Some believe it is a new state of matter. Another school of thought says it is a new state of mind. It is a super solid, theorised long ago and observed in 2004 by scientists at Penn State University in the United States. It is a state of matter different from the three we all know about — solid, liquid and gas — as well as from the few exotic states that have been specially created in the laboratory. However, there were many doubting Thomases in the scientific community. For them, a super solid was still a figment of the imagination.
But now two Canadian scientists have come up with more evidence about the existence of this intriguing matter.
At the University of Alberta, physicist John Beamish and his student James Day cooled helium under such high pressure that it became a solid. This is normal behaviour, expected under simple laws of physics. But when solid helium was cooled to extremely low temperatures, it started to exhibit unusual behaviour. When the scientists subjected it to a shearing force, the supercooled helium seemed far stiffer than expected. This seems to be a trivial observation by ordinary standards, but for physicists it was almost like watching a miracle, except for the fact that they might be able to explain why it happens.
Scientists call it an important breakthrough in understanding matter. The knowledge, like what happened with super fluids, could one day be useful in making commercial products. Beamish and Day published their results in the current issue of the journal Nature.
In technical terms, a super solid is a solid with super fluid properties. A super fluid itself cannot be described so easily. It is a fluid that exhibits bizarre properties. A beaker of super fluid helium, for example, is actually a mixture of normal helium and super fluid helium. The normal helium continues to behave in normal ways, while the super fluid portion has zero viscosity and infinite thermal conductivity, among other things. If you heat a portion of super fluid helium, the heat instantly flows to the other end. If you immerse a capillary tube in it, super fluid helium flows up and out of the beaker. Part of it flows up the sides of the beaker and forms a coating. If you rotate the beaker, the super fluid stands still.
When these properties are transferred to solids, things take a different turn. A solid cannot flow, and so it cannot exhibit some of the properties that we notice in super fluids. However, when cooled to a fraction of a degree above absolute zero under pressure, a portion of helium becomes super solid and loses friction completely. This loss of friction showed up in the original experiment of Moses Chan and colleagues at Penn State. They had spun super cooled crystals of helium like in a merry go round, and found that a part of it behaved as if it was not there. The crystal suddenly became lighter, but the missing mass reappeared when it was slightly heated. This meant that the mass had not actually disappeared but only lost its friction.
Physicists are interested in super solidity because it shows up aspects of nature not seen elsewhere, and are thus difficult to study. For one, the phenomenon forces us to rethink our definitions of solids and liquids, particularly in the bizarre world of quantum mechanics. In fact, it is quantum mechanics that provides an explanation for this phenomenon.
According to this theory, matter can be considered either particles or waves. When seen as waves, it provides a good but still weird explanation of super solidity. When cooled beyond a point, the waves that constitute the atoms of solid helium expand and join hands with one another. Ultimately they become one giant wave, or part of one giant atom.
The discovery by Beamish is all the more important because very few physicists work in this area. No one, for example, does research in this field in India. Even in Canada, Beamish and his team are the only ones doing experimental research in super solids. Irrespective of whether the research has immediate use or not, the scientists seem to have taken one more step in the affirmative in answering the question: do super solids exist?