Editing String theory (section)

== Criticism ==

=== Number of solutions ===
{{main|String theory landscape}}

To construct models of particle physics based on string theory, physicists typically begin by specifying a shape for the extra dimensions of spacetime. Each of these different shapes corresponds to a different possible universe, or "vacuum state", with a different collection of particles and forces. String theory as it is currently understood has an enormous number of vacuum states, typically estimated to be around {{math|10<sup>500</sup>}}, and these might be sufficiently diverse to accommodate almost any phenomenon that might be observed at low energies.<ref>[[#Woit|Woit]], pp. 240–242</ref>

Many critics of string theory have expressed concerns about the large number of possible universes described by string theory. In his book ''Not Even Wrong'', [[Peter Woit]], a lecturer in the mathematics department at [[Columbia University]], has argued that the large number of different physical scenarios renders string theory vacuous as a framework for constructing models of particle physics. According to Woit,

{{quote|The possible existence of, say, {{math|10<sup>500</sup>}} consistent different vacuum states for superstring theory probably destroys the hope of using the theory to predict anything. If one picks among this large set just those states whose properties agree with present experimental observations, it is likely there still will be such a large number of these that one can get just about whatever value one wants for the results of any new observation.<ref name="Woit 2006, p. 242">[[#Woit|Woit]], p. 242</ref>}}

Some physicists believe this large number of solutions is actually a virtue because it may allow a natural anthropic explanation of the observed values of [[physical constant]]s, in particular the small value of the cosmological constant.<ref name="Woit 2006, p. 242"/> The [[anthropic principle]] is the idea that some of the numbers appearing in the laws of physics are not fixed by any fundamental principle but must be compatible with the evolution of intelligent life. In 1987, [[Steven Weinberg]] published an article in which he argued that the cosmological constant could not have been too large, or else [[galaxy|galaxies]] and intelligent life would not have been able to develop.<ref name=Weinberg/> Weinberg suggested that there might be a huge number of possible consistent universes, each with a different value of the cosmological constant, and observations indicate a small value of the cosmological constant only because humans happen to live in a universe that has allowed intelligent life, and hence observers, to exist.<ref>[[#Woit|Woit]], p. 243</ref>

String theorist Leonard Susskind has argued that string theory provides a natural anthropic explanation of the small value of the cosmological constant.<ref name=Susskind2005/> According to Susskind, the different vacuum states of string theory might be realized as different universes within a larger [[multiverse]]. The fact that the observed universe has a small cosmological constant is just a tautological consequence of the fact that a small value is required for life to exist.<ref>[[#Woit|Woit]], pp. 242–243</ref> Many prominent theorists and critics have disagreed with Susskind's conclusions.<ref>[[#Woit|Woit]], p. 240</ref> According to Woit, "in this case [anthropic reasoning] is nothing more than an excuse for failure. Speculative scientific ideas fail not just when they make incorrect predictions, but also when they turn out to be vacuous and incapable of predicting anything."<ref>[[#Woit|Woit]], p. 249</ref>

=== Compatibility with dark energy ===
It remains unknown whether string theory is compatible with a metastable, positive [[cosmological constant]].
Some putative examples of such solutions do exist, such as the model described by Kachru ''et al''. in 2003.<ref>{{cite journal |last1=Kachru |first1=Shamit |last2=Kallosh |first2=Renata |last3=Linde |first3=Andrei |last4=Trivedi |first4=Sandip P. |title=de Sitter Vacua in String Theory |journal=[[Phys. Rev. D]] |date=2003 |volume=68 |issue=4 |page=046005 |doi=10.1103/PhysRevD.68.046005 |arxiv=hep-th/0301240 |bibcode=2003PhRvD..68d6005K |s2cid=119482182 | issn=0556-2821 }}</ref> In 2018, a group of four physicists advanced a controversial conjecture which would imply that [[Swampland (physics)|no such universe exists]]. This is contrary to some popular models of [[dark energy]] such as [[Lambda-CDM model|Λ-CDM]], which requires a positive vacuum energy. However, string theory is likely compatible with certain types of [[quintessence (physics)|quintessence]], where dark energy is caused by a new field with exotic properties.<ref>{{cite web |last1=Wolchover |first1=Natalie |title=Dark Energy May Be Incompatible With String Theory |url=https://www.quantamagazine.org/dark-energy-may-be-incompatible-with-string-theory-20180809/ |website=[[Quanta Magazine]] |publisher=Simons Foundation |access-date=2 April 2020 |date=9 August 2018 |archive-date=15 November 2020 |archive-url=https://web.archive.org/web/20201115210807/https://www.quantamagazine.org/dark-energy-may-be-incompatible-with-string-theory-20180809/ |url-status=live }}</ref>

=== Background independence ===
{{main|Background independence}}

One of the fundamental properties of Einstein's general theory of relativity is that it is [[background independence|background independent]], meaning that the formulation of the theory does not in any way privilege a particular spacetime geometry.<ref>[[#Smolin|Smolin]], p. 81</ref>

