
#redirect String theory
String theory is a physical theory whose fundamental building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that were the basis of most earlier physics. For this reason, string theories are able to avoid problems associated with the presence of pointlike particles in a physical theory. Detailed study of string theories has revealed that they describe not just strings but other objects, variously including points, membranes, and higher-dimensional objects. As discussed below, it is important to realize that no string theory has yet made firm predictions that would allow it to be experimentally tested. The term 'string theory' properly refers to both the 26-dimensional bosonic string theory and to the 10-dimensional superstring theory discovered by adding supersymmetry. Nowadays, 'string theory' usually refers to the supersymmetric variant while the earlier is given its full name, 'bosonic string theory'. Interest in string theory is driven largely by the hope that it will prove to be a theory of everything. It is one viable solution for quantum gravity, and in addition to gravity it can naturally describe interactions similar to electromagnetism and the other forces of nature. Superstring theories also include fermions, the building blocks of matter. It is not yet known whether string theory is able to describe a universe with the precise collection of forces and matter that we observe, nor how much freedom to choose those details the theory will allow. On a more concrete level, string theory has led to advances in the mathematics of Knot_theory, Calabi-Yau spaces and many other fields. Much exciting new mathematics in recent years has its origin in string theory. String theory has also led to much insight into supersymmetry gauge theory, a subject which includes possible extensions of the standard model. ==History== String theory was originally invented to explain certain peculiarities of hadron behavior. In certain particle-accelerator experiments, physicists observed that a hadron's angular momentum was exactly proportional to the square of its energy. No simple model of the hadron, such as picturing it as a set of smaller particles held together by spring-like forces, was able to explain these relationships. In order to account for these "Regge trajectories", physicists turned to a model where each hadron was in fact a rotating string, moving in accordance with Einstein's special relativity. The concepts which resulted became one component of bosonic string theory, which is still the first version taught to many students. (The original need for a viable theory of hadrons has been fulfilled by quantum chromodynamics, the study of quarks and their interactions. It is now hoped that string theory or some descendant of it will provide a more fundamental knowledge behind quarks themselves.) Bosonic string theory is formulated in terms of the Nambu-Goto action, a mathematical quantity which can be used to predict how strings move through space and time. By applying the ideas of quantum mechanics to the Nambu-Goto action—a procedure known as quantization (physics)—one can deduce that each string can vibrate in many different ways, and that each vibrational state appears to be a different particle. The mass the particle has, and the fashion with which it can interact, are determined by the way the string vibrates—in essence, by the "note" which the string sounds. The scale of notes, each corresponding to a different kind of particle, is termed the "spectrum" of the theory. These early models included both ''open'' strings, which have two distinct endpoints, and ''closed'' strings, where the endpoints are joined to make a complete loop. The two types of string behave in slightly different ways, yielding two spectra. Not all modern string theories use both types; some incorporate only the closed variety. However, the bosonic theory has problems. Most importantly, as the name implies, the spectrum of particles contains only bosons, particles like the photon which obey particular rules of behavior. While bosons are a critical ingredient in the Universe, they are certainly not its only constituents. Investigating how a string theory may include fermions in its spectrum led to supersymmetry, a mathematical relation between bosons and fermions which is now an independent area of study. String theories which include fermionic vibrations are now known as superstring theory; several different kinds have been described. In the 1990s, Edward Witten and others found strong evidence that the different superstring theories were different limits of an unknown 11-dimensional theory called M-theory. These discoveries sparked the second superstring revolution. (Several meanings of the "M" have been proposed; physicists joke that the true meaning will only be chosen when the theory is finally understood.) Many recent developments in the field relate to D-branes, objects which physicists discovered must also be included in any theory which includes open strings of the super string theory. ==Basic properties== While understanding the details of string and superstring theories requires considerable mathematical sophistication, some qualitative properties of quantum strings can be understood in a fairly intuitive fashion. For example, quantum strings have tension, much like regular strings made of twine; this tension is considered a fundamental parameter of the theory. The tension of a quantum string is closely related to its size. Consider a closed loop of string, left to move through space without external forces. Its tension will tend to contract it into a smaller and smaller loop. Classical intuition suggests that it might shrink to a single point, but this would violate Werner Heisenberg's uncertainty principle. The characteristic size of the string loop will be a balance between the tension force, acting to make it small, and the uncertainty effect, which keeps it "stretched". Consequently, the minimum size of a string must be related to the string tension. ==Extra dimensions== One intriguing feature of string theory is that it predicts the number of dimensions which the universe should possess. Nothing in James Clerk Maxwell's theory of electromagnetism or Albert Einstein's theory of relativity makes this kind of prediction; these theories require physicists to insert the number of dimensions "by hand". Instead, string theory allows one to compute the number of spacetime dimensions from first principles. Technically, this happens because Lorentz invariance can only be satisfied in a certain number of dimensions. This is roughly like saying that if we measure the distance between two points, then rotate our observer by some angle and measure again, the observed distance only stays the same if the universe has a particular number of dimensions. The only problem is that when the calculation is done, the universe's dimensionality is not four as one may expect (three axes of space and one of time), but twenty-six. More precisely, bosonic string theories are 26-dimensional, while superstring and M-theories turn out to involve 10 or 11 dimensions. However, these models appear to contradict observed phenomena. Physicists usually solve this problem in one of two different ways. The first is to Kaluza-Klein theory the extra dimensions; i.e., the 6 or 7 extra dimensions are so small as to be undetectable in our phenomenal experience. We achieve the 6-dimensional model's resolution with Calabi-Yau spaces. In 7 dimensions, they are termed G2 manifold. Essentially these extra dimensions are "compactified" by causing them to loop back upon themselves. A standard analogy for