics , whereas the other fundamental forces are described within the framework of quantum mechanics. A quantum theory of gravity is needed in order to reconcile general relativity with the principles of quantum mechanics, but difficulties arise when one attempts to apply the usual prescriptions of quantum theory to the force of gravity. In addition to

3135-635: Is mainly concerned with finding out the measurable implications of physical models . It is the practice of observing celestial objects by using telescopes and other astronomical apparatus. Most astrophysical observations are made using the electromagnetic spectrum . Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A few gravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect. Neutrino observatories have also been built, primarily to study

3230-461: Is the speed of light , k is the Boltzmann constant , ħ is the reduced Planck constant , G is Newton's constant , and A is the surface area of the event horizon. Like any physical system, a black hole has an entropy defined in terms of the number of different microstates that lead to the same macroscopic features. The Bekenstein–Hawking entropy formula gives the expected value of

3325-405: Is to find a solution of the theory that reproduces the observed spectrum of elementary particles, with a small cosmological constant , containing dark matter and a plausible mechanism for cosmic inflation . While there has been progress toward these goals, it is not known to what extent string theory describes the real world or how much freedom the theory allows in the choice of details. One of

3420-658: The Danish Space Board . On 1 January 2005 the DRI and the geodesy part of Kort & Matrikelstyrelsen merged to form the Danish National Space Center . The DRI prepared instruments/components for: As of 2010, the Ørsted is still in orbit and returning data. Astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of

3515-516: The Lambda-CDM model , are the Big Bang , cosmic inflation , dark matter, dark energy and fundamental theories of physics. The roots of astrophysics can be found in the seventeenth century emergence of a unified physics, in which the same laws applied to the celestial and terrestrial realms. There were scientists who were qualified in both physics and astronomy who laid the firm foundation for

3610-619: The cosmic microwave background . Emissions from these objects are examined across all parts of the electromagnetic spectrum , and the properties examined include luminosity , density , temperature , and chemical composition. Because astrophysics is a very broad subject, astrophysicists apply concepts and methods from many disciplines of physics, including classical mechanics , electromagnetism , statistical mechanics , thermodynamics , quantum mechanics , relativity , nuclear and particle physics , and atomic and molecular physics . In practice, modern astronomical research often involves

3705-654: The derived category of coherent sheaves on a complex algebraic variety , or the Fukaya category of a symplectic manifold . The connection between the physical notion of a brane and the mathematical notion of a category has led to important mathematical insights in the fields of algebraic and symplectic geometry and representation theory . Prior to 1995, theorists believed that there were five consistent versions of superstring theory (type I, type IIA, type IIB, and two versions of heterotic string theory). This understanding changed in 1995 when Edward Witten suggested that

3800-408: The first superstring revolution in 1984, many physicists turned to string theory as a unified theory of particle physics and quantum gravity. Unlike supergravity theory, string theory was able to accommodate the chirality of the standard model, and it provided a theory of gravity consistent with quantum effects. Another feature of string theory that many physicists were drawn to in the 1980s and 1990s

3895-408: The 17th century, natural philosophers such as Galileo , Descartes , and Newton began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the same natural laws . Their challenge was that the tools had not yet been invented with which to prove these assertions. For much of the nineteenth century, astronomical research was focused on

Danish Space Research Institute - Misplaced Pages Continue

3990-460: The Austrian physicist Ludwig Boltzmann , who showed that the thermodynamic properties of a gas could be derived from the combined properties of its many constituent molecules . Boltzmann argued that by averaging the behaviors of all the different molecules in a gas, one can understand macroscopic properties such as volume, temperature, and pressure. In addition, this perspective led him to give

4085-503: The BFSS matrix model is exactly equivalent to M-theory. The BFSS matrix model can therefore be used as a prototype for a correct formulation of M-theory and a tool for investigating the properties of M-theory in a relatively simple setting. The development of the matrix model formulation of M-theory has led physicists to consider various connections between string theory and a branch of mathematics called noncommutative geometry . This subject

4180-477: The Sun, Moon, planets, comets, meteors, and nebulae; and on instrumentation for telescopes and laboratories. Around 1920, following the discovery of the Hertzsprung–Russell diagram still used as the basis for classifying stars and their evolution, Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars , in his paper The Internal Constitution of the Stars . At that time,

