Shulou(Shulou.com)11/24 Report--

There are three common states of matter: solid, liquid, and gas, which are different states occupied by the same element or molecule according to the strength of its chemical bonds. Strong bonds in solids keep the material rigid, but when heated these bonds break and leave weaker bonds that allow particles to flow with each other. Heating it further, the weak bonds break and the particles fly freely through space. See, it's a gas.

Some of you may also know that if we continue to heat up, electrons separate from atoms, breaking all molecular bonds in the process and creating plasma. Thus we seem to conclude that the states of matter are simply the different states in which atoms can be. But wait, the quarks inside protons and neutrons are "matter," so what state are they in? Does it depend on the state of matter of the atoms to which they belong?

To answer these questions, we'd better figure out what the state of matter is. If one were to reason from the states of matter learned in school (solid, liquid, gas, and plasma), a simple pattern would be obvious. Changes in temperature lead to changes in state or phase, and phase transitions occur at specific temperatures and pressures of the material. Consider the two-dimensional relationship between phase and temperature and pressure and plot it into a diagram, which we call a phase diagram.

The phase diagram shows us that the state of matter is much more complex than solids, liquids, and gases. On the one hand, matter exists in secret hidden states. For example, at temperatures and pressures above the critical point, the boundary between gas and liquid becomes blurred, and we have a supercritical fluid with both properties.

Quark-gluon What happens if we keep raising the temperature of the plasma? Plasmas are still composed of composite particles: electrons and nuclei. Just as we tear apart atoms when making plasmas, we can tear apart nuclei if we raise the temperature high enough. We would need a high temperature of about 7 trillion Kelvin, the so-called Hagedorn temperature, because the binding energy of nucleons is extremely high.

When we reach that temperature, quarks are stripped from nucleons, producing quark-gluon plasma, which is the next state we can get. The interaction between gluons and quarks is still important, so quark-gluon plasma behaves more like a liquid. We often make this stuff in particle accelerators, but in very small quantities. In the very early universe, however, everything was quark-gluon plasma, and probably in the cores of massive neutron stars as well.

If quark-gluon plasma is liquid, does that mean it can freeze? Yes, and in its frozen form it is a nucleon, or more generally a hadron: protons, neutrons, and combinations of quarks. Hadrons are actually "crystals" of quark-gluon plasma, which is matter in "solid" form.

Quark phase diagrams For quantum chromodynamics, quark matter is also referred to as QCD matter, which is the physics of quark-gluon interactions. Quark matter has its own state of matter. Let's look at its phase diagram. Note that its phase diagram differs from the one mentioned above in that the pressure is replaced by a baryon chemical potential, which indicates how much energy quarks can absorb or emit.

At the top of the phase diagram, quark-gluon plasma is actually an analogy to gas in atomic matter, even though it behaves more fluidly. In the lower left corner of the phase diagram, hadrons are so-called "solids." If we move to the right on this graph, we drill into neutron stars. First, the individual quark "crystals" fuse together to form a so-called neutron fluid, and then the neutrons dissolve, and we end up with a very strange form of liquid quark matter.

The other most familiar states of matter can be explained by the interaction of particles under classical forces. But once you bring quantum mechanics into it, many strange states of matter become possible. For example, in degenerate matter such as Bose-Einstein condensates, all quantum states are occupied, leading to some surprising and useful emergent properties such as superconductivity and superfluidity.

Time crystals are the newest and perhaps the strangest quantum state of matter. These are configurations of entangled particles that oscillate between states even though they have no energy. In conventional thermodynamics, the lowest energy corresponds to absolute zero temperature, which in turn implies zero motion of the particles. The lowest energy state of time crystals involves actual motion, which makes them thermodynamically different from other states of matter, and therefore they qualify as their own state of matter.

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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