this is to consider multidimensional space as a garden hose. If we view the hose from a sufficient distance, it appears to have only one dimension, its length. This is akin to the 4 macroscopic dimensions we are accustomed to dealing with every day. If, however, one approaches the hose, one discovers that it contains a second dimension, its circumference. This "extra dimension" is only visible within a relatively close range to the hose, just as the extra dimensions of the Calabi-Yau space are only visible at extremely small distances, and thus are not easily detected. (Of course, everyday garden hoses exist in three spatial dimensions, but for the purpose of the analogy, we neglect its thickness and consider only motion on the ''surface'' of the hose. A point on the hose's surface can be specified by two numbers, a distance along the hose and a distance along the circumference, just as points on the Earth's surface can be uniquely specified by latitude and longitude. In either case, we say that the object has two spatial dimensions. Like the Earth, garden hoses have an interior, a region that requires an extra dimension; however, unlike the Earth, a Calabi-Yau space has no interior.) Another possibility is that we are stuck in a 3+1 dimensional subspace of the full universe, where the "3+1" reminds us that time is a different kind of dimension than space. Because it involves mathematical objects called D-branes, this is known as a Brane cosmology theory. In either case, gravity acting in the hidden dimensions produces other non-gravitational forces such as electromagnetism. In principle, therefore, it is possible to deduce the nature of those extra dimensions by requiring consistency with the standard model, but this is not yet a practical possibility. ==Problems== As of 2005, string theory is unverified. It is by no means the only theory currently being developed which suffers from this difficulty; any new development can pass through a stage of uncertainty before it becomes conclusively accepted or rejected. As Richard Feynman noted in ''The Character of Physical Law,'' the key test of a scientific theory is whether its consequences agree with the measurements we take in experiments. It does not matter who invented the theory, "what his name is", or even how aesthetically appealing the theory may be—"if it disagrees with experiment, it's wrong." (Of course, there are subsidiary issues: something may have gone wrong with the experiment, or perhaps the person computing the consequences of the theory made a mistake. All these possibilities must be checked, which may take a considerable time.) No version of string theory has yet made a prediction which differs from those made by other theories—at least, not in a way that could be checked by a currently feasible experiment. In this sense, string theory is still in a "larval stage": it possesses many features of mathematical interest, and it may yet become supremely important in our understanding of the Universe, but it requires further developments before it is accepted or falisfied. These developments may be in the theory itself, such as new methods of performing calculations and deriving predictions, or they may be advances in experimental science, which make formerly ungraspable quantities measurable. Human beings do not have the technology to observe strings (which are said to be roughly of Planck length, about 10-35 meters across). Eventually, we may be able to observe strings in a meaningful way, or at least to gain substantial insight by observing cosmological phenomena which may elucidate string physics. Another problem is that, like quantum field theory, much of string theory is still only formulated perturbatively (''i.e.,'' as a series of approximations rather than as an exact solution). Although nonperturbative techniques have progressed considerably—including conjectured complete definitions in space-times satisfying certain asymptotics—a full nonperturbative definition of the theory is still lacking. == Related topics == * List of string theory topics * superstring theory * M-theory * F-theory * quantum gravity * Kaluza-Klein theory * conformal field theory * supersymmetry, supergravity * Loop quantum gravity * Graviton == References == ===Popular books and articles=== * Davies, Paul, and Julian R. Brown. ''Superstrings: A Theory of Everything?''. Cambridge University Press (1988). ISBN 0-521-43775-X. * Brian Greene, ''The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory'', W.W. Norton & Company; Reissue edition (2003) ISBN 0-393-05858-1. * Gribbin, John, ''The Search for Superstrings, Symmetry, and the Theory of Everything''. London, Great Britain: Little Brown and Company (1998). ISBN 0-316-32975-4. * Michio Kaku, ''Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the Tenth Dimension.'' New York, Oxford University Press (1994). ISBN 0-195-08514-0. ===Textbooks=== * Michael Green, John Schwarz and Edward Witten, ''Superstring theory'', Cambridge University Press (1987). The original textbook. ** Vol. 1: Introduction, ISBN 0-521-35752-7. ** Vol. 2: Loop amplitudes, anomalies and phenomenology, ISBN 0-521-35753-5. * Johnson, Clifford, ''D-branes'', Cambridge University Press (2003). ISBN 0-521-80912-6. * Joseph Polchinski, ''String Theory'', Cambridge University Press (1998). A modern textbook. ** Vol. 1: An introduction to the bosonic string, ISBN 0-521-63303-6. ** Vol. 2: Superstring theory and beyond, ISBN 0-521-63304-4. * Zwiebach, Barton. ''A First Course in String Theory.'' Cambridge University Press (2004). ISBN 0-521-83143-1. ===External links=== *[http://superstringtheory.com/ Superstringtheory.com] - A popular online guide. *[http://www.americanscientist.org/template/AssetDetail/assetid/18638 Is string theory even wrong?] - A very interesting criticism of string theory. *[http://tena4.vub.ac.be/beyondstringtheory/ Beyond String Theory] - An ongoing project, illuminating many aspects of string theory and related topics. *[http://www.pbs.org/wgbh/nova/elegant/ The Elegant Universe] - NOVA documentary by Brian Greene. Various images, texts, videos and animations explaining string theory. *[http://www.msnbc.com/news/201650.asp The Symphony of Everything: a short interactive introduction to string theory.] *[http://schwinger.harvard.edu/~sps/ SCI.physics.STRINGS] - The home page of a newsgroup dedicated to string theory *[http://xxx.arxiv.org/abs/hep-th/0311044 Resource Letter] - A good guide for students to the string theory literature. *[http://www.math.columbia.edu/~woit/blog/ A popular blog on string theory] String theory Protoscience Cosmology fa:نظریه ریسمان la:Theoria Chordarum simple:String theory vi:Lý thuyết dây