4275-430: The Sun. Cosmic rays consisting of very high-energy particles can be observed hitting the Earth's atmosphere. Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available, spanning centuries or millennia . On the other hand, radio observations may look at events on

4370-493: The basis for our understanding of elementary particles, which are modeled as excitations in the fundamental fields. In quantum field theory, one typically computes the probabilities of various physical events using the techniques of perturbation theory . Developed by Richard Feynman and others in the first half of the twentieth century, perturbative quantum field theory uses special diagrams called Feynman diagrams to organize computations. One imagines that these diagrams depict

4465-494: The behaviors of a star) and computational numerical simulations . Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen. Theorists in astrophysics endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute

4560-408: The challenges of string theory is that the full theory does not have a satisfactory definition in all circumstances. The scattering of strings is most straightforwardly defined using the techniques of perturbation theory , but it is not known in general how to define string theory nonperturbatively . It is also not clear whether there is any principle by which string theory selects its vacuum state ,

4655-409: The compact extra dimensions must be shaped like a Calabi–Yau manifold . A Calabi–Yau manifold is a special space which is typically taken to be six-dimensional in applications to string theory. It is named after mathematicians Eugenio Calabi and Shing-Tung Yau . Another approach to reducing the number of dimensions is the so-called brane-world scenario. In this approach, physicists assume that

4750-419: The context of heterotic strings in four dimensions and by Chris Hull and Paul Townsend in the context of the type IIB theory. Theorists also found that different string theories may be related by T-duality. This duality implies that strings propagating on completely different spacetime geometries may be physically equivalent. At around the same time, as many physicists were studying the properties of strings,

4845-510: The current science of astrophysics. In modern times, students continue to be drawn to astrophysics due to its popularization by the Royal Astronomical Society and notable educators such as prominent professors Lawrence Krauss , Subrahmanyan Chandrasekhar , Stephen Hawking , Hubert Reeves , Carl Sagan and Patrick Moore . The efforts of the early, late, and present scientists continue to attract young people to study

SECTION 50

#1732779987101

4940-425: The dual description. For example, a string has momentum as it propagates around a circle, and it can also wind around the circle one or more times. The number of times the string winds around a circle is called the winding number . If a string has momentum p and winding number n in one description, it will have momentum n and winding number p in the dual description. For example, type IIA string theory

5035-461: The eleven-dimensional theory is a supermembrane theory but there are some reasons to doubt that interpretation, we will non-committally call it the M-theory, leaving to the future the relation of M to membranes." In the absence of an understanding of the true meaning and structure of M-theory, Witten has suggested that the M should stand for "magic", "mystery", or "membrane" according to taste, and

5130-443: The end of the 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves through optical, x-ray, and gamma wavelengths. In the 21st century, it further expanded to include observations based on gravitational waves . Observational astronomy is a division of the astronomical science that is concerned with recording and interpreting data, in contrast with theoretical astrophysics , which

5225-402: The entropy of a black hole is instead proportional to the surface area of its event horizon , the boundary beyond which matter and radiation are lost to its gravitational attraction. When combined with ideas of the physicist Stephen Hawking , Bekenstein's work yielded a precise formula for the entropy of a black hole. The Bekenstein–Hawking formula expresses the entropy S as where c

5320-479: The entropy of a black hole, but by the 1990s, physicists still lacked a derivation of this formula by counting microstates in a theory of quantum gravity. Finding such a derivation of this formula was considered an important test of the viability of any theory of quantum gravity such as string theory. In a paper from 1996, Andrew Strominger and Cumrun Vafa showed how to derive the Bekenstein–Hawking formula for certain black holes in string theory. Their calculation

5415-449: The extra dimensions are assumed to "close up" on themselves to form circles. In the limit where these curled up dimensions become very small, one obtains a theory in which spacetime has effectively a lower number of dimensions. A standard analogy for this is to consider a multidimensional object such as a garden hose. If the hose is viewed from a sufficient distance, it appears to have only one dimension, its length. However, as one approaches

