The unknowns of cognitive enhancement Can science and policy catch up with practice ?

23 OCTOBER 2015 • VOL 350 ISSUE 6259 379 crystal momentum space. This, however, is not important, as a similar effect should be observed in Weyl semimetals, where opposite-chirality fermions exist at distinct points in momentum space. The way that the chiral anomaly manifests in Na 3 Bi is through magnetoresistance (a dependence of the electrical resistance of the material on an applied magnetic field). The physical picture of the chiral anomaly, when applied to a Dirac or Weyl semimetal, implies a magnetic field– dependent contribution to the resistance, which is negative (the resistance is reduced and the material becomes a better conductor when the magnetic field is applied) and quadratic in the field (10, 11). The effect also exists only when the current is aligned with the direction of the field (the magnetoresistance is longitudinal), survives up to a temperature of about 90 K, and is large (quickly rising to more than 100% as the temperature decreases below 90 K). These features are unusual and cannot be explained by any other known mechanism but the chiral anomaly. What makes the observed effect important, apart from the analogy to particle physics, is that the chiral anomaly is a purely quantum mechanical phenomenon without any clas-