Introduction to M-theory: Difference between revisions

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{{Short description|Candidate "Theory of Everything"}}

{{Introductory article|M-theory}}

{{StringIntroductory article|M-theory|cTopic=TheoryString theory}}

{{multiple|

{{inlinerefs|date=August 2013}}

{{More citations needed|date= December 2018}}

{{Missing information|article|actual introductory technical information that goes beyond history and popular science content|date=November 2023}}

In non-technical terms, '''M-theory''' presents an idea about the basic substance of the [[universe]].

}}

{{String theory|cTopic= Theory}}

In non-technical terms, [[M-theory]] presents an idea about the basic substance of the [[universe]]. Although a complete mathematical formulation of M-theory is not known, the general approach is the leading contender for a universal "[[Theory of Everything]]" that unifies gravity with other forces such as [[electromagnetism]]. M-theory aims to unify [[introduction to quantum mechanics|quantum mechanics]] with [[introduction to general relativity|general relativity]]'s gravitational force in a mathematically consistent way. In comparison, other theories such as [[loop quantum gravity]] are considered by [[physicist]]s and researchers to be less elegant, because they posit [[gravity]] to be completely different from forces such as the electromagnetic force.<ref name=atlantic>{{cite news|last1= Wolchover|first1= Natalie|title= The Best Explanation for Everything in the Universe|url= https://www.theatlantic.com/science/archive/2017/12/string-theory-everything/548774/|access-date= 7 February 2018|work= The Atlantic|date= December 2017|archive-date= 15 November 2020|archive-url= https://web.archive.org/web/20201115210213/https://www.theatlantic.com/science/archive/2017/12/string-theory-everything/548774/|url-status= live}}</ref><ref name=quanta>{{cite news|title= Physicists and Philosophers Debate the Boundaries of Science {{!}} Quanta Magazine|url= https://www.quantamagazine.org/physicists-and-philosophers-debate-the-boundaries-of-science-20151216|access-date= 7 February 2018|work= Quanta Magazine|date= 16 December 2015|archive-date= 15 November 2020|archive-url= https://web.archive.org/web/20201115210250/https://www.quantamagazine.org/physicists-and-philosophers-debate-the-boundaries-of-science-20151216|url-status= live}}</ref><ref>{{cite news

|last1= Devlin

|first1= Hannah

|title= Tying loose ends? Gravitational waves could solve string theory, study claims

|url= https://www.theguardian.com/science/2017/jul/05/gravitational-waves-string-theory

|access-date= 7 February 2018

|work= The Guardian

|date= 5 July 2017

|language= en

|archive-date= 15 November 2020

|archive-url= https://web.archive.org/web/20201115210235/https://www.theguardian.com/science/2017/jul/05/gravitational-waves-string-theory

|url-status= live

}}</ref>

==Background==

In the early years of the 20th century, the [[atom]]&nbsp;– long believed to be the smallest building-block of [[matter]]&nbsp;– was proven to consist of even smaller components called [[protons]], [[neutrons]] and [[electrons]], which are known as [[subatomic particles]]. BeginningOther insubatomic theparticles 1960s,began otherbeing subatomicdiscovered particlesin werethe discovered1960s. In the 1970s, it was discovered that protons and neutrons (and other [[hadron]]s) are themselves made up of smaller particles called [[quarks]]. The [[Quantum mechanics|QuantumStandard theoryModel]] is the set of rules that describes the interactions of these particles.

In the 1980s, a new mathematical model of [[theoretical physics]], called [[string theory]], emerged. It showed how all the particles,different andsubatomic allparticles ofknown theto forms of energy in the universe,science could be constructed by hypothetical one-dimensional "strings", infinitesimal building-blocks that have only the dimension of length, but not height noror width. Further,These stringstrings theoryvibrate suggestedin thatmultiple thedimensions universeand, isdepending madeon uphow ofthey multiplevibrate, [[dimension]]s.they Height,might width,be andseen length constitutein three-dimensional space, andas timematter, giveslight aor totalgravity. ofIn fourstring observable dimensions; howevertheory, string theories initially supported theevery possibilityform of tenmatter dimensionsis – the remaining six of which we cannot detect directly. This was later increasedsaid to 11be dimensionsthe based on various interpretationsresult of the 10-dimensional theory that led to five partial theories as described below. Super-gravity theory also played a significant part in establishing the necessityvibration of the 11th dimensionstrings.

However, for string theory to be mathematically consistent, the strings must live in a universe with ten [[dimension]]s. String theory explains our perception of the universe to have four dimensions (three space dimensions and one time dimension) by imagining that the extra six dimensions are "curled up", to be so small that they can't be observed day-to-day. The technical term for this is ''[[Compactification (physics)|compactification]].'' These dimensions are usually made to take the shape of mathematical objects called [[Calabi–Yau manifold]]s.

These "strings" vibrate in multiple dimensions, and depending on how they vibrate, they might be seen in three-dimensional space as matter, light, or gravity. It is the vibration of the string which determines whether it appears to be matter or energy, and every form of matter or energy is the result of the vibration of strings.

Five major string theories were developed and found to be mathematically consistent with the principle of all matter being made of strings. Having five different versions of string theory was seen as a puzzle.

String theory, as mentioned above, ran into a problem: another version of the equations was discovered, then another, and then another. Eventually, there were five major string theories. The main differences between each theory were principally the number of dimensions in which the strings developed, and their characteristics (some were open loops, some were closed loops, etc.). Furthermore, all these theories appeared to be correct. Scientists were not comfortable with five seemingly contradictory sets of equations to describe the same thing.

InSpeaking 1994,at athe string[[Strings theorist(conference)|Strings]] named'95 conference at the University of Southern California, [[Edward Witten]] of the [[Institute for Advanced Study]] and other important researchers [[second superstring revolution|consideredsuggested that]] that the five different versions of string theory might be describing the same thing seen from different perspectives.<ref>{{Cite Theyweb proposed|url=http://physics.usc.edu/Strings95/program.html a|title=University unifyingof theorySouthern calledCalifornia, "M-theory"Los Angeles, inFuture whichPerspectives thein "M"String isTheory, notMarch specifically13-18, defined1995, butE. isWitten: generallySome understoodproblems toof standstrong forand "membrane".weak Thecoupling words|access-date=2017-04-08 "matrix",|archive-date=2020-11-15 "master",|archive-url=https://web.archive.org/web/20201115210233/http://physics.usc.edu/Strings95/program.html "mother",|url-status=live "monster",}}</ref> "mystery",He "magic"proposed havea alsounifying beentheory claimed.called "[[M-theory]]", which brought all of the string theories together. It did this by asserting that strings are reallyan 1-dimensional slicesapproximation of acurled-up 2two-dimensional membranemembranes vibrating in an 11-dimensional space[[spacetime]]. According to Witten, the ''M'' could stand for "magic", "mystery", or "membrane" according to taste, and the true meaning of the title should be decided when a better understanding of the theory is discovered.<ref>{{cite journal |last=Duff |first=Michael |date=1996 |author-link=Michael Duff (physicist) |title=M-theory (the theory formerly known as strings) |journal=International Journal of Modern Physics A |volume=11 |issue=32 |pages=6523–41 |bibcode=1996IJMPA..11.5623D |doi=10.1142/S0217751X96002583 |arxiv = hep-th/9608117 |s2cid=17432791 }}</ref>

==Status==

M-theory is not complete, and the mathematics of the approach are not yet well understood. M-theory is a theory of quantum gravity; and as all others it has not gained experimental evidence that would confirm its validity.<ref name=atlantic/> It also does not single out our observable universe as being special, and so does not aim to predict from first principles everything we can measure about it.

M-theory is not complete, but the underlying structure of the mathematics has been established and is in agreement with all the string theories. Furthermore, it has passed many tests of internal mathematical consistency.

Nevertheless, some physicists are drawn to M-theory because of its degree of uniqueness and rich set of mathematical properties, triggering the hope that it may describe our world within a single framework.

Until some way is found to observe the yet hypothetical higher dimensions, which are needed for consistency reasons, M-theory has a very difficult time making predictions that can be tested in a laboratory.{{Citation needed|date=December 2011}} Technologically, it may never be possible for it to be experimentally confirmed.{{Citation needed|date=December 2011}}

One feature of M-theory that has drawn great interest is that it naturally predicts the existence of the [[graviton]], a [[Spin (physics)|spin-2]] particle hypothesized to mediate the gravitational force. Furthermore, M-theory naturally predicts a phenomenon that resembles [[black hole evaporation]]. Competing unification theories such as [[asymptotically safe gravity]], [[An Exceptionally Simple Theory of Everything|E8 theory]], [[noncommutative geometry]], and [[causal fermion systems]] have not demonstrated any level of mathematical consistency.

