Particles on the brain

So that last post got me wondering about the difference between bosons and fermions. Is a proton a boson or a fermion? A lone proton in a beam of protons is a fermion, because it's got spin 1/2. But a proton in a non-integer spin system, such as a deuterium nucleus (NOT a deuterium atom), is a boson.

Proton + Electron = Hydrogen atom, fermion
Proton + Neutron + Electron = Deuterium atom, fermion
Proton + Neutron = Deuteron particle, boson

What does this mean? A few things. Fermions are subject to the Pauli Exclusion principle, which states that no two fermions can occupy the same quantum state. This rule does not apply to bosons. At normal temperatures, the two types of particles are interacting so quickly that the difference is less important. All that needs to be taken home is that the interaction of the two types of paticles is necessary to constitute the universe.

At very low temps, the action or absence of the Pauli Exclusion Principle plays a much bigger role.

Very low temperature bosons can all occupy the same state. Since most bosons can constitute matter, a very low temperature (homogenous) group of bosons becomes a "new" phase of matter, a superfluid. Superfluids are as close to regular fluids as plasmas are to gases. Similar, but just barely. Superfluids have almost no energy and the whole thing occupies the same quantum state, so the quantum state of the whole thing changes at once and is apparent from a macroscopic viewpoint. As a result of having so little energy, superfluids interact strangely.

A normal fluid, when placed in a container in a gravimetric field, is pulled to the bottom of the container until the bottom is covered by a single layer of molecules that push on the sides of the container. The container, in turn, pushes back against the fluid and gives it a shape indicative of the container. If energy is added to the system, say by stiring the container up, the fluid will no longer be in the shape of the container (it will be shaped like a whirlpool). However, as the fluid and the container interact, friction reduces the energy in the system until it reaches a stable state, with the fluid in the same shape as the container.

This state is deceptively energetic, a glass of water is holding very much energy. If you poke a hole in the side of it near the bottom, the fluid (which is pushing on the walls of the container), shoots out with gusto. This is because the fluid is moving but contained.

Many superfluids don't take the shape of their containers, because they are incapable of losing energy through friction, they just don't have the energy to spare. As a result, when a superfluid pushes on the walls of the container and vice versa, there is energy to force the superfluid into a shape, but the superfluid is moving, and friction can't stop it. So you know what happens? The superfluid isn't held down by gravity, and "slides up" the walls and out of the container!

Fermions have a different thing going on at low energy levels. Fermions don't all pack together in the same quantum state like bosons, they are all different from each other and don't get along. A low-energy fermion system is much more energetic than bosons prepared in a similar manner. As a result, low-energy fermions push against each other strongly, even at absolute zero, which gives such a system a high pressure that not even gravity can overcome. Many large stars are constituted thusly, that's why they don't implode.