5510-591: The five theories were just special limiting cases of an eleven-dimensional theory called M-theory. Witten's conjecture was based on the work of a number of other physicists, including Ashoke Sen , Chris Hull , Paul Townsend , and Michael Duff . His announcement led to a flurry of research activity now known as the second superstring revolution . In the 1970s, many physicists became interested in supergravity theories, which combine general relativity with supersymmetry. Whereas general relativity makes sense in any number of dimensions, supergravity places an upper limit on

5605-451: The founders of the discipline, James Keeler , said, astrophysics "seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space– what they are, rather than where they are", which is studied in celestial mechanics . Among the subjects studied are the Sun ( solar physics ), other stars , galaxies , extrasolar planets , the interstellar medium and

5700-472: The framework of quantum mechanics. One important example of a matrix model is the BFSS matrix model proposed by Tom Banks , Willy Fischler , Stephen Shenker , and Leonard Susskind in 1997. This theory describes the behavior of a set of nine large matrices. In their original paper, these authors showed, among other things, that the low energy limit of this matrix model is described by eleven-dimensional supergravity. These calculations led them to propose that

5795-579: The history and science of astrophysics. The television sitcom show The Big Bang Theory popularized the field of astrophysics with the general public, and featured some well known scientists like Stephen Hawking and Neil deGrasse Tyson . String theory In physics , string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings . String theory describes how these strings propagate through space and interact with each other. On distance scales larger than

SECTION 60

#1732779987101

5890-424: The hose, one discovers that it contains a second dimension, its circumference. Thus, an ant crawling on the surface of the hose would move in two dimensions. Compactification can be used to construct models in which spacetime is effectively four-dimensional. However, not every way of compactifying the extra dimensions produces a model with the right properties to describe nature. In a viable model of particle physics,

5985-487: The late 1960s as a theory of the strong nuclear force , before being abandoned in favor of quantum chromodynamics . Subsequently, it was realized that the very properties that made string theory unsuitable as a theory of nuclear physics made it a promising candidate for a quantum theory of gravity. The earliest version of string theory, bosonic string theory , incorporated only the class of particles known as bosons . It later developed into superstring theory , which posits

6080-443: The micro-level. By the late 1970s, these two frameworks had proven to be sufficient to explain most of the observed features of the universe , from elementary particles to atoms to the evolution of stars and the universe as a whole. In spite of these successes, there are still many problems that remain to be solved. One of the deepest problems in modern physics is the problem of quantum gravity . The general theory of relativity

6175-505: The months following Witten's announcement, hundreds of new papers appeared on the Internet confirming different parts of his proposal. Today this flurry of work is known as the second superstring revolution. Initially, some physicists suggested that the new theory was a fundamental theory of membranes, but Witten was skeptical of the role of membranes in the theory. In a paper from 1996, Hořava and Witten wrote "As it has been proposed that

6270-597: The number of dimensions. In 1978, work by Werner Nahm showed that the maximum spacetime dimension in which one can formulate a consistent supersymmetric theory is eleven. In the same year, Eugene Cremmer , Bernard Julia , and Joël Scherk of the École Normale Supérieure showed that supergravity not only permits up to eleven dimensions but is in fact most elegant in this maximal number of dimensions. Initially, many physicists hoped that by compactifying eleven-dimensional supergravity , it might be possible to construct realistic models of our four-dimensional world. The hope

6365-442: The observable universe is a four-dimensional subspace of a higher dimensional space. In such models, the force-carrying bosons of particle physics arise from open strings with endpoints attached to the four-dimensional subspace, while gravity arises from closed strings propagating through the larger ambient space. This idea plays an important role in attempts to develop models of real-world physics based on string theory, and it provides

6460-434: The one-dimensional diagram representing the path of a point particle by a two-dimensional (2D) surface representing the motion of a string. Unlike in quantum field theory, string theory does not have a full non-perturbative definition, so many of the theoretical questions that physicists would like to answer remain out of reach. In theories of particle physics based on string theory, the characteristic length scale of strings

6555-422: The paths of point-like particles and their interactions. The starting point for string theory is the idea that the point-like particles of quantum field theory can also be modeled as one-dimensional objects called strings. The interaction of strings is most straightforwardly defined by generalizing the perturbation theory used in ordinary quantum field theory. At the level of Feynman diagrams, this means replacing

