In the framework of quantum field theory, matter particles are indeed described as excitations of their respective quantum fields. These fields permeate all of spacetime and can exhibit different energy states or excitations. The question of what initially caused the excitation of these fields and why they do not return to their unexcited state is an interesting one.
In quantum field theory, the initial excitation of a field can arise through various mechanisms. One possibility is through interactions with other fields or particles. For example, in the early universe, during the process of cosmological inflation, quantum fluctuations in the fields can become amplified and lead to the creation of particles and excitations.
Additionally, fields can also be excited by external sources or energy inputs. For instance, in particle accelerators, high-energy collisions can provide the necessary energy to excite the fields and create new particles. Similarly, in natural processes such as radioactive decay or particle interactions, the decay or collision can result in the excitation of the fields.
Now, regarding why these excitations do not immediately return to their unexcited state, it relates to the dynamics and properties of the quantum field itself. The behavior of quantum fields is governed by fundamental laws and equations of motion, such as the equations of quantum field theory. These equations determine how the fields evolve over time and how their excitations propagate.
In some cases, the excitations of the field may naturally decay or relax back to their unexcited state through processes like particle interactions or emission of radiation. However, there are situations where the excitations can be stable and persist for extended periods or indefinitely. This stability can arise due to various factors, such as conservation laws or symmetries that protect certain states from decaying.
For example, in particle physics, stable particles like electrons or protons are considered to be the ground or lowest energy states of their respective fields and are protected by conservation laws like electric charge or baryon number. These particles do not spontaneously decay because there are no lower energy states or allowed decay modes available to them.
In summary, the initial excitation of a quantum field can occur through various mechanisms, including interactions or external inputs. The persistence of these excitations depends on the specific dynamics of the field, including conservation laws or symmetries that protect certain states from decaying.