Quantum science allows for collectivebehavior that runs counter to human intuition. For example, at very low temperatures helium-4 atoms, in their wavelike manifestation, can begin to overlap.

When this happens the atoms are indistinguishable and indeed constitute a single quantum state. In this state liquid helium-4 will flow without friction. Comparably chilled, quantum-condensed dilute gases (Bose-Einstein condensates, or BEC) also exhibit superfluid behavior.

What about solids? Can they "flow" without friction? Last year Moses Chan (Penn State) announced the results of an experiment in which solid helium-4 was revolved like a merry-go-round. It appeared that when the bulk was revolved at least part of the solid remained stationary. In effect part of the solid was passing through the rest of the solid without friction.

Chan interpreted this to mean that a fraction of the sample had become superfluid (see www.aip.org/pnu/2004/split/669-1.html and www.aip.org/pnu/2004/split/699-2.html). Now, Chan sees evidence for superfluid behavior in solid hydrogen as well.

Speaking at this week's meeting of the American Physical Society (APS) in Los Angeles, Chan said that his hydrogen results are preliminary and that further checks are needed to be made before ruling out alternative explanations. The concept of what it means to be a solid, Chan said, needs to be re-examined.