Question: What is superfluidity and how is it possible that a fermionic condensate can flow at low velocities without dissipating any energy—i.e. zero viscosity??

David Freeborn answered on 20 Jun 2013:

Hi Sanban- awesome question!

Superfluidity is a really cool example of quantum mechanics affecting things in an observable way on the every day level.
There’s no “classical physics” way to understand superfluidity. It can only be explained with quantum physics.

In qunatum physics, every particle sits in a particular “quantum state” that tells us how much energy the particle has, and its other properties.

In quantum field theory, there are two classifications of particle, fermions and bosons. These two types of particle behave differently at low temperatures. In particular, no two fermions can sit in the same quantum state (this is called the Pauli exclusion principle, and also explains how atoms work), but two bosons can. If two identical particles have the same quantum state, the particles are quantum mechanically identical and all the particles in the same quantum state will behave as the same particle.

When we cool a liquid of Bosons down to almost absolute zero, more and more of the particles sit in the lowest energy level. This is because cooling a liquid is in effect taking energy away from it. If we could reach absolute zero, all the Bosons would be in the same energy level, so the liquid would behave as a single quantum state.

Because the liquid is behaving like a single quantum state, none of the particles are able to interact. It’s almost as if there is only one particle. This is how you get the cool effects of superfluidity.

You can see some awesome videos of superfluids here: