In quantum field theory, the concepts of excitation and particle are related but have distinct meanings. Let's explore each term individually:
- Excitation: In quantum field theory, a field is a mathematical construct that describes a physical quantity, such as the electromagnetic field or the Higgs field. Quantum fields can exist in various states, and an excitation refers to a departure from the field's ground state or vacuum state.
Think of the field as a calm ocean. The ground state represents a flat, undisturbed surface of the ocean. An excitation would then correspond to a wave or disturbance propagating through the water, altering its state from the calm, flat surface. Similarly, in a quantum field, an excitation represents an energy perturbation or disturbance in the field that carries information and propagates through space and time.
Excitations in quantum fields are quantized, meaning they come in discrete units called quanta or quanta of excitation. These quanta are often referred to as particles, which brings us to the next point.
- Particle: In quantum field theory, particles are the quanta or units of excitation in quantum fields. They are the smallest indivisible units of the field's energy and momentum. However, it's important to note that the concept of a particle in quantum field theory differs from the classical notion of a localized, billiard-ball-like object.
According to quantum field theory, particles are not fixed entities with definite positions and trajectories. Instead, they are manifestations of field excitations. The field's behavior and its excitations determine the properties of the associated particles, such as their mass, charge, and spin.
Particles can be created or annihilated by interactions between quantum fields. For example, in high-energy collisions, particles can be created from the energy of the interacting fields, while in other processes, particles can be destroyed or converted into different types of particles.
In summary, excitation refers to a departure from the ground state of a quantum field, while particles are the discrete units of these excitations. Particles are not classical objects with fixed positions but rather manifestations of quantum field excitations with specific properties.