Superfluidity

Superfluidity is a unique property of certain fluids that have zero viscosity. Viscosity is a term used to describe a fluid's resistance to flowing. Because a superfluid has no viscosity, it can flow without losing any kinetic energy. This means that if you stir a superfluid, it creates tiny whirlpools called vortices. These vortices will continue to rotate indefinitely because there is no friction to stop them.

This phenomenon represents a state of matter where quantum effects become visible on a large scale.

This strange state occurs in two isotopes of helium: helium-3 and helium-4. To reach this state, the helium must be cooled to cryogenic temperatures. These are extremely low temperatures used in scientific research. In helium-4, the process happens at relatively high temperatures compared to helium-3. This is because each atom of helium-4 is a boson particle, which has an integer spin. Helium-3 atoms are fermion particles. For helium-3 to become a superfluid, the particles must pair up to act like bosons. This pairing process is very similar to how electrons pair up in superconductivity.

When helium becomes a superfluid, it exhibits behaviors that seem to defy gravity. One such behavior is the formation of a Rollin film. This is a thin, invisible film of liquid that can creep up the inside walls of a container. If the container is not sealed, the superfluid will creep up the sides and out of the vessel.

The liquid moves to find its own level, which can cause the levels in two connected containers to equalize. This movement happens because the liquid has no resistance to its own flow.

The history of superfluidity is marked by several major scientific breakthroughs. On August 2, 1911, Heike Kamerlingh Onnes may have observed the transition to the superfluid phase. This occurred on the same day he observed superconductivity in mercury. Later, in 1937, Pyotr Kapitsa discovered superfluidity in helium-4. John F. Allen and Don Misener discovered it independently at the same time. Kapitsa was the only one of these three to receive the Nobel Prize for the discovery. In 1941, Lev Landau developed the accepted macroscopic theory for helium-4. Landau later received a Nobel Prize in 1962 for this theoretical work.

Scientific research has expanded from liquid helium to ultracold atomic gases. In 2000, researchers observed quantum vortices in an ultracold bosonic gas using rubidium-87. Later, in April 2005, Wolfgang Ketterle and his team at MIT observed similar vortices in lithium-6. They achieved this at a temperature of only 50 nK. In 1999, Lene Hau used sodium atoms to create a condensate that could slow or even stop light. Her team used this system to create superfluid analogues of tornadoes and shock waves. These experiments show how superfluidity can be studied in many different types of matter.

Astrophysicists also use the concept of superfluidity to understand the universe. Arkady Migdal first proposed that superfluidity might exist inside neutron stars. He suggested that nucleons could form Cooper pairs due to a long-range attractive nuclear force. This would lead to superfluidity and superconductivity within the star. Other scientists have proposed a theory of superfluid dark matter. This theory suggests that dark matter might exist in a superfluid state at certain scales. This state could help explain galaxy rotation curves and the Tully-Fisher relation. It might also help explain the cosmic microwave background.

Finally, superfluidity connects to the most fundamental theories of physics. Superfluid Vacuum Theory (SVT) is an approach that views the physical vacuum as a superfluid. The goal of SVT is to create a model that unifies gravity with quantum mechanics. This would make it a candidate for a theory of quantum gravity. Scientists hope this could describe all fundamental interactions and particles as parts of one entity. Even in the natural world, we see patterns that mimic these changes. The rapid changes in flight patterns during starling murmurations look like the phase changes seen in superfluids.