Introduction to M-theory: Difference between revisions

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

{{Introductory article|M-theory|String theory}}

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

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

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

{{String theory|cTopic= Theory}}

In non-technical terms, [[M-theory]] presents an idea about the basic substance of the [[universe]]. As of 2023, science has produced no experimental evidence to support the conclusion that M-theory is a description of the real world. 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/students 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

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In the 1980s, a new mathematical model of [[theoretical physics]], called [[string theory]], emerged. It showed how all the different subatomic particles known to science could be constructed by hypothetical one-dimensional "strings", infinitesimal building-blocks that have only the dimension of length, but not height or width. 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. In string theory, every form of matter is said to be the result of the vibration of strings.

However, for string theory to be mathematically consistent, the strings must belive in a universe ofwith ten [[dimension]]s. ThisString theory opposesexplains our perception of the universe to have four dimensions: (three space dimensions (height, width, and length) and one time dimension. String theorists explained the fact that only 4 dimensions are perceived, as opposed to ten,) by sayingimagining that the extra six dimensions wereare "curled up", to be so small that they could notcan't be observed day-to-day. The technical term for this is ''[[Compactification (physics)|compactification]].'' PhysicistsThese speculateddimensions thatare theusually compactifiedmade dimensionsto tooktake the shape of mathematical objects called [[Calabi–Yau manifold]]s.

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 described above ran into a problem: another version of the equations was discovered, then another, and then another. Eventually, five major string theories were developed. The main differences between the theories 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 workable. Scientists were not comfortable with five seemingly contradictory sets of equations to describe the same thing.

Speaking at the string[[Strings theory(conference)|Strings]] '95 conference at the University of Southern California in 1995, [[Edward Witten]] of the [[Institute for Advanced Study]] [[second superstring revolution|suggested that]] the five different versions of string theory might be describing the same thing seen from different perspectives.<ref>{{Cite web |url=http://physics.usc.edu/Strings95/program.html |title=University of Southern California, Los Angeles, Future Perspectives in String Theory, March 13-18, 1995, E. Witten: Some problems of strong and weak coupling |access-date=2017-04-08 |archive-date=2020-11-15 |archive-url=https://web.archive.org/web/20201115210233/http://physics.usc.edu/Strings95/program.html |url-status=live }}</ref> He proposed a unifying theory called "[[M-theory]]", which brought all of the string theories together. It did this by asserting that strings are reallyan one-dimensional slicesapproximation of acurled-up two-dimensional membranemembranes vibrating in an 11-dimensional [[spacetime]]. According to Witten, the ''M'' could stand for "magic", "mystery", "matrix", 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 mathematics of the approach has been explored in great detail. However, so far no experimental support for the M-theory exists.<ref name=atlantic/> Some physicists are skeptical that this approach will ever lead to a physical theory describing our real world, due to fundamental issues.<ref name="Smolin_Response">[[Lee Smolin]], April 2007:[https://web.archive.org/web/20130621085800/http://www.kitp.ucsb.edu/joep/links/on-some-criticisms-of-string-theory/lee-smolins-response Response to review of ''The Trouble with Physics''] by [[Joe Polchinski]].</ref>

Nevertheless, some [[cosmologists]]physicists are drawn to M-theory because of its mathematicaldegree of [[elegance]]uniqueness and relativerich simplicityset of mathematical properties, triggering the hope that the simplicity is a reason why it may describe our world within a single framework.

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. Another approach to quantum gravity is [[loop quantum gravity]], a non-unifying theory; many physicists consider loop quantum gravity to be less elegant than M-theory because it posits gravity to be completely different from the other fundamental forces.<ref name=atlantic/><ref name=quanta/>

==See also==