States of matter: The unthinkable forms beyond solid, liquid and gas

From time crystals to supersolids, we keep discovering extraordinary new kinds of matter – now the true challenge is being able to predict what we’ll find next

Physics 11 August 2021

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Brett Ryder

THE tenets of physics can seem carved in stone. The speed of light is a constant. There are four fundamental forces. Theoretically, rules like these are open to revision. But new contenders had better come with a chisel and a very big hammer.

You would be forgiven for thinking this confidence also applies to something as fundamental as the different states of matter. As we learned in school, there are three of them: solid, liquid, gas. Right?

Actually, these are only the start. We now know of all sorts of exotic states, from superconductors to Bose-Einstein condensates, quantum spin liquids to topological insulators. The sheer number is as bewildering as their names. Strangely, no one can give you a definitive list: there could be as few as four of them or perhaps thousands.

Sorting this mess out isn’t just a matter of satisfying our curiosity. If we can pin down exactly what constitutes a state of matter, we should be better able to predict and discover new ones. That would not only have great technological benefits, but it could also give us fresh ways to probe the nature of reality.

Such predictive power is central to how physics typically works: we predicted the Higgs boson existed and built a particle collider to find it. But where states of matter are concerned, precision and prediction have eluded us – until now, thanks to the recent discovery of a whole new class of matter.

The rules we are taught at school to define states of matter, based on nothing more complex than shape, seem simple enough. A solid has a fixed shape. …

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