Editing String theory (section)

=== 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>