6650-457: The physical state that determines the properties of our universe. These problems have led some in the community to criticize these approaches to the unification of physics and question the value of continued research on these problems. The application of quantum mechanics to physical objects such as the electromagnetic field , which are extended in space and time, is known as quantum field theory . In particle physics, quantum field theories form

6745-455: The physicist, Gustav Kirchhoff , and the chemist, Robert Bunsen , had demonstrated that the dark lines in the solar spectrum corresponded to bright lines in the spectra of known gases, specific lines corresponding to unique chemical elements . Kirchhoff deduced that the dark lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere. In this way it

6840-546: The problem of developing a consistent theory of quantum gravity, there are many other fundamental problems in the physics of atomic nuclei , black holes , and the early universe. String theory is a theoretical framework that attempts to address these questions and many others. The starting point for string theory is the idea that the point-like particles of particle physics can also be modeled as one-dimensional objects called strings . String theory describes how strings propagate through space and interact with each other. In

6935-408: The quantum aspects of black holes, and work on string theory has attempted to clarify these issues. In late 1997 this line of work culminated in the discovery of the anti-de Sitter/conformal field theory correspondence or AdS/CFT. This is a theoretical result that relates string theory to other physical theories which are better understood theoretically. The AdS/CFT correspondence has implications for

7030-401: The quantum aspects of gravity. String theory has proved to be an important tool for investigating the theoretical properties of black holes because it provides a framework in which theorists can study their thermodynamics . In the branch of physics called statistical mechanics , entropy is a measure of the randomness or disorder of a physical system. This concept was studied in the 1870s by

7125-405: The routine work of measuring the positions and computing the motions of astronomical objects. A new astronomy, soon to be called astrophysics, began to emerge when William Hyde Wollaston and Joseph von Fraunhofer independently discovered that, when decomposing the light from the Sun, a multitude of dark lines (regions where there was less or no light) were observed in the spectrum . By 1860

7220-487: The solar spectrum with any known elements. He thus claimed the line represented a new element, which was called helium , after the Greek Helios , the Sun personified. In 1885, Edward C. Pickering undertook an ambitious program of stellar spectral classification at Harvard College Observatory , in which a team of woman computers , notably Williamina Fleming , Antonia Maury , and Annie Jump Cannon , classified

7315-627: The source of stellar energy was a complete mystery; Eddington correctly speculated that the source was fusion of hydrogen into helium, liberating enormous energy according to Einstein's equation E = mc . This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed of hydrogen (see metallicity ), had not yet been discovered. In 1925 Cecilia Helena Payne (later Cecilia Payne-Gaposchkin ) wrote an influential doctoral dissertation at Radcliffe College , in which she applied Saha's ionization theory to stellar atmospheres to relate

7410-516: The spectra recorded on photographic plates. By 1890, a catalog of over 10,000 stars had been prepared that grouped them into thirteen spectral types. Following Pickering's vision, by 1924 Cannon expanded the catalog to nine volumes and over a quarter of a million stars, developing the Harvard Classification Scheme which was accepted for worldwide use in 1922. In 1895, George Ellery Hale and James E. Keeler , along with

7505-408: The spectral classes to the temperature of stars. Most significantly, she discovered that hydrogen and helium were the principal components of stars, not the composition of Earth. Despite Eddington's suggestion, discovery was so unexpected that her dissertation readers (including Russell ) convinced her to modify the conclusion before publication. However, later research confirmed her discovery. By

7600-405: The string scale, a string looks just like an ordinary particle, with its mass , charge , and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton , a quantum mechanical particle that carries the gravitational force . Thus, string theory is a theory of quantum gravity . String theory

7695-455: The string. String theory's application as a form of quantum gravity proposes a vibrational state responsible for the graviton , a yet unproven quantum particle that is theorized to carry gravitational force. One of the main developments of the past several decades in string theory was the discovery of certain 'dualities', mathematical transformations that identify one physical theory with another. Physicists studying string theory have discovered

7790-432: The strings appearing in type IIA superstring theory. Speaking at a string theory conference in 1995, Edward Witten made the surprising suggestion that all five superstring theories were in fact just different limiting cases of a single theory in eleven spacetime dimensions. Witten's announcement drew together all of the previous results on S- and T-duality and the appearance of higher-dimensional branes in string theory. In

