Quantum Criticality in an Ising Chain: Experimental Evidence for Emergent E8 Symmetry

It is not often that an exact theory can describe a many-particle quantum-mechanical system, but one of the few exceptions is the behavior of a string of ferromagnets—an Ising chain—at magnetic field strengths that separate different types of ordering. Its excitations were predicted 20 years ago to be governed by the symmetry group E8, one of the most intriguing objects in mathematics. Now, Coldea et al. (p. [177][1]) report direct experimental confirmation of this result in a quasi-one-dimensional Ising ferromagnet CoNb2O6, which they probed by neutron scattering. Two of the eight predicted excitations could be observed. Moreover, the ratio of the two lowest excitations is in quantitative agreement with the so-called “golden ratio” predicted by theory. [1]: /lookup/doi/10.1126/science.1180085