So, the electrical engineer in me says that this is because any movement of the magnet would induce an electric potential in the superconductor. But because such a potential would create an effectively infinite amount of current requiring an infinite amount of work, the magnet is unable to move.
Or from a "cause-and-effect" viewpoint, movement of the magnet induces a current loop in the superconductor, the creation of which creates an opposing magnetic force.
Either way I believe this is the same principle behind electric motor braking (e.g. when you short-circuit the inputs of a motor). I believe you would also see a similar effect by dropping a magnet down a tube encircled with many (or perhaps one spiral) loop of wire -- the magnet's descent will be slowed by eddy currents in the loops.
Edit: Aha, this is just half the story. The superconductor is prevented from spinning due to flux pinning: http://en.wikipedia.org/wiki/Flux_pinning (or what the researchers call quantum trapping / quantum locking)
You can get clever with pulsed electromagnets and active coils, but eddy current braking works just fine with a permanent magnet and a thick piece of copper: http://www.youtube.com/watch?v=nrw-i5Ku0mI
But, it clearly does not take an infinite amount of force to move the "locked" superconductor, because you can adjust it by hand. The real question is, how much force does it take?
Right, I believe this is a function of how much of the magnetic flux does not encounter the superconductor. But IANAP, just a former EE, so I don't know that this is 100% correct.
...I really want to hear from an actual physicist about the "quantum trapping" explanation. I feel like I must be oversimplifying but I can't find any inconsistencies in my model.
Or from a "cause-and-effect" viewpoint, movement of the magnet induces a current loop in the superconductor, the creation of which creates an opposing magnetic force.
Either way I believe this is the same principle behind electric motor braking (e.g. when you short-circuit the inputs of a motor). I believe you would also see a similar effect by dropping a magnet down a tube encircled with many (or perhaps one spiral) loop of wire -- the magnet's descent will be slowed by eddy currents in the loops.
Edit: Aha, this is just half the story. The superconductor is prevented from spinning due to flux pinning: http://en.wikipedia.org/wiki/Flux_pinning (or what the researchers call quantum trapping / quantum locking)