it hits a specified target on the moon with a margin of error no greater than the thickness of an amoeba.’ 16 Many physicists find this ‘unnatural and abhorrent’ 17 , as it lacks a physical mechanism for the alteration of the parameters, and believe a theory of everything should allow calculations that predict these values itself rather than relying on measurement of values and fine-tuning of parameters. However, supersymmetry can solve the hierarchy problem without unnatural fine-tuning; the Higgs boson interacts with virtual super-partners, which provide quantum corrections and stabilizes the Higgs at a lower mass. 18 The observed mass of 125 GeV would in fact be in the range some supersymmetric theories predict, providing some indirect evidence for those theories. 19 A second problem supersymmetry could solve is to unify the three non-gravitational forces. If the strengths of these forces are extrapolated back to very high energies, as were present in the early universe, the strengths converge to nearly a single value. In 1974, Steven Weinberg showed that in fact the forces would not converge to exactly the same point but unify with each other at different points, never becoming one unified force. 20 Many physicists again found it hard to believe that the universe would create forces that were almost the same strength in the early universe but did not quite unify by a tiny amount; they had an intuition that the forces would start from the Big Bang as the same force but would separate as the universe cooled. However, add in supersymmetry and the small discrepancy in strengths disappears and the three forces become one strength at high energies. The forces unify with supersymmetry as the new super-partners provide additional quantum fluctuations; this causes the strengths of the forces to become the same with amazing accuracy. 21 Weinberg said, ‘If you have two curves, it's not surprising that they intersect somewhere but if you have three curves that intersect at the same point, then that's not trivial’ 22 , showing that he considers this unification to be more than a coincidence; force unification therefore provides another reason for believing in supersymmetry. The next physics beyond the Standard Model is dark matter. Physicists can measure how much mass they can observe in galaxies and calculate how fast the galaxies should rotate. However, they found that galaxies rotate faster than they predicted so there must be more matter present than we can observe; the extra matter must therefore not emit, absorb or interact with electromagnetic radiation. Physicists now know dark matter makes up 85% of matter in the universe and believe it to be mostly made up of weakly interacting massive particles (WIMPs). 23 The Standard Model does not have any particles that we believe to contribute significantly to dark matter; although neutrinos do not interact via electromagnetism, they seem to have too small a mass to make them a large part of dark matter. 24 However, supersymmetry could resolve dark matter and once again explain physics beyond the standard model. It predicts the existence of particles called neutralinos, which include photinos, zinos and higgsinos ( Figure 1 ). Neutralinos are a good candidate for WIMPs as they are predicted not to interact with the strong or electromagnetic forces, the lightest neutralinos would be stable and they should be massive enough to account for a significant amount of dark matter. 25 Supersymmetry could also potentially incorporate quantum mechanics and general relativity into one theory of everything. Currently quantum mechanics and general relativity are incompatible, so gravity cannot be unified with the three fundamental forces in the Standard Model. 11-dimensional supergravity was the first supersymmetric theory that incorporated gravity; it showed that using supersymmetry to turn a particle into its super-partner and back was mathematically equivalent to moving the particle through space-time. 26 This connected quantum properties with space-time, producing a theory connecting gravity to quantum mechanics. We cannot observe these extra
16 (Greene, 1999, pp. 174-175) 17 (Ananthaswamy, In SUSY we trust: What the LHC is really looking for, 2009) 18 (Baggott, 2012) 19 (Castelvecchi, 2012) 20 (Ananthaswamy, In SUSY we trust: What the LHC is really looking for, 2009) 21 (Greene, 1999) 22 (Ananthaswamy, In SUSY we trust: What the LHC is really looking for, 2009) 23 (Powell, 2013) 24 (Rees, 1999) 25 (Baggott, 2012) 26 (Duff, 2011)
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