One of the main criticisms of string theory from early on is that it is not manifestly background-independent. In string theory, one must typically specify a fixed reference geometry for spacetime, and all other possible geometries are described as perturbations of this fixed one. In his book ''[[The Trouble With Physics]]'', physicist [[Lee Smolin]] of the [[Perimeter Institute for Theoretical Physics]] claims that this is the principal weakness of string theory as a theory of quantum gravity, saying that string theory has failed to incorporate this important insight from general relativity.<ref>[[#Smolin|Smolin]], p. 184</ref>

Others have disagreed with Smolin's characterization of string theory. In a review of Smolin's book, string theorist Joseph Polchinski writes

{{quote|[Smolin] is mistaking an aspect of the mathematical language being used for one of the physics being described. New physical theories are often discovered using a mathematical language that is not the most suitable for them... In string theory, it has always been clear that the physics is background-independent even if the language being used is not, and the search for a more suitable language continues. Indeed, as Smolin belatedly notes, [AdS/CFT] provides a solution to this problem, one that is unexpected and powerful.<ref name="Polchinski 2007"/>}}

Polchinski notes that an important open problem in quantum gravity is to develop holographic descriptions of gravity which do not require the gravitational field to be asymptotically anti-de Sitter.<ref name="Polchinski 2007" /> Smolin has responded by saying that the AdS/CFT correspondence, as it is currently understood, may not be strong enough to resolve all concerns about background independence.<ref>{{cite web|url=http://www.kitp.ucsb.edu/joep/links/on-some-criticisms-of-string-theory/lee-smolins-response |title=Response to review of The Trouble with Physics by Joe Polchinski|access-date=December 31, 2015 |url-status=dead |archive-url=https://web.archive.org/web/20151105234427/http://www.kitp.ucsb.edu/joep/links/on-some-criticisms-of-string-theory/lee-smolins-response |archive-date=November 5, 2015 |author=Smolin, Lee |date=April 2007|publisher=kitp.ucsb.edu}}</ref>

=== Sociology of science ===

Since the superstring revolutions of the 1980s and 1990s, string theory has been one of the dominant paradigms of high energy theoretical physics.<ref>[[#Penrose|Penrose]], p. 1017</ref> Some string theorists have expressed the view that there does not exist an equally successful alternative theory addressing the deep questions of fundamental physics. In an interview from 1987, [[Nobel laureate]] [[David Gross]] made the following controversial comments about the reasons for the popularity of string theory:

{{quote|The most important [reason] is that there are no other good ideas around. That's what gets most people into it. When people started to get interested in string theory they didn't know anything about it. In fact, the first reaction of most people is that the theory is extremely ugly and unpleasant, at least that was the case a few years ago when the understanding of string theory was much less developed. It was difficult for people to learn about it and to be turned on. So I think the real reason why people have got attracted by it is because there is no other game in town. All other approaches of constructing grand unified theories, which were more conservative to begin with, and only gradually became more and more radical, have failed, and this game hasn't failed yet.<ref>[[#Woit|Woit]], pp. 224–225</ref>}}

Several other high-profile theorists and commentators have expressed similar views, suggesting that there are no viable alternatives to string theory.<ref>[[#Woit|Woit]], Ch. 16</ref>

Many critics of string theory have commented on this state of affairs. In his book criticizing string theory, Peter Woit views the status of string theory research as unhealthy and detrimental to the future of fundamental physics. He argues that the extreme popularity of string theory among theoretical physicists is partly a consequence of the financial structure of academia and the fierce competition for scarce resources.<ref>[[#Woit|Woit]], p. 239</ref> In his book ''[[The Road to Reality]]'', mathematical physicist [[Roger Penrose]] expresses similar views, stating "The often frantic competitiveness that this ease of communication engenders leads to [[bandwagon effect]]s, where researchers fear to be left behind if they do not join in."<ref>[[#Penrose|Penrose]], p. 1018</ref> Penrose also claims that the technical difficulty of modern physics forces young scientists to rely on the preferences of established researchers, rather than forging new paths of their own.<ref>[[#Penrose|Penrose]], pp. 1019–1020</ref> Lee Smolin expresses a slightly different position in his critique, claiming that string theory grew out of a tradition of particle physics which discourages speculation about the foundations of physics, while his preferred approach, [[loop quantum gravity]], encourages more radical thinking. According to Smolin,

{{quote|String theory is a powerful, well-motivated idea and deserves much of the work that has been devoted to it. If it has so far failed, the principal reason is that its intrinsic flaws are closely tied to its strengths—and, of course, the story is unfinished, since string theory may well turn out to be part of the truth. The real question is not why we have expended so much energy on string theory but why we haven't expended nearly enough on alternative approaches.<ref>[[#Smolin|Smolin]], p. 349</ref>}}

Smolin goes on to offer a number of prescriptions for how scientists might encourage a greater diversity of approaches to quantum gravity research.<ref>[[#Smolin|Smolin]], Ch. 20</ref>