Today is the future.  Specifically, it’s the day to which Marty McFly and Emmett “Doc” Brown travel in the 1989 film Back to the Future II.

But where are our hoverboards?

It turns out: They do exist! They’re just not exactly what the movie predicted they would be.

Real hoverboards, such as the one the University of Geneva in Switzerland, can levitate a human being. But they can’t travel anywhere that a human wants to go; they need to follow a magnetic track.

Real hoverboards work by exploiting the principles of quantum mechanics and the bizarre interplay between superconductors and magnetic fields.

A superconductor is a material with zero electrical resistance. The superconducting magnets in the Large Hadron Collider, for instance, can carry 1000 times more current than a toaster, but they generate neither heat nor light.

Superconductors also exhibit another bizarre property, thanks to the weird and wonderful laws of quantum mechanics: They expel external magnetic fields.

This standoff between magnets and superconductors is what allows the modern hoverboard to levitate. Here’s how it works:

• Magnets must maintain their North-South magnetic field lines.
• A superconductor placed on a magnet interrupts those field lines.
• The magnet uses its field lines to lift the superconductor out of its way.
• The superconductor locks onto the magnetic field lines shoved between its atoms.
• The superconductor is suspended.

When this happens, the magnet’s North-South poles then become the tracks along which the superconductor slides. Consequently, superconductor hoverboards are less like a skateboard and more like a train.

All known superconducting materials gain their superconducting superpowers only at extremely low temperatures—around minus 230 degrees Fahrenheit (125 Kelvin) or colder. So real-world hoverboards need to be doused with super-cooled liquid nitrogen around every 30 minutes to maintain their extremely low temperature. 

Today’s hoverboards are pretty neat, but it turns out they would be of little help in a futuristic chase scene.