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Subir Sachdev, William Witczak-Krempa, and Erik Sørensen are condensed matter physicists. They study exotic but tangible systems, such as superfluids. And their latest paper about one such system has a black hole in it.

How did a black hole get into a condensed matter paper? "Well, it's a long story," says Sachdev, who is a professor at Harvard and a Distinguished Visiting Research Chair at Perimeter Institute.

It's a long story, he might add, that in a way starts with him: he was one of the first condensed matter physicists to venture into the strange land of string theory, where the black holes live. But that is getting ahead of the tale.

"Let's start here," Sachdev says. "Condensed matter physicists study the behaviour of electrons in many materials -- semiconductors, metals, and exotic materials like superconductors."

Normally, these physicists can model the behaviour of a material as if electrons were moving freely around inside it. Even if that's not what's actually happening, because of complex interactions, it makes the model easy to understand and the calculations easier to do. Electrons (and occasionally other particles) used in this kind of short-hand model are called quasi-particles.

However, there are a handful of systems that cannot be described by considering electrons (or any other kind of quasi-particle) moving around.

"What we try to do is understand a quantum system where you have electricity without electrons," says Sachdev. "Of course, the system does have electrons in it, but the behaviour of the system doesn't look like electrons moving at all. What you see is not even particles, but some lumps of quantum excitations that are doing strange quantum things."

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