This morning I sat in a room that was full of confusion. The small group of physicists gathered for the session on “supersolids” was thoroughly perplexed.

Two years ago, researchers from Penn State University reported the first evidence for a supersolid – a strange new form of matter which is a solid but which, through quantum effects, can flow like a liquid.

Today we learnt that other groups have now repeated the result, but their experiments have also raised new questions.

In the original experiments, a team led by Moses Chan showed that a block of solid helium chilled to within a whisker of absolute zero – around 50 milliKelvin – started to behave a bit like a superfluid. As the helium was rotated, some of the material (about 1%) appeared to decouple, flowing through the solid so that it stayed still as the rest of the structure turned. The results of these experiments were published in Nature and Science .

At this meeting, it was reported that three other teams have repeated the rotation experiments, seeing a smaller but similar effect to Chan’s group. This seems to confirm the existence of the supersolid state.

However, some other types of experiment have not been able to detect supersolidity. John Beamish of the University of Alberta in Canada had failed to persuade solid helium to show any “supersolid” properties when it was put under pressure .

What does it mean? Does it make the evidence for supersolidity less solid?

I cornered Beamish after the talk to find out. Intuitively, said Beamish, you might expect a solid that can flow to do so when you squeeze it, but he points out that rotation has a special status in quantum mechanics. Maybe the rotation is essential for supersolid behaviour.

Another set of experiments, carried out by John Reppy of Cornell University in Ithaca, New York and his colleagues, suggested that the supersolid effect seen during rotation disappeared altogether if the helium crystal was annealed – meaning that it was kept at a temperature just below its melting point.

Annealing tends to eliminate any defects in the crystal, so these results suggest that defects might underpin supersolidity. But the data are brand new (presented in a talk that wasn’t on the schedule) and there are some discrepancies with Chan’s results which the two teams will have to hammer out.

Whatever is happening in the “supersolid” helium, theorists can’t yet explain it. “I don’t think people understand at all what’s going on,” the session chair told me.

Thankfully, there’s nothing like a mystery to motivate more research – let’s give them until next year’s APS meeting to work it out.