Some [[cosmologists]] are drawn to M-theory because of its mathematical elegance and relative simplicity. Physicist and author [[Michio Kaku]] has remarked that M-theory may present us with a "[[Theory of Everything]]" which is so concise that its underlying formula would fit on a T-shirt.<ref>

{{cite web

|last1=Kaku |first1=M.

|author1-link=Michio Kaku

|year=

|title=M-Theory: The Mother of all SuperStrings

|url=http://mkaku.org/home/?page_id=262

|accessdate=2013-08-06

}}</ref> [[Stephen Hawking]] originally believed that M-theory may be the ultimate theory but later suggested that the search for understanding of mathematics and physics will [[Gödel's incompleteness theorems|never be complete]].<ref>

{{cite web

|last1=Hawking |first1=S.

|author1-link=Stephen Hawking

|date=3 October 2003

|url=http://www.damtp.cam.ac.uk/strings02/dirac/hawking/

|title=Gödel and the end of physics

|accessdate=2013-08-06

}}</ref>

[[Stephen Hawking]] and [[Leonard Mlodinow]], in the popular scientific book ''[[The Grand Design (book)|The Grand Design]]'', take a philosophical position to support a view of the universe as a [[multiverse]], and define it in the book as [[model-dependent realism]] which along with a sum-over-histories approach (see [[Path integral formulation]] of [[Quantum mechanics]]) to the universe as a whole, is used to claim that M-theory is the only candidate for a complete theory of the universe.

==See also==

* [[SuperstringHistory of string theory]]

==References==

{{Reflistreflist|02}}

==Further reading==

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|last=Greene |first=B.

|author1-link=Brian Greene

|yeardate=1999

|title=[[The Elegant Universe|The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory]]

|publisher=[[W.W. Norton]]

|isbn=978-0-375-70811-14

|title-link=The Elegant Universe

}}

}}

*{{cite book

* {{cite webbook

|last=Greene |first=B.

|yeardate=2004

|title=[[The Fabric of the Cosmos|The Fabric of the Cosmos: Space, Time, and the Texture of Reality]]

|publisher=[[Alfred A. Knopf]]

|isbn= 978-0-375-41288-32

|title-link=The Fabric of the Cosmos

}}

|bibcode=2004fcst.book.....G

*{{cite journal

}}

* {{cite bookjournal

|last1=Miemic |first1=A.

|last2=Schnakenburg |first2=I.

|yeardate=2006

|title=Basics of M-theory

|journal=[[Fortschritte der Physik]]

|volume=54 |issue=1 |pages=5-725–72

|arxiv=hep-th/0509137

|bibcode=2006ForPh..54....5M

|doi=10.1002/prop.200510256

|s2cid=98007313

}}

}}

*{{cite book

* {{cite webbook

|last=Musser |first=G.

|authorlinkauthor-link=George Musser

|yeardate=2008

|title=The Complete Idiot's Guide to String Theory

|publisher=[[Alpha Books]]

|isbn=978-1-59257-702-6

}}

* {{cite book

|last1=Smolin |first1=L.

|author1-link=Lee Smolin

|yeardate=2006

|title=[[The Trouble with Physics]]

|publisher=[[Houghton Mifflin]]

|isbn=978-0-618-55105-7

|title-link=The Trouble with Physics

}}

}}

* {{cite book

|last1=Woit
|first1=P.

|author1-link=Peter Woit

|yeardate=2006

|title=Not Even Wrong: The Failure of String Theory and the Continuing Challenge to Unify the Laws of Physics

|isbn=978-0-465-09275-68

|publisher=[[Basic Books]]

|url-access=registration

}}

|url=https://archive.org/details/notevenwrongfail00woit

}}

==External links==

{{Spoken Wikipedia|En-Introduction_to_M-theory-article.ogg|date=2014-11-06}}

* [httphttps://www.pbs.org/wgbh/nova/series/the-elegant-universe/ The Elegant Universe] -– A Threethree-Hourhour miniseries with [[Brian Greene]] by NOVA (original PBS Broadcast Dates: October 28, 8-10 p.m. and November 4, 8-9 p.m., 2003). Various images, texts, videos and animations explaining string theory and M-theory.

*[http://superstringtheory.com/ Superstringtheory.com] - The "Official String Theory Web Site", created by Patricia Schwarz. Excellent references on string theory and M-theory for the layperson and expert.

{{String theory topics |state=collapsed}}

{{Introductory article|M-theoryscience articles}}

{{DEFAULTSORT:Introduction To M-Theory}}

[[Category:TheoreticalPhilosophy physicsof science]]

[[Category:Physical cosmology]]

[[Category:String theory]]