How "Second Sound" Helps the Large Hadron Collider Work

Superfluids have many extraordinary properties. One of them is called "second sound" even though it doesn't have anything to do with sound waves. It does have something to do with making the Large Hadron Collider work, though. Find out why.


Superfluids are special materials that, when cooled to a certain point, have no viscosity. Viscosity means internal friction, and it restricts the flow of the fluid. Without any restrictions to their internal flow, superfluids can do all kinds of crazy things, like climbing the walls of a container - as you can see in the video above.

A little-known, and generally misnamed characteristic of superfluids is "second sound." The name gets under the skin of many physicists because sound waves aren't traveling through the fluid. Heat waves are.

Heat doesn't usually travel as a wave. It moves through a fluid in a process called diffusion. Start a fire under a pot of water and the lowest layer of water will heat up first. The atoms in this layer will start moving fast, colliding with the cooler atoms next to them, which will heat up and collide with the next group of atoms, and so on. While the fluid is heating, the heat establishes a gradient, with the hottest water at the bottom and the coolest at the top. The heat gradually moves upwards through the water until it's all one temperature. Heat in a superfluid doesn't establish a gradient. It moves as one single wave of heat outwards from the source - like the sound of a bell struck once, moving outward over the surrounding area.

This single pulse of heat leaves the superfluid mostly unaffected by the momentary rise in temperature. Instead of heating from bottom to top, waves of heat move up through the fluid, but the superfluid stays cool. That's very convenient. The Large Hadron Collider uses liquid helium to cool its magnets. As the superfluid helium circulates, its unique response to heat allows it to gain only 0.1 K for every kilometer it travels. Without this property of helium, the LHC wouldn't work.

[Sources: Second Sound in a Superfluid, Nothing.]



That's really neat. I'm guessing this is a convective process rather than conductive? Meaning it wouldn't work in space? How fast do these waves move? Does it end up looking like a gradient when exposed to a constant heat source like the LHC magnets?