dimensions in our four-dimensional world as they are curled up to unobservable sizes. 27 However, supergravity turned out not to be a complete theory, as it did not fully describe the weak force. Physicists next looked at uniting supersymmetry and string theory into superstring theory. This is where particles are not point-like but are one-dimensional strings moving in 10-dimensional space- time; superstrings also united quantum mechanics with general relativity and solved the problem with the weak force from supergravity, but it still had problems. 28 Physicists found there were five superstring theories, which all gave mathematically consistent results, and they could not work out which was the correct one. In 1995, Edward Witten showed that the five 10-dimensional superstring theories and 11-dimensional supergravity were in fact all different expressions of the same supersymmetric theory, M-theory; this gave physicists confidence that M-theory could be the theory of everything. 29 Thus, supersymmetry could be necessary to unite quantum mechanics with general relativity and produce a theory of everything. Despite the many attractive properties of supersymmetry, there are also reasons why it may not explain the physics beyond the Standard Model. There is no reason, beyond physicists’ intuition and ideas of beauty, why the universe has to satisfy every possible symmetry or have force unification. Secondly, no direct evidence has ever been found for supersymmetry. The LHC has currently not found evidence for the existence of any super-partners; squarks with masses below 1.1 TeV and gluinos with masses below 500 GeV have been excluded with 95% confidence levels. 30 Different supersymmetric theories predict super-partners with different masses, so some theories expect super-partners more massive than we can observe. However, to explain much of the physics beyond the Standard Model we need to find super-partners with masses not too much more than the mass of the Higgs boson. 31 To be a candidate for dark matter, at least some super-partners cannot be much more than about 300 GeV, which we should have already found. 32 Similarly, to solve the hierarchy problem requires low mass super-partners, with masses around 1 TeV; if the super-partners have greater masses, then we would need to fine-tune various parameters to stabilize the Higgs’ mass, creating an ‘unnatural’ theory, which was what supersymmetry was created to prevent. 33 In addition, the Higgs boson found at the LHC has a mass of 125 GeV, which is too heavy for the simplest supersymmetric models and only predicted by more complicated theories. Finally, supersymmetry does not have to be true to explain the physics beyond the Standard Model; various non-supersymmetric theories could each solve separate problems. 34 Supersymmetry is a beautiful idea, which could amazingly explain much of the physics beyond the Standard Model in a single theory. It could prevent unnatural fine-tuning by naturally stabilizing the Higgs’ mass, unify the forces with high accuracy and explain dark matter. It is also an integral part of M-theory, which could be a theory of everything and unify general relativity with quantum mechanics. I think it is unlikely that one idea could simultaneously explain all of this without being a fundamental property of the universe; however, scientific theories are based on evidence and ultimately with no evidence supersymmetry is just an idea and mathematical equations with no true physical relevance to the universe. I am optimistic that supersymmetry can explain the physics beyond the Standard Model but unless super-partners are found at around 1 TeV then supersymmetric theories cannot be the explanation without being unnaturally fine-tuned themselves. Therefore, the next few years, after the LHC’s 2015 upgrade to higher energies, may be the last chance for natural supersymmetry to prove itself before it must be abandoned.
27 (Hey & Walters, 2003) 28 (Duff, 2011)
29 (Hawking, 2001, pp. 56-57) 30 (CMS Collaboration, 2011) 31 (Lykken & Spiropulu, 2014) 32 (Castelvecchi, 2012) 33 (Ananthaswamy, The truth hurts: LHC breaks supersymmetry's beauty, 2011) 34 (Gefter, 2011)
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