Thursday, February 09, 2017

Physicists Devise "Black Hole" on a Chip

Black holes are one of the best-known and most intriguing concepts in astrophysics. They're places where a literally unstoppable force—usually the domain of philosophers—manifests. They've given rise to countless thought experiments and what-ifs, provided a theoretical tool to probe the nature of our universe, and inspired generations of scientists and science enthusiasts alike to stretch their imaginations to the extreme...and beyond.

But in spite of all that, we have a hard time studying these astronomical behemoths. Being perfectly absorptive, they're hard to spot in space—and when we do find one, it's usually because it's shredding a star or colliding with another black hole, spewing out enormous amounts of energy in the process. These events can teach us a lot, but that kind of violent reaction—along with the enormous distance between here and there—makes it difficult to study some of the more subtle processes that we expect to see from black holes, such as Hawking radiation.

To that end, scientists have long been at work designing artificial analogs of black holes—things that create a similar inescapable pull, but without having to cram a few suns' worth of mass into a small space. While some of these analogs already exist, a new paper just published in Physical Review Letters proposes a new way to manufacture a "point of no return", this time as part of an electronic circuit.

Imagine throwing a stone into a pond, watching the circle of ripples propagate away from the point of impact. These ripples are gravity waves (not to be confused with gravitational waves!)—so called because the energy they contain is expressed as a displacement of water from its equilibrium position in Earth's gravitational field: in its ground state, the water's surface is flat. Add energy to the system, though, and some of the water ends up at a higher potential energy, while an equal amount ends up lower.

Now, let's put a hole in the bottom of the pond, right where the stone landed. The gravity waves continue to roll outward from the stone's point of impact on the surface, but now the water that they're traveling through is being sucked backward toward their point of origin. Make that hole wide enough, and the water is rushing backwards faster than the waves propagate outward—et voilà! You've got an event horizon in a pond. The new method, proposed by a collaboration of theoretical physicists from four different institutions, aims to create a similar phenomenon using spin waves in magnetic materials.

A substance acts like a magnet when the atoms that comprise it are lined up, all pointing the same direction. It's not a perfect model, but think of the Bohr atom, with particle-like electrons whizzing around a nucleus the same way our planet revolves around the sun. In an ordinary magnet like the kind on your fridge, all the "planets" orbit their respective "stars" in the same plane, and all going around in the same direction.

In rare earth magnets, the super-powerful kind (usually much shinier than the fridge variety), there's the added factor that the electrons' spins are aligned as well. Again, the analogy isn't perfect, but in our planetary model of magnetic atoms above, a rare earth magnet would be a collection of stars where the planets are all rotating the same direction, in addition to orbiting their stars in the same direction.

In the video below, skewers attached to a piece of tape provide a great visual analogy for electron spins in a magnetic wire. They're all lined up at first, but when one electron is disturbed, it passes that disturbance on down the line, creating a spin wave.

If there's no current in a magnetic wire, spin waves in it would move something like the "gummy bear waves" in the video above, bouncing back and forth and eventually dissipating. But a current is analogous to reeling that tape in from one end. If you reel it in fast enough, you can pull those waves toward yourself even when they're trying to travel away, and this is essentially what the new study's authors have proposed.
 Spin waves, dragged by the electric current, are trapped in a way similar to light being attracted by a black hole.
Image Credit: David Torriko,

By using a wire that's thicker at one end than the other—which accelerates the electrons as they get near the thin end—the new study's authors have proposed that we should be able to create a kind of spin wave event horizon at a certain point in the wire. Whether this boundary would behave like an actual event horizon, complete with an analog of Hawking radiation, remains to be seen, but the scientists are optimistic. If it does, it could bode well for quantum computing: entanglement, a cornerstone of quantum computing, is also a key feature of Hawking radiation, so this method may provide future researchers a way to create and experiment with an exotic kind of entanglement.

Stephen Skolnick

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