7885-434: The study of black holes and quantum gravity, and it has been applied to other subjects, including nuclear and condensed matter physics . Since string theory incorporates all of the fundamental interactions, including gravity, many physicists hope that it will eventually be developed to the point where it fully describes our universe, making it a theory of everything . One of the goals of current research in string theory

7980-405: The true meaning of the title should be decided when a more fundamental formulation of the theory is known. In mathematics, a matrix is a rectangular array of numbers or other data. In physics, a matrix model is a particular kind of physical theory whose mathematical formulation involves the notion of a matrix in an important way. A matrix model describes the behavior of a set of matrices within

8075-499: The type I theory includes both open strings (which are segments with endpoints) and closed strings (which form closed loops), while types IIA, IIB and heterotic include only closed strings. In everyday life, there are three familiar dimensions (3D) of space: height, width and length. Einstein's general theory of relativity treats time as a dimension on par with the three spatial dimensions; in general relativity, space and time are not modeled as separate entities but are instead unified to

8170-499: The universe; origin of cosmic rays ; general relativity , special relativity , quantum and physical cosmology (the physical study of the largest-scale structures of the universe), including string cosmology and astroparticle physics . Astronomy is an ancient science, long separated from the study of terrestrial physics. In the Aristotelian worldview, bodies in the sky appeared to be unchanging spheres whose only motion

8265-489: The universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Relativistic astrophysics serves as a tool to gauge the properties of large-scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole ( astro )physics and the study of gravitational waves . Some widely accepted and studied theories and models in astrophysics, now included in

8360-466: The value of continued research on string theory unification. In the 20th century, two theoretical frameworks emerged for formulating the laws of physics. The first is Albert Einstein 's general theory of relativity , a theory that explains the force of gravity and the structure of spacetime at the macro-level. The other is quantum mechanics , a completely different formulation, which uses known probability principles to describe physical phenomena at

8455-492: The vibrational states of a string corresponds to the graviton, a quantum mechanical particle that carries the gravitational force. The original version of string theory was bosonic string theory , but this version described only bosons , a class of particles that transmit forces between the matter particles, or fermions . Bosonic string theory was eventually superseded by theories called superstring theories . These theories describe both bosons and fermions, and they incorporate

8550-478: Was based on the observation that D-branes—which look like fluctuating membranes when they are weakly interacting—become dense, massive objects with event horizons when the interactions are strong. In other words, a system of strongly interacting D-branes in string theory is indistinguishable from a black hole. Strominger and Vafa analyzed such D-brane systems and calculated the number of different ways of placing D-branes in spacetime so that their combined mass and charge

8645-450: Was its high degree of uniqueness. In ordinary particle theories, one can consider any collection of elementary particles whose classical behavior is described by an arbitrary Lagrangian . In string theory, the possibilities are much more constrained: by the 1990s, physicists had argued that there were only five consistent supersymmetric versions of the theory. Although there were only a handful of consistent superstring theories, it remained

8740-406: Was proved that the chemical elements found in the Sun and stars were also found on Earth. Among those who extended the study of solar and stellar spectra was Norman Lockyer , who in 1868 detected radiant, as well as dark lines in solar spectra. Working with chemist Edward Frankland to investigate the spectra of elements at various temperatures and pressures, he could not associate a yellow line in

8835-446: Was that Strominger and Vafa considered only extremal black holes in order to make the calculation tractable. These are defined as black holes with the lowest possible mass compatible with a given charge. Strominger and Vafa also restricted attention to black holes in five-dimensional spacetime with unphysical supersymmetry. Although it was originally developed in this very particular and physically unrealistic context in string theory,

8930-550: Was that such models would provide a unified description of the four fundamental forces of nature: electromagnetism, the strong and weak nuclear forces , and gravity. Interest in eleven-dimensional supergravity soon waned as various flaws in this scheme were discovered. One of the problems was that the laws of physics appear to distinguish between clockwise and counterclockwise, a phenomenon known as chirality . Edward Witten and others observed this chirality property cannot be readily derived by compactifying from eleven dimensions. In

9025-448: Was uniform motion in a circle, while the earthly world was the realm which underwent growth and decay and in which natural motion was in a straight line and ended when the moving object reached its goal . Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; either Fire as maintained by Plato , or Aether as maintained by Aristotle . During

#100899