It seems likely that observable phenomena will be emergent ones from the complex interactions of strings in any real-world scenario. Presumably string theorists are using chaos theory and computer simulation to deduce these emergent phenomena? I can't imagine that anyone expects to find a nice analytical solution that manifests itself in the observable universe when, at quantum scales, virtually every observable thing is an incredibly complex system. ---- Has anyone explored the relationship between string theory and the Transactional Interpretation of quantum mechanics? Both involve the representation of particles as standing waves in n-space (although I agree that the TI much simpler). The TI states that it is the interference of advance & retarded waves that creates these standing waves. This seems to elegantly resolve every quantum paradox, while at the same time making use of a previously disregarded solution to Maxwell's equations. Does string theory still disregard waves that propagate backwards in time...? ---- The article says '..measure distance between two points.. rotate that observer'- are we talking about rotating the observer about its own central axis, or rotating it about some third axis? This is confusing User:24.176.6.165 07:23, 29 Dec 2004 (UTC) The article says that in string theory, spacetime has either 10, 11 or 26 dimensions. But http://superstringtheory.com/basics/basic5a.html says: That sounds crazy -- because bosonic strings live in 26 dimensions but supersymmetric string theories live in 10 dimensions. But '''the extra 16 dimensions of the bosonic side of the theory aren't really spacetime dimensions'''. Heterotic string theories are supersymmetric string theories living in ten spacetime dimensions. :I believe that the quote you've given comes specifically from a discussion of "heterotic" string theories. They are called "heterotic" because left-moving and right-moving excitations (think of them as waves moving around the closed string) look very different. The left-movers look like excitations of the bosonic string and the right-movers look like excitations of the supersymmetric string. That's the context of the statement you quoted above. (For the record, the comment about 11 dimensions refers to M-theory.) -- User:Steuard 02:21, 26 Mar 2004 (UTC) ---- Shouldn't the title be string field theory? Or am I mistaken for something else? -- User:TakuyaMurata :Google would suggest not - 211,000 hits for "string theory" and 5,860 for "string field theory". I've never heard it called string field theory. User:Angela 20:03, Oct 18, 2003 (UTC) ::String field theory is an alternative formalism for string theory. It's ultimately the same thing. User:Mporter Jan 29, 2004 (AEST) ---- I have a question concerning this theory. The idea of 10, 11 or 26 dimensions, or evens strings themselves, doesn't seem very elegant. So, can we say there are no strings or 11 etc. dimensions, but it is our only way to describe the world as it is? (Because the mind is unable to understand it in another way than "string" or "dimension"?) E.g. there are no Calabi-Yau-Spaces, but we can describe the thing that is as if there where Calabi-Yau-Spaces? So: reality stays beautiful, because it is our mind that is not capable enough to think of a real elegant theory. I like this more than saying "there are 11 dimensions". User:Hhc2 14:19, 21 Oct 2003 (UTC) :On the one hand, one way of doing physics is not to bother asking wether Nature ''really'' behaves the way your model says as long as it gives the right answer to experiments. This is mainly due to Quantum Mechanics where people stopped asking wether particles ''really'' travelled along two different paths at the same time as long as the probabilities they got where the good ones. Your position is thus perfectly acceptable form this point of view. But on the other hand you should accept the idea that Nature is not reducible to what our senses tell us. For example can you find an absolute reason why spacetime should be 4 dimensional ? All you can say is that at your (low-energy) scale, Nature ''seems'' to be 4 dimensional, right. But you cannot infer the answer, say, just after the Big Bang. There's no reason why the dimensionality of spacetime should be an absolute concept. So you can distinguish between Nature and our different representations of it but Nature still could change considerably with the scale. Stating the contrary would be a bit anthropocentristic :) LeYaYa 8 Feb 2004 :: I am glad that - after some months - somebody has answered my question! I would like to point out that my intention was not to reduce nature / the reality to a level that I can understand it. That indeed would be anthropocentric. I wanted to express the regrettable status of my mind, which ist not able to think at the same time in 11 dimensions and call it beautiful or elegant. ::Reading the popular articles on string theory, I have the impression that they talk about strings as existing objects, that the average reader must come to the conclusion that the universe consists of some sort of very small spaghetti, only to small to see them. hhc2 6. march 04 == Fixed article a bit == I don't have the time nor the energy to write a long essay, but I fixed a lot misconceptions and ouright errors in the original page. ---------- A link or quick defininition for the term "perturbative" in this context would be helpful for the layman. ----- Although the general structure of this article is quite good, I think we should word it more carefully. All the arguments given are mathematical (and valid), but in physics it is important to tie them in with experiments. Or at least clearly show their "speculative character." Here, it sounds like string theory is already a given fact... User:Awolf002 01:26, 6 Apr 2004 (UTC) I like that added "hint" about missing tests. Much better now User:Steuard, right? User:Awolf002 19:04, 8 Apr 2004 (UTC) == Two questions and clarifications == I'm new at this, and a little confused, but intrigued. The article says, "string theories are able to avoid problems associated with the presence of pointlike particles in a physical theory. Detailed study of string theories has revealed that they contain not just strings but other objects, variously including points, membranes, and higher-dimensional objects." Can anyone explain to me what sort of problems string theories are able to avoid? Also, how is it possible that each string "contains" other objects? I thought the whole point of a string theory is that the strings are the smallest, indivisible building blocks (as opposed to conventional physical models, which hold that the smallest building blocks are three-dimensional)? I'd love to understand this better. --User:Seneca644 03:56, 6 Jul 2004 (UTC) :One of the most obvious (to physicists) problems has to do with the infinities that show up when you try to describe a point charge. With a finite amount of electric charge concentrated at a single infinitely small point, the electric field at that point is infinitely large. If the same charge is instead distributed continuously along a string, no infinitely small point along the string has a non-zero charge, so the problem goes away. (The actual calculations used in string theory are rarely this straightforward, but I think the principle is pretty much the same.) :As for your second question, that's just an unclear phrase in the article (embarassingly, I think I wrote it). The strings don't contain other objects, the theories do. I've just changed the word "contain" to "describe", which should make the meaning much clearer.