How does our understanding of physics change at extremely high energies and temperatures, such as those present during the first few moments of the universe?
Our understanding of physics changes at extremely high energies and temperatures, such as those present during the first few moments of the universe, because at these energies and temperatures the behavior of matter and energy becomes dominated by the principles of quantum mechanics and relativity, rather than classical mechanics.

At such high energies, particles can be created and destroyed spontaneously, and the behavior of particles becomes governed by the principles of quantum field theory, rather than classical mechanics. This means that particles can exist in multiple states at the same time, and can be entangled with one another, leading to phenomena like quantum tunneling and quantum teleportation.

Additionally, at these high energies and temperatures, the fundamental forces of nature can become unified, leading to a single force that governs the behavior of all particles. This theory, known as grand unification theory, has yet to be fully developed and tested, but is a promising avenue for exploring the behavior of matter and energy at high energies.

Finally, at extremely high temperatures, matter can become so hot and dense that it exists in a state known as a quark-gluon plasma, where individual particles lose their identity and become part of a soup of particles that behave collectively. This state of matter has been studied in laboratory experiments, and can give us insights into the behavior of matter in the early universe.
Can you tell me more about quark-gluon plasma? Why does it behave collectively and why only at high temperatures? What does this have to do with quantum mechanics?
Quark-gluon plasma (QGP) is a state of matter that is believed to exist at extremely high temperatures and densities, where quarks and gluons, which are elementary particles that make up protons and neutrons, are no longer confined inside individual particles but are instead free to move and interact collectively. QGP is thought to be a state of matter that existed shortly after the Big Bang, during the early moments of the universe, and is also created in high-energy collisions of heavy ions in particle accelerators, such as those achieved in experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).

At high temperatures and densities, the strong nuclear force, which is mediated by gluons and holds quarks together inside protons and neutrons, weakens, and quarks and gluons can become deconfined, meaning they are no longer confined within hadrons (particles made of quarks, such as protons and neutrons) but are free to move and interact over large distances. This results in the formation of a new state of matter, the QGP, where quarks and gluons behave collectively and exhibit unique properties.

One of the key aspects of QGP is its collective behavior, which arises due to the principles of quantum mechanics. Quantum mechanics describes the behavior of particles at the atomic and subatomic level, and it is inherently probabilistic and allows for quantum entanglement, superposition, and coherence. In the case of QGP, the collective behavior arises from the interactions of large numbers of quarks and gluons, which can exhibit emergent properties that are not present in individual particles. Quantum entanglement, where the properties of particles become correlated and interconnected, can play a role in the collective behavior of QGP, leading to phenomena such as rapid thermalization and hydrodynamical flow.

The high temperatures required to create QGP are necessary to provide enough energy to overcome the strong nuclear force that normally confines quarks and gluons inside hadrons. At lower temperatures, quarks and gluons are confined inside hadrons, and the collective behavior of QGP is not observed. However, at high temperatures, the energy density is sufficient to deconfine quarks and gluons and create QGP, where they behave collectively due to the weakening of the strong nuclear force.

The study of QGP provides insights into the fundamental properties of the strong nuclear force, the behavior of matter at extreme temperatures and densities, and the early universe's conditions shortly after the Big Bang. It also has implications for our understanding of the nature of phase transitions, thermodynamics, and the fundamental laws of quantum mechanics in the context of high-energy physics.