Final answer:
We do not talk about Bose-Einstein condensation of phonons or photons because phonons are not individual entities and photons, while bosons, do not undergo traditional condensation due to their non-conservation and ease of creation and destruction.
Step-by-step explanation:
The reason we do not talk about the Bose-Einstein condensation of phonons or photons is tied to the nature of these particles and the conditions under which condensation occurs. Phonons, being quantized vibrations existing within a crystal lattice, are collective excitations and not individual particles per se. They cannot exist independently and thus don't undergo Bose-Einstein condensation as individual entities. Photons, on the other hand, are bosons, and under specific conditions, such as in a laser, they can exhibit behaviors similar to condensation where many photons occupy the same quantum state. However, this is not the same as the traditional Bose-Einstein condensation observed in cold atom systems because photons are not conserved particles and are easily created and destroyed. This process is more akin to photon stimulated emission rather than condensation in the strict sense.
Understanding these phenomena requires a grasp of quantum mechanics and the principles that govern the behavior of subatomic particles. BCS theory, which models superconductivity, shows how electron pairs known as Cooper pairs can act like bosons at low temperatures and form a condensate, giving rise to superconductivity. This is different from phonons or photons since the electron pairs are formed within a fermionic context, with constraints imposed by the Pauli exclusion principle before pairing and condensation.