--User:Steuard 22:21, Jul 9, 2004 (UTC) ---- I don't know about other people's hoses, but my garden hose has 3 dimensions, one more than this article seems to indicate. Is it some kind of gateway to another universe perhaps? Is it leaking into another brane like gravitons supposedly do? User:Infradig 04:46, 1 Sep 2004 (UTC) == Image == Is anyone able either to obtain royalty-free string images like those cool ones on that nova documentary, or render something? That'd add to the article. [[User:Maestrosync|[maestro]]] 04:47, 4 Dec 2004 (UTC) == "String theory, as with any current theory of quantum gravity, is unverifiable, and therefore it is also unfalsifiable." == GAHHH!!! I just had a two hour argument with my agnostic friend (I am an atheist) about this. Just because an idea is created does not make it unfalsifiable! Theists always use this argument and it is flawed. It is the exact point of the Invisible Pink Unicorn--that becuase it is created, you can not disprove it, even though we all know she is a falsehood (blessed be her holy hooves). It's because the moment an idea is created, and you vehemently defend it, people say it can not be disproven. By this token, Gimli exists, because I say so, and therefore, it is unfalsifiable. User:Lockeownzj00 20:13, 5 Feb 2005 (UTC) :I think you misunderstand something here. Unfalsifiability is bad, from a philosophical viewpoint, for the reasons you stated above. Of course, an Invisible Pink Unicorn cannot exist, because invisibility precludes colour. *shakes head* User:BovineBeast By Rob: JUST because the grasp of the human mind cannot process a consept and theory so complicated, doesn't make it untrue!, But what we perceve is what we belive. ==Superstring theory== Shouldn't this article be titled "Superstring theory"? The phrase "string theory" is a popular shortening of the original term. -- 02:41, 1 Mar 2005 (UTC) == what about the philosophy of a sphere? == we know , a mathematical back ground concerning the origin of any sphere is a bit complex too ,a sphere is that it has no start and no end ,. absolutely we can find the end of a sphere [ sphere of radius R ] it is a preety simple question [ if parameters of diff.. eq n provide] ; but are our minds are ready enough to find the origin of any sphere. if we say a point matter then also that same is too a sphere , if we say a differential of a space then the ouestion is redirected to the constant k of our space time concept. here word sphere ,i mean to say any originating ball or a body [equivalently], a potential field, uniform lump matter of same constituent originating in space time co-ordinates . what ever we do , how hard we try , my above question is a question to all explorers of fundamental science, including me. :i think , as per mathematics says, we can only talk about matter formation[ hence energy] or the meta physics, if and only if the above puzzle is cracked through mathematics , hence getting nearer and nearer to the truth . this will certainly risk free the possibility of any wrong truth/ model of any prediction what ever we make towards universe. further questions; connect: e2t_solar@rediffmail.com do you have a problem : i too is having a part of problem let us solve together [ contact me if interested in my question] (unsigned I prabhat) ---- I say improving the article will make masses happier. --User:Coolcat User talk:Coolcat 15:47, 20 Mar 2005 (UTC) == Trouble with this article == I think this article is very biased. "As of 2005, string theory is unverifiable. It is by no means the only theory currently being developed which suffers from this difficulty; any new development can pass through a stage of unverifiability before it becomes conclusively accepted or rejected. As Richard Feynman noted in The Character of Physical Law, the key test of a scientific theory is whether its consequences agree with the measurements we take in experiments. It does not matter who invented the theory, "what his name is", or even how aesthetically appealing the theory may be—"if it disagrees with experiment, it's wrong." String theory's perdiction of cosmological constant is wrong. (55 orders of magnitude) In fact, some would argue string theory is not even a theory given that even if the amazing particle accelerators which would supposedly show evidence of the smaller dimensions could be built, string theorists still don't predictions on what exactly would be seen. Please see http://www.math.columbia.edu/%7Ewoit/strings.pdf ==Question about time== Time is not a dimension, right? Right? Can somebody please explain, when I search information some sites tell me that time is the fourth dimension and some tell that it's not a dimension. Is it a matter of opinion until we understand a little more about how the world works? :It is the perspective of essentially all theoretical physics since Einstein that time is a dimension. However, you should probably find an actual discussion forum if you want a more complete explanation (perhaps you could try the Usenet group "sci.physics", which you can access through [http://groups.google.com/ Google Groups]); this page is only intended for discussion of how to improve the content of the article on String theory.--User:Steuard 20:52, Apr 21, 2005 (UTC) ==garden hose thing== thank god. without the garden hose thing i'd have been lost. User:70.177.90.39 03:06, 11 May 2005 (UTC) == Question == Recently someone made a new theory based on the sting theory and the kazula klein theory, where everything is explained only using flexion of the space. I cant remember who this is. Does anybody know this theory? == How does string theory explain matter/anti-matter anihilation? == As a non-physicist following string theory through Scientific American and Nova, one question I've always wanted to ask about string theory is "How does string theory describe the anihilation of matter and anti-matter". I would expect that there must be some mathematical symetry between the vibration mode of a string that results in an electron vs. that of a string that results in a positron. When two such vibrating strings collide, just what happens? Thanks, Mark Z. == Extra Dimentions == I read with interest the theory that extra spatial dimentions exist. However, this view is at variance with one expressed by Professor Stephen Hawking, who remarks that "if there were more than three spatial dimentions, the orbits of planets around the sun or electrons around a nucleus would be unstable and they would tend to spiral inward" --Derek R Crawford. :That description of gravity would certainly be accurate if the extra spatial dimensions were "accessible" to us in the same way that the usual three are: the (approximate) force of gravity in D spatial dimensions falls off at large radius r as r^-(D-1). (I suspect that the comments about electron orbits aren't as relevant once quantum mechanics gets involved: QM doesn't allow for "spiraling inwards" at that scale.) But we already know that any extra dimensions would have to be different somehow, because we can't see them. :The most common explanation in string theory is that the extra dimensions are "curled up" in small circles (or some other compact space). I've tried to explain that concept a little bit on [http://www.slimy.com/~steuard/research/MITClub2004/slide09.html this slide] in an overview of string theory that I wrote. The slide compares a human being's perception of a tightrope to an ant's: the person can move in only one dimension, but the ant can move in two. :If that were the case, then measurements on very small scales in our universe ''would'' see gravity falling off faster than r^-2. But once the radius of the extra dimension became small compared to r, we would be back to r^-2 again (you can imagine that the small circle has "filled up" with gravitational field: there's nowhere else for it to dissipate in that direction). Last I heard, direct measurements have only verified that r^-2 gravity holds down to about 0.1 mm (which is quite large, when you think about it). So there's lots of room for the extra dimensions required by string theory.--User:Steuard 19:41, Jun 21, 2005 (UTC)
String theory is a branch of theoretical physics that attempts to build a theory of quantum gravity using one-dimensional strings rather than zero-dimensional point particles as fundamental building blocks. The name ''string theory'' is somewhat of a misnomer since the modern theory also includes higher dimensional objects known as branes. :''For more information, please read the main article about String theory.'' Quantum field theory Theoretical physicsGravity Protoscience vi:Category:Lý thuyết dây
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#redirect String theory
