Question: If we ever reach absolute zero where all movement of particles stops, will this create a fifth phase of matter or will it still be called a solid even if the particles have stopped moving?

Arttu Rajantie answered on 23 Jun 2011:

Because of the uncertainty principle of quantum mechanics, you cannot stop to motion of the particles completely. However, the quantum nature of the particle becomes important and therefore the behaviour of the particles changes completely from what we are used to. The actual behaviour depends on the particles and their interactions, and there can many interesting phenomena that are specific to some types of particles, but one can get a good idea what happens by considering particles that do not interact at all.

First of all, there are two types of particles in quantum mechanics, bosons and fermions. Atoms are bosons if the total number of electrons, protons and neutrons is even, and fermions if it is odd. When cooled to low enough temperatures, bosons become a Bose-Einstein condensate, in which most of the particles are in the ground state, which is the closest you can get to stopping their motion. The Bose-Einstein condensate is basically a macroscopic quantum state, and it behaves like fluid with some strange properties. There is a sharp transition to the Bose-Einstein condensate phase, so it can very well be thought of as a new phase of matter.

In contrast, fermions obey the so called Pauli exclusion principle, which says that there can be only one fermion in each quantum state. Therefore, instead of all of them going into the ground state, they fill all the lowest states in the system. This state is very different from the Bose-Einstein condensate, but is has interesting properties too, which are again purely due to quantum mechanics. For example, it has a very high pressure. In fact, this pressure is what supports white dwarf stars and neutron stars against gravitational collapse.