Quantum matter is everywhere, from the interiors ofneutron stars to the electrons in everyday metals. Likeordinary, classical matter, it is made up of many inter-acting particles. In classical matter, however, it is poss-ible to think of each particle as an individual entity,whereas in quantum matter Heisenberg's uncertaintyprinciple prevents us from telling individual particlesapart: their behaviour can only be described collectively.In spite of this, many types of quantum matter are wellunderstood from a theoretical point of view. For exam-ple, the "electron liquid" that is responsible for the flowof electricity through ordinary metals, the magneticproperties of many insulating materials and the normaland superfluid phases of helium at very low tempera-tures have all succumbed to the probing of theorists.
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