Figure 1
Illustration: (Left) Alan Stonebraker

Figure 1: (Left) Andronikashvili torsional oscillator. Viscous fluid is dragged by the moving disks, increasing the moment of inertia and reducing the oscillation frequency. In the superfluid phase, there is a frictionless component that does not move with the disks. The decoupling of this mass raises the oscillator frequency, giving a direct measurement of the superfluid density. (Right) What happens when solid helium is plastically deformed? Black curve: torsional oscillator frequency before deforming the helium. The increase at low temperatures suggests decoupling of some of the solid helium, i.e., supersolidity. Blue curve: the expected behavior if the defects produced by deformation enhance the supersolid fraction. Red curve: the behavior if the effect of the new defects is to elastically soften the crystal.