Since the 1970s, scientists have believed that helium, when extremely cold, can become a "supersolid" -- taking on characteristics of both a frictionless liquid (a superfluid) and a crystalline solid. But recent Cornell research says this might be science fiction.

John Reppy, the John Wetherill Professor of Physics Emeritus, reports in a recent edition of Physical Review Letters (June 25) that "evidence" for supersolid helium may have been premature. His work is also the subject of a "Viewpoint" in the journal Physics.

"We do know a lot," Reppy said. "But then something like this happens where nature is much more complex, even with very simple systems, than the human mind likes to think."

Reppy challenges experiments by Pennsylvania State University scientists published in 2004, in which they filled a hollow device called a torsional oscillator with helium cooled to near absolute zero (-273 degrees Celsius). They reported a change in the rate of oscillation as the helium got colder, leading them to conclude that some of the helium particles detached from the walls of the oscillator and began sliding around each other freely. This was evidence of supersolid helium, they said.

Reppy, who has studied the quantum properties of helium isotopes for decades, argues that this change in the oscillation rate was caused by the helium deforming -- not the helium becoming a supersolid. He places a new spin on the theory: Helium may actually be softening at high temperatures, rather than hardening at low temperatures.

"Basically what you're seeing is a high-temperature phenomenon that disappears at low temperatures, rather than some new phenomenon that appears at low temperatures," Reppy explained.

The research was funded by the National Science Foundation and the NSF-supported Cornell Center for Materials Research.