For three years as an undergraduate physics student performing at science magic shows and open houses, I convinced students (and sometimes their parents) that I was some kind of magician by levitating a small cube-shaped magnet. The magnet floated above a superconductor by only a centimeter or so, but that was ample space to wave a piece of paper between the two to prove there were literally no strings attached. Tapping one edge of the cube sent it spinning in place, and even if you pushed the magnet down it resolutely bounced back up again—if it failed to do so, it meant the superconductor needed to be colder.
A simple recipe for this scientifically grounded spell would be a splash of liquid nitrogen for cooling a ceramic superconductor resting in a Styrofoam basin and a magnet that produces a strong, permanent magnetic field made of rare earth elements.
The levitation works, thanks to superconductivity, which could be understood through basic principals of conductivity. Certain elements and materials, aptly called conductors, serve as an electrical conduit, which means electrons can pass through them with relative ease. These electrons still bump into the atoms that make up the conductor and lose a bit of energy with each collision. But, when cooled to a sufficiently chilly temperature, the electrons can flow freely through the conductor without any collisions. That’s because electrons pair up at extremely low temperatures (whereas heat would break the tentative bond between them). Although their bonds are weak, there’s strength in numbers: Pairing up makes it so the collisions that would normally leech energy from the electron flow have no effect because the collisions are weaker than the electrons’ bond.
A superconductor’s critical temperature—how cold it needs to be for these pairings to be possible—depends on its material. Metallic superconductors such as pure aluminum or niobium, for example, have extremely low critical temperatures, typically only a few degrees above absolute zero. Using one of them for an at-home experiment isn’t an option, however, unless you happen to have a lot of liquid helium lying around. (Liquid helium boils at 4.2 kelvins or about –270 degrees Celsius, only a few degrees shy of absolute zero). Fortunately, there’s an alternative: high-temperature superconductors, which are ceramics made from multiple elements that allow electrons to flow freely under slightly higher than most critical temperatures.
77 K (about –196 degrees C) doesn’t seem like a day at the tropics, but in the world of superconductors it’s downright toasty. It’s also the temperature at which liquid nitrogen—much more accessible than liquid helium—boils. For most high-temperature ceramic superconductors, such as those made of yttrium barium copper oxide (YBCO) or bismuth strontium calcium copper oxide (BSCCO), liquid nitrogen can be used to cool them below their critical temperatures.
We’ve got two pieces of the puzzle at hand now: a high-temperature superconductor and enough liquid nitrogen to keep it cool. But how can we float a magnet above the cooled superconductor? (Or vice versa: in our video with Richard Garriott, he floated a cooled superconductor above a bed of rare earth magnets.)
Quantum magnetic levitation boils down to something called the Meissner effect, which only occurs when a material is cold enough to behave like a superconductor. At normal temperatures, magnetic fields can pass through the material normally. Once it is cold enough to exhibit superconductivity, however, those magnetic fields get expelled. Any magnetic fields that were passing through must instead move around it. When a magnet is placed above a superconductor at critical temperature, the superconductor pushes away its field by acting like a magnet with the same pole causing the magnet to repel, that is, “float”—no magical sleight of hand required.