In school, you might have learned that there are three or even four states of matter; the classical states of matter. But as our understanding of the universe has expanded, we have realized that matter can organize itself in more ways than we imagined. Although there is still disagreement as to how many states actually exist, so far over 15 have either been demonstrated in the lab or have enough circumstantial evidence that scientific theories hinge on them existing.
What is a state of matter?
A state of matter is when a certain quantity of a substance has chemical and physical properties that are uniform. It is possible for this substance to go from one state to another through a change of phase. We can think of the melting of an ice cube as a simple phase change.
Classical states of matter: solid, liquid, gas, plasma
In the classical states of matter, we have a solid, liquid, gas, and plasma. These states are observed in normal conditions in everyday life and are defined in terms of volume, shape, and the general property of a substance. A solid has a shape and a volume, and its constituent particles are tightly packed together. In liquids, particles have weaker forces between them and, for that reason, they have a (mostly) constant volume but take the shape of the receptacle they are filling.
Both gas and plasma have neither a fixed shape nor volume. The difference is that plasmas are electrically conductive, produce electric currents, and react strongly to electromagnetic forces. And while we are all familiar with gases, plasma is equally common and is seen in lightning, sparks, fluorescent lights, stars, and certain flames too.
Without getting into extremely weird physics, we can see that these four states might not represent all the categories that are out there. Both solids and liquids have subcategories, and some of them seem to cross the narrow definitions we have given them. Liquid crystals and liquid glass are among these peculiar states, but they are not alone.
“We all observe in everyday life at least three states of matter: solid, liquid, and gas. But even within what we would consider as well-known states of matter, nature can be tricking us," Dr Guillaume Nataf, from Cambridge University's Department of Materials Science & Metallurgy, told IFLScience.
"For example, Ig Nobel laureates John Mainstone and Thomas Parnell performed a long-term experiment that measures the flow of a piece of pitch – bitumen – over many years, showing that even at room temperature bitumen flows and therefore belongs to the liquid state of matter. Other intriguing examples are numerous.”
In the last century, scientists have come to realize that there are more states beyond the ones that we are (more or less) familiar with. To find and study them, however, we need to go to extremes. Pressure, heat, and cold can push substances into configurations with bizarre properties.
The fifth state of matter
The so-called fifth state of matter is the Bose-Einstein condensate, which happens only in a very dilute gas of particles known as bosons and when the temperature is close to absolute zero.
Under these conditions, the entire gas stops behaving like it is made of individual particles and instead behaves like a single macroscopic quantum system. It is obtained only in extremely low-density scenarios (one-hundred-thousandth the density of air) and ultra-low temperatures (a fraction of a degree above absolute zero). At this limiting condition, quantum mechanic effects become dominant and we see peculiar states.
Superfluids, supersolids, and superconductors
Staying at ultra-low temperatures, we can also experience superfluids – a second liquid state where the substance can flow without friction. One of the most curious consequences of this state is that superfluids are capable of climbing out of the containers they are placed in.
You can also have supersolids, those that move without friction, and superconductors, which are materials that have zero electrical resistance below a certain temperature. There are also Rydberg polarons, where it’s possible to have atoms inside other atoms.
On the other end of the scale at high temperatures, we start with supercritical fluids, when it is so hot and the pressure is high enough that it's impossible to distinguish if a fluid is a gas or a liquid. Increasing the pressure significantly, we get to the core of white dwarfs, which are likely made of electron-degenerate matter. In these stars, electrons are in a degenerate gas form, which is a perfect heat conductor and behaves like a solid.
Continuing to increase the pressure on matter, we reach neutron-degenerate matter, which is seen only in neutron stars. There, protons and electrons are so tightly packed together that they turn into neutrons due to beta decay. Beyond that state, there’s the quark-gluon plasma, when even the building blocks of particles are no longer constrained into tight configurations.
Condensed matter physics
A field that has been particularly prolific in discovering new states is condensed matter physics, either solid or liquid. Some can be found in superconductors and are made-up of quasiparticles, a phenomenon that behaves like a particle without being one. The exciton and the dropleton form in similar ways, with electrons forming a bound state with a positively-charged “hole” where the electron should be. The exciton behaves a bit like a simple atom, while the dropleton is the first quasiparticle that has shown liquid-like behavior.
The quantum question: what constitutes a new state of matter?
Quantum properties can also be crucial to discern states of matter. The way things like particles’ spin interact might lead to different states, such as a quantum spin liquid or spin ice. So it is not surprising that researchers have been finding new configurations while investigating other phenomena.
There’s also a lot of discussion about what constitutes a new state of matter and if peculiar quantum configurations within an ordinary solid, for example, should be considered states or not. This is an intriguing debate and makes it difficult to give a straightforward answer on how many states are out there.
“It is generally said that in order to understand matter, it is necessary to have a precise definition of its different states," Dr Nataf told IFLScience. "However, knowing if something belongs to a new state of matter or not doesn't necessarily help in understanding new fundamental concepts or finding applications for it. Therefore, I believe that advances can be made independently of classification systems."
In the near future, more new states of matter will certainly be discovered and perhaps some exciting new applications will also be unlocked from the ones we are already familiar with.