In the context of quantum field theory, the ground state is often referred to as the vacuum state. This terminology might seem a bit confusing since we typically associate the term "vacuum" with empty space. However, in quantum field theory, the vacuum state does not mean empty or devoid of anything. Instead, it refers to the state of lowest possible energy.
To understand why it's called the vacuum state, we need to delve into the concept of quantum fluctuations. According to quantum mechanics, even in the absence of any particles or excitations, empty space is not truly empty. It is filled with a sea of virtual particles and their corresponding antiparticles that continuously pop in and out of existence. These fluctuations arise due to the inherent uncertainty in quantum mechanics.
When we consider a field theory, such as the electromagnetic field or the Higgs field, these fluctuations are described by the field itself. The ground state, or vacuum state, of the theory represents the state with the lowest possible energy. It is the state where the field fluctuations have minimal energy. These fluctuations can be seen as the virtual particle-antiparticle pairs appearing and annihilating rapidly.
In a sense, the vacuum state is the background state against which we measure the excitations or particles that arise from it. Any particle or excitation in the theory is considered to be an "excited" state above the vacuum. Thus, the vacuum state is analogous to the lowest energy state or the baseline state from which all other states emerge.
So, while the term "vacuum" might give the impression of emptiness, it is used in quantum field theory to describe the state of minimum energy, where field fluctuations and virtual particles are still present.