String theory
String theory is a physical theory whose fundamental building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that were the basis of most earlier physics. For this reason, string theories are able to avoid problems associated with the presence of pointlike particles in a physical theory. Detailed study of string theories has revealed that they describe not just strings but other objects, variously including points, membranes, and higher-dimensional objects. As discussed below, it is important to realize that no string theory has yet made firm predictions that would allow it to be experimentally tested. The term 'string theory' properly refers to both the 26-dimensional bosonic string theory and to the 10-dimensional superstring theory discovered by adding supersymmetry. Nowadays, 'string theory' usually refers to the supersymmetric variant while the earlier is given its full name, 'bosonic string theory'. Interest in string theory is driven largely by the hope that it will prove to be a theory of everything. It is one viable solution for quantum gravity, and in addition to gravity it can naturally describe interactions similar to electromagnetism and the other forces of nature. Superstring theories also include fermions, the building blocks of matter. It is not yet known whether string theory is able to describe a universe with the precise collection of forces and matter that we observe, nor how much freedom to choose those details the theory will allow. On a more concrete level, string theory has led to advances in the mathematics of Knot_theory, Calabi-Yau spaces and many other fields. Much exciting new mathematics in recent years has its origin in string theory. String theory has also led to much insight into supersymmetry gauge theory, a subject which includes possible extensions of the standard model. ==History== String theory was originally invented to explain certain peculiarities of hadron behavior. In certain particle-accelerator experiments, physicists observed that a hadron's angular momentum was exactly proportional to the square of its energy. No simple model of the hadron, such as picturing it as a set of smaller particles held together by spring-like forces, was able to explain these relationships. In order to account for these "Regge trajectories", physicists turned to a model where each hadron was in fact a rotating string, moving in accordance with Einstein's special relativity. The concepts which resulted became one component of bosonic string theory, which is still the first version taught to many students. (The original need for a viable theory of hadrons has been fulfilled by quantum chromodynamics, the study of quarks and their interactions. It is now hoped that string theory or some descendant of it will provide a more fundamental knowledge behind quarks themselves.) Bosonic string theory is formulated in terms of the Nambu-Goto action, a mathematical quantity which can be used to predict how strings move through space and time. By applying the ideas of quantum mechanics to the Nambu-Goto action—a procedure known as quantization (physics)—one can deduce that each string can vibrate in many different ways, and that each vibrational state appears to be a different particle. The mass the particle has, and the fashion with which it can interact, are determined by the way the string vibrates—in essence, by the "note" which the string sounds. The scale of notes, each corresponding to a different kind of particle, is termed the "spectrum" of the theory. These early models included both ''open'' strings, which have two distinct endpoints, and ''closed'' strings, where the endpoints are joined to make a complete loop. The two types of string behave in slightly different ways, yielding two spectra. Not all modern string theories use both types; some incorporate only the closed variety. However, the bosonic theory has problems. Most importantly, as the name implies, the spectrum of particles contains only bosons, particles like the photon which obey particular rules of behavior. While bosons are a critical ingredient in the Universe, they are certainly not its only constituents. Investigating how a string theory may include fermions in its spectrum led to supersymmetry, a mathematical relation between bosons and fermions which is now an independent area of study. String theories which include fermionic vibrations are now known as superstring theory; several different kinds have been described. In the 1990s, Edward Witten and others found strong evidence that the different superstring theories were different limits of an unknown 11-dimensional theory called M-theory. These discoveries sparked the second superstring revolution. (Several meanings of the "M" have been proposed; physicists joke that the true meaning will only be chosen when the theory is finally understood.) Many recent developments in the field relate to D-branes, objects which physicists discovered must also be included in any theory which includes open strings of the super string theory. ==Basic properties== While understanding the details of string and superstring theories requires considerable mathematical sophistication, some qualitative properties of quantum strings can be understood in a fairly intuitive fashion. For example, quantum strings have tension, much like regular strings made of twine; this tension is considered a fundamental parameter of the theory. The tension of a quantum string is closely related to its size. Consider a closed loop of string, left to move through space without external forces. Its tension will tend to contract it into a smaller and smaller loop. Classical intuition suggests that it might shrink to a single point, but this would violate Werner Heisenberg's uncertainty principle. The characteristic size of the string loop will be a balance between the tension force, acting to make it small, and the uncertainty effect, which keeps it "stretched". Consequently, the minimum size of a string must be related to the string tension. ==Extra dimensions== One intriguing feature of string theory is that it predicts the number of dimensions which the universe should possess. Nothing in James Clerk Maxwell's theory of electromagnetism or Albert Einstein's theory of relativity makes this kind of prediction; these theories require physicists to insert the number of dimensions "by hand". Instead, string theory allows one to compute the number of spacetime dimensions from first principles. Technically, this happens because Lorentz invariance can only be satisfied in a certain number of dimensions. This is roughly like saying that if we measure the distance between two points, then rotate our observer by some angle and measure again, the observed distance only stays the same if the universe has a particular number of dimensions. The only problem is that when the calculation is done, the universe's dimensionality is not four as one may expect (three axes of space and one of time), but twenty-six. More precisely, bosonic string theories are 26-dimensional, while superstring and M-theories turn out to involve 10 or 11 dimensions. However, these models appear to contradict observed phenomena. Physicists usually solve this problem in one of two different ways. The first is to Kaluza-Klein theory the extra dimensions; i.e., the 6 or 7 extra dimensions are so small as to be undetectable in our phenomenal experience. We achieve the 6-dimensional model's resolution with Calabi-Yau spaces. In 7 dimensions, they are termed G2 manifold. Essentially these extra dimensions are "compactified" by causing them to loop back upon themselves. A standard analogy for this is