A matter of solidity
A material that oozes through itself presents a super physics puzzle
Solids are supposed to be the reliable state of matter.
Gases are flighty and flitting and expand to fill any available space. Liquids will also mold themselves to whatever shape they occupy, from soda bottle to swimming pool. Solids, though, are steadfast and unyielding, stable and dependable, like the rise and fall of tides, the guidance of the North Star or the love of a dog.
But that truism may turn out not to be so true. In the past few years, physicists have learned of a solid that doesn’t adhere to fixed rules. Within this solid — helium at very low temperatures — some atoms appear to leave their rigidly defined positions and begin moving through the rest of the material without friction. In essence, the atoms are simultaneously solid and fluid, and the material slides through itself.