How the love-hate relationship between superconductors and magnets causes levitation

Superconductors are known for their complete lack of resistance to electrical flow. They also have less widely-known quirk — they make magnets levitate.

Superconductors are fantastically useful in laboratory and medical settings. Provided the temperature is low enough, they have no resistance to electrical current. They are literally the perfect conductors. But they're really boring to look at. Some are chilled by liquid nitrogen, so a photographer can get a nice shot of steam coming up around the superconductor, but it's still just steam swirling around a chunk of ordinary-looking material. To liven up the image, most semi-conductors are pictured with another object levitating above them. That something is a magnet.

Magnets are known for their attractive properties. What is it about a superconductor that makes them defy gravity in order to get away? And if they are being repelled, why don't they just fly away from the superconductor? Why do they just hover?

It turns out that the magnet and the superconductor are both attracted to and repelled by each other. As the temperature drops and the superconductor kicks into gear, The Meissner Effect comes into play. The Meissner Effect is the tendency for a superconductor to expel magnetic fields from its interior. The field is generated by the magnet, so the superconductor pushes the magnet away. (Or the magnet pushes the superconductor away.)

If the magnet gets forced closer to the superconductor, though, some of the magnetic field lines do manage to sink into the superconductor due to imperfections on its surface. This part of the field gets trapped, and when the two objects are released, it exerts a pull to keep them where they were. They still feel the repulsion, though, and so the pull and push counteract each other, and the two objects reach an equilibrium a certain distance apart. Picking up the magnet will get the superconductor to follow. They're stuck in their own, personal disfunctional relationship.

[Via How it Works, Hyperphysics, and the University of Oslo.]