to consider multidimensional space as a garden hose. If we view the hose from a sufficient distance, it appears to have only one dimension, its length. This is akin to the 4 macroscopic dimensions we are accustomed to dealing with every day. If, however, one approaches the hose, one discovers that it contains a second dimension, its circumference. This "extra dimension" is only visible within a relatively close range to the hose, just as the extra dimensions of the Calabi-Yau space are only visible at extremely small distances, and thus are not easily detected. (Of course, everyday garden hoses exist in three spatial dimensions, but for the purpose of the analogy, we neglect its thickness and consider only motion on the ''surface'' of the hose. A point on the hose's surface can be specified by two numbers, a distance along the hose and a distance along the circumference, just as points on the Earth's surface can be uniquely specified by latitude and longitude. In either case, we say that the object has two spatial dimensions. Like the Earth, garden hoses have an interior, a region that requires an extra dimension; however, unlike the Earth, a Calabi-Yau space has no interior.) Another possibility is that we are stuck in a 3+1 dimensional subspace of the full universe, where the "3+1" reminds us that time is a different kind of dimension than space. Because it involves mathematical objects called D-branes, this is known as a Brane cosmology theory. In either case, gravity acting in the hidden dimensions produces other non-gravitational forces such as electromagnetism. In principle, therefore, it is possible to deduce the nature of those extra dimensions by requiring consistency with the standard model, but this is not yet a practical possibility. ==Problems== As of 2005, string theory is unverified. It is by no means the only theory currently being developed which suffers from this difficulty; any new development can pass through a stage of uncertainty before it becomes conclusively accepted or rejected. As Richard Feynman noted in ''The Character of Physical Law,'' the key test of a scientific theory is whether its consequences agree with the measurements we take in experiments. It does not matter who invented the theory, "what his name is", or even how aesthetically appealing the theory may be—"if it disagrees with experiment, it's wrong." (Of course, there are subsidiary issues: something may have gone wrong with the experiment, or perhaps the person computing the consequences of the theory made a mistake. All these possibilities must be checked, which may take a considerable time.) No version of string theory has yet made a prediction which differs from those made by other theories—at least, not in a way that could be checked by a currently feasible experiment. In this sense, string theory is still in a "larval stage": it possesses many features of mathematical interest, and it may yet become supremely important in our understanding of the Universe, but it requires further developments before it is accepted or falisfied. These developments may be in the theory itself, such as new methods of performing calculations and deriving predictions, or they may be advances in experimental science, which make formerly ungraspable quantities measurable. Human beings do not have the technology to observe strings (which are said to be roughly of Planck length, about 10-35 meters across). Eventually, we may be able to observe strings in a meaningful way, or at least to gain substantial insight by observing cosmological phenomena which may elucidate string physics. Another problem is that, like quantum field theory, much of string theory is still only formulated perturbatively (''i.e.,'' as a series of approximations rather than as an exact solution). Although nonperturbative techniques have progressed considerably—including conjectured complete definitions in space-times satisfying certain asymptotics—a full nonperturbative definition of the theory is still lacking. == Related topics == * List of string theory topics * superstring theory * M-theory * F-theory * quantum gravity * Kaluza-Klein theory * conformal field theory * supersymmetry, supergravity * Loop quantum gravity * Graviton == References == ===Popular books and articles=== * Davies, Paul, and Julian R. Brown. ''Superstrings: A Theory of Everything?''. Cambridge University Press (1988). ISBN 0-521-43775-X. * Brian Greene, ''The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory'', W.W. Norton & Company; Reissue edition (2003) ISBN 0-393-05858-1. * Gribbin, John, ''The Search for Superstrings, Symmetry, and the Theory of Everything''. London, Great Britain: Little Brown and Company (1998). ISBN 0-316-32975-4. * Michio Kaku, ''Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the Tenth Dimension.'' New York, Oxford University Press (1994). ISBN 0-195-08514-0. ===Textbooks=== * Michael Green, John Schwarz and Edward Witten, ''Superstring theory'', Cambridge University Press (1987). The original textbook. ** Vol. 1: Introduction, ISBN 0-521-35752-7. ** Vol. 2: Loop amplitudes, anomalies and phenomenology, ISBN 0-521-35753-5. * Johnson, Clifford, ''D-branes'', Cambridge University Press (2003). ISBN 0-521-80912-6. * Joseph Polchinski, ''String Theory'', Cambridge University Press (1998). A modern textbook. ** Vol. 1: An introduction to the bosonic string, ISBN 0-521-63303-6. ** Vol. 2: Superstring theory and beyond, ISBN 0-521-63304-4. * Zwiebach, Barton. ''A First Course in String Theory.'' Cambridge University Press (2004). ISBN 0-521-83143-1. ===External links=== *[http://superstringtheory.com/ Superstringtheory.com] - A popular online guide. *[http://www.americanscientist.org/template/AssetDetail/assetid/18638 Is string theory even wrong?] - A very interesting criticism of string theory. *[http://tena4.vub.ac.be/beyondstringtheory/ Beyond String Theory] - An ongoing project, illuminating many aspects of string theory and related topics. *[http://www.pbs.org/wgbh/nova/elegant/ The Elegant Universe] - NOVA documentary by Brian Greene. Various images, texts, videos and animations explaining string theory. *[http://www.msnbc.com/news/201650.asp The Symphony of Everything: a short interactive introduction to string theory.] *[http://schwinger.harvard.edu/~sps/ SCI.physics.STRINGS] - The home page of a newsgroup dedicated to string theory *[http://xxx.arxiv.org/abs/hep-th/0311044 Resource Letter] - A good guide for students to the string theory literature. *[http://www.math.columbia.edu/~woit/blog/ A popular blog on string theory] String theory Protoscience Cosmology fa:نظریه ریسمان la:Theoria Chordarum simple:String theory vi:Lý thuyết dây
String theory
