| MadSci Network: Physics |
Hi Thomas,
No, we don't just make these things up. At the broadest level, there are either three or four states of matter, depending upon whether or not you consider a plasma to be a special type of gas, or its own state. While I personally tend to view plasmas as a subset of the gaseous state, I believe that most scientists would say that there are four states of matter--solid, liquid, gas, and plasma.
The states are basically divided by the criteria of whether the material has a definite shape, and whether it has a definite volume. Plasmas would be separated out because they interact with electromagnetic fields very differently than neutral gases. I can't go through all of the other states that you mention, but many of them would essentially be considered subsets of one of the four states of matter, based upon other differences in their behavior.
Take superfluids, for example, and liquid helium in particular. Below a critical temperature, liquid helium loses its viscosity. Viscosity is a measure of the resistance of a liquid to flowing--cold molasses has high viscosity, while the viscosity of water is much lower. With zero viscosity, a superfluid can do very strange things, such as spontaneously flowing out of an unsealed container. A very unusual liquid. But, because it has no fixed shape while having a fixed volume, it is definitely a liquid.
Another example would be an electron degenerate gas, such as in a white dwarf star. In those systems, the matter is so tightly packed that the electrons are essentially beginning to overlap one another (in a fuzzy, quantum mechanical sense). Because no two electrons can have the exact same properties (location and energy) at the same time, that means that if the matter is to be squeezed any harder, some electrons must be moved to higher energies. As a result, a degenerate gas has an extremely high resistance to being squeezed, even if it's very cold. Not at all the sort of behavior that we expect in a gas. But it's still a gas, because it has no fixed shape or volume.
On the other hand, some of the states that you mention don't apply to the overall matter. For example, the Quantum Hall State has nothing to do with the state of matter, but involves interactions between the electrons within a material (a semiconductor). We would call the material a solid whether or not its electrons are in a Quantum Hall state. The only differences are in the electromagnetic properties of the semiconductor.
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