It seems likely that observable phenomena will be emergent ones from the complex interactions of strings in any real-world scenario. Presumably string theorists are using chaos theory and computer simulation to deduce these emergent phenomena? I can't imagine that anyone expects to find a nice analytical solution that manifests itself in the observable universe when, at quantum scales, virtually every observable thing is an incredibly complex system. ---- Has anyone explored the relationship between string theory and the Transactional Interpretation of quantum mechanics? Both involve the representation of particles as standing waves in n-space (although I agree that the TI much simpler). The TI states that it is the interference of advance & retarded waves that creates these standing waves. This seems to elegantly resolve every quantum paradox, while at the same time making use of a previously disregarded solution to Maxwell's equations. Does string theory still disregard waves that propagate backwards in time...? ---- The article says '..measure distance between two points.. rotate that observer'- are we talking about rotating the observer about its own central axis, or rotating it about some third axis? This is confusing User:24.176.6.165 07:23, 29 Dec 2004 (UTC) The article says that in string theory, spacetime has either 10, 11 or 26 dimensions. But http://superstringtheory.com/basics/basic5a.html says: That sounds crazy -- because bosonic strings live in 26 dimensions but supersymmetric string theories live in 10 dimensions. But '''the extra 16 dimensions of the bosonic side of the theory aren't really spacetime dimensions'''. Heterotic string theories are supersymmetric string theories living in ten spacetime dimensions. :I believe that the quote you've given comes specifically from a discussion of "heterotic" string theories. They are called "heterotic" because left-moving and right-moving excitations (think of them as waves moving around the closed string) look very different. The left-movers look like excitations of the bosonic string and the right-movers look like excitations of the supersymmetric string. That's the context of the statement you quoted above. (For the record, the comment about 11 dimensions refers to M-theory.) -- User:Steuard 02:21, 26 Mar 2004 (UTC) ---- Shouldn't the title be string field theory? Or am I mistaken for something else? -- User:TakuyaMurata :Google would suggest not - 211,000 hits for "string theory" and 5,860 for "string field theory". I've never heard it called string field theory. User:Angela 20:03, Oct 18, 2003 (UTC) ::String field theory is an alternative formalism for string theory. It's ultimately the same thing. User:Mporter Jan 29, 2004 (AEST) ---- I have a question concerning this theory. The idea of 10, 11 or 26 dimensions, or evens strings themselves, doesn't seem very elegant. So, can we say there are no strings or 11 etc. dimensions, but it is our only way to describe the world as it is? (Because the mind is unable to understand it in another way than "string" or "dimension"?) E.g. there are no Calabi-Yau-Spaces, but we can describe the thing that is as if there where Calabi-Yau-Spaces? So: reality stays beautiful, because it is our mind that is not capable enough to think of a real elegant theory. I like this more than saying "there are 11 dimensions". User:Hhc2 14:19, 21 Oct 2003 (UTC) :On the one hand, one way of doing physics is not to bother asking wether Nature ''really'' behaves the way your model says as long as it gives the right answer to experiments. This is mainly due to Quantum Mechanics where people stopped asking wether particles ''really'' travelled along two different paths at the same time as long as the probabilities they got where the good ones. Your position is thus perfectly acceptable form this point of view. But on the other hand you should accept the idea that Nature is not reducible to what our senses tell us. For example can you find an absolute reason why spacetime should be 4 dimensional ? All you can say is that at your (low-energy) scale, Nature ''seems'' to be 4 dimensional, right. But you cannot infer the answer, say, just after the Big Bang. There's no reason why the dimensionality of spacetime should be an absolute concept. So you can distinguish between Nature and our different representations of it but Nature still could change considerably with the scale. Stating the contrary would be a bit anthropocentristic :) LeYaYa 8 Feb 2004 :: I am glad that - after some months - somebody has answered my question! I would like to point out that my intention was not to reduce nature / the reality to a level that I can understand it. That indeed would be anthropocentric. I wanted to express the regrettable status of my mind, which ist not able to think at the same time in 11 dimensions and call it beautiful or elegant. ::Reading the popular articles on string theory, I have the impression that they talk about strings as existing objects, that the average reader must come to the conclusion that the universe consists of some sort of very small spaghetti, only to small to see them. hhc2 6. march 04 == Fixed article a bit == I don't have the time nor the energy to write a long essay, but I fixed a lot misconceptions and ouright errors in the original page. ---------- A link or quick defininition for the term "perturbative" in this context would be helpful for the layman. ----- Although the general structure of this article is quite good, I think we should word it more carefully. All the arguments given are mathematical (and valid), but in physics it is important to tie them in with experiments. Or at least clearly show their "speculative character." Here, it sounds like string theory is already a given fact... User:Awolf002 01:26, 6 Apr 2004 (UTC) I like that added "hint" about missing tests. Much better now User:Steuard, right? User:Awolf002 19:04, 8 Apr 2004 (UTC) == Two questions and clarifications == I'm new at this, and a little confused, but intrigued. The article says, "string theories are able to avoid problems associated with the presence of pointlike particles in a physical theory. Detailed study of string theories has revealed that they contain not just strings but other objects, variously including points, membranes, and higher-dimensional objects." Can anyone explain to me what sort of problems string theories are able to avoid? Also, how is it possible that each string "contains" other objects? I thought the whole point of a string theory is that the strings are the smallest, indivisible building blocks (as opposed to conventional physical models, which hold that the smallest building blocks are three-dimensional)? I'd love to understand this better. --User:Seneca644 03:56, 6 Jul 2004 (UTC) :One of the most obvious (to physicists) problems has to do with the infinities that show up when you try to describe a point charge. With a finite amount of electric charge concentrated at a single infinitely small point, the electric field at that point is infinitely large. If the same charge is instead distributed continuously along a string, no infinitely small point along the string has a non-zero charge, so the problem goes away. (The actual calculations used in string theory are rarely this straightforward, but I think the principle is pretty much the same.) :As for your second question, that's just an unclear phrase in the article (embarassingly, I think I wrote it). The strings don't contain other objects, the theories do. I've just changed the word "contain" to "describe", which should make the meaning much clearer.--User:Steuard 22:21, Jul 9, 2004 (UTC) ---- I don't know about other people's hoses, but my garden hose has 3 dimensions, one more than this article seems to indicate. Is it some kind of gateway to another universe perhaps? Is it leaking into another brane like gravitons supposedly do? User:Infradig 04:46, 1 Sep 2004 (UTC) == Image == Is anyone able either to obtain royalty-free string images like those cool ones on that nova documentary, or render something? That'd add to the article. [[User:Maestrosync|[maestro]]] 04:47, 4 Dec 2004 (UTC) == "String theory, as with any current theory of quantum gravity, is unverifiable, and therefore it is also unfalsifiable." == GAHHH!!! I just had a two hour argument with my agnostic friend (I am an atheist) about this. Just because an idea is created does not make it unfalsifiable! Theists always use this argument and it is flawed. It is the exact point of the Invisible Pink Unicorn--that becuase it is created, you can not disprove it, even though we all know she is a falsehood (blessed be her holy hooves). It's because the moment an idea is created, and you vehemently defend it, people say it can not be disproven. By this token, Gimli exists, because I say so, and therefore, it is unfalsifiable. User:Lockeownzj00 20:13, 5 Feb 2005 (UTC) :I think you misunderstand something here. Unfalsifiability is bad, from a philosophical viewpoint, for the reasons you stated above. Of course, an Invisible Pink Unicorn cannot exist, because invisibility precludes colour. *shakes head* User:BovineBeast By Rob: JUST because the grasp of the human mind cannot process a consept and theory so complicated, doesn't make it untrue!, But what we perceve is what we belive. ==Superstring theory== Shouldn't this article be titled "Superstring theory"? The phrase "string theory" is a popular shortening of the original term. -- 02:41, 1 Mar 2005 (UTC) == what about the philosophy of a sphere? == we know , a mathematical back ground concerning the origin of any sphere is a bit complex too ,a sphere is that it has no start and no end ,. absolutely we can find the end of a sphere [ sphere of radius R ] it is a preety simple question [ if parameters of diff.. eq n provide] ; but are our minds are ready enough to find the origin of any sphere. if we say a point matter then also that same is too a sphere , if we say a differential of a space then the ouestion is redirected to the constant k of our space time concept. here word sphere ,i mean to say any originating ball or a body [equivalently], a potential field, uniform lump matter of same constituent originating in space time co-ordinates . what ever we do , how hard we try , my above question is a question to all explorers of fundamental science, including me. :i think , as per mathematics says, we can only talk about matter formation[ hence energy] or the meta physics, if and only if the above puzzle is cracked through mathematics , hence getting nearer and nearer to the truth . this will certainly risk free the possibility of any wrong truth/ model of any prediction what ever we make towards universe. further questions; connect: e2t_solar@rediffmail.com do you have a problem : i too is having a part of problem let us solve together [ contact me if interested in my question] (unsigned I prabhat) ---- I say improving the article will make masses happier. --User:Coolcat User talk:Coolcat 15:47, 20 Mar 2005 (UTC) == Trouble with this article == I think this article is very biased. "As of 2005, string theory is unverifiable. It is by no means the only theory currently being developed which suffers from this difficulty; any new development can pass through a stage of unverifiability before it becomes conclusively accepted or rejected. As Richard Feynman noted in The Character of Physical Law, the key test of a scientific theory is whether its consequences agree with the measurements we take in experiments. It does not matter who invented the theory, "what his name is", or even how aesthetically appealing the theory may be—"if it disagrees with experiment, it's wrong." String theory's perdiction of cosmological constant is wrong. (55 orders of magnitude) In fact, some would argue string theory is not even a theory given that even if the amazing particle accelerators which would supposedly show evidence of the smaller dimensions could be built, string theorists still don't predictions on what exactly would be seen. Please see http://www.math.columbia.edu/%7Ewoit/strings.pdf ==Question about time== Time is not a dimension, right? Right? Can somebody please explain, when I search information some sites tell me that time is the fourth dimension and some tell that it's not a dimension. Is it a matter of opinion until we understand a little more about how the world works? :It is the perspective of essentially all theoretical physics since Einstein that time is a dimension. However, you should probably find an actual discussion forum if you want a more complete explanation (perhaps you could try the Usenet group "sci.physics", which you can access through [http://groups.google.com/ Google Groups]); this page is only intended for discussion of how to improve the content of the article on String theory.--User:Steuard 20:52, Apr 21, 2005 (UTC) ==garden hose thing== thank god. without the garden hose thing i'd have been lost. User:70.177.90.39 03:06, 11 May 2005 (UTC) == Question == Recently someone made a new theory based on the sting theory and the kazula klein theory, where everything is explained only using flexion of the space. I cant remember who this is. Does anybody know this theory? == How does string theory explain matter/anti-matter anihilation? == As a non-physicist following string theory through Scientific American and Nova, one question I've always wanted to ask about string theory is "How does string theory describe the anihilation of matter and anti-matter". I would expect that there must be some mathematical symetry between the vibration mode of a string that results in an electron vs. that of a string that results in a positron. When two such vibrating strings collide, just what happens? Thanks, Mark Z. == Extra Dimentions == I read with interest the theory that extra spatial dimentions exist. However, this view is at variance with one expressed by Professor Stephen Hawking, who remarks that "if there were more than three spatial dimentions, the orbits of planets around the sun or electrons around a nucleus would be unstable and they would tend to spiral inward" --Derek R Crawford. :That description of gravity would certainly be accurate if the extra spatial dimensions were "accessible" to us in the same way that the usual three are: the (approximate) force of gravity in D spatial dimensions falls off at large radius r as r^-(D-1). (I suspect that the comments about electron orbits aren't as relevant once quantum mechanics gets involved: QM doesn't allow for "spiraling inwards" at that scale.) But we already know that any extra dimensions would have to be different somehow, because we can't see them. :The most common explanation in string theory is that the extra dimensions are "curled up" in small circles (or some other compact space). I've tried to explain that concept a little bit on [http://www.slimy.com/~steuard/research/MITClub2004/slide09.html this slide] in an overview of string theory that I wrote. The slide compares a human being's perception of a tightrope to an ant's: the person can move in only one dimension, but the ant can move in two. :If that were the case, then measurements on very small scales in our universe ''would'' see gravity falling off faster than r^-2. But once the radius of the extra dimension became small compared to r, we would be back to r^-2 again (you can imagine that the small circle has "filled up" with gravitational field: there's nowhere else for it to dissipate in that direction). Last I heard, direct measurements have only verified that r^-2 gravity holds down to about 0.1 mm (which is quite large, when you think about it). So there's lots of room for the extra dimensions required by string theory.--User:Steuard 19:41, Jun 21, 2005 (UTC)
String theory
String theory is a branch of theoretical physics that attempts to build a theory of quantum gravity using one-dimensional strings rather than zero-dimensional point particles as fundamental building blocks. The name ''string theory'' is somewhat of a misnomer since the modern theory also includes higher dimensional objects known as branes. :''For more information, please read the main article about String theory.'' Quantum field theory Theoretical physicsGravity Protoscience vi:Category:Lý thuyết dây
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