According to quantum field theory, the vacuum is not simply "pure nothingness." It is a complex and dynamic state that is subject to quantum fluctuations. These fluctuations give rise to the concept of vacuum energy.
In quantum mechanics, the Heisenberg uncertainty principle allows for temporary violations of energy conservation as long as they occur for a sufficiently short period of time. This means that energy can fluctuate within certain limits, even in empty space. These fluctuations arise due to the inherent uncertainty in the values of various quantum fields that permeate space.
Virtual particles, which are temporary fluctuations in these quantum fields, continuously pop in and out of existence in the vacuum. These particles have associated energies, and their creation and annihilation contribute to the overall energy content of the vacuum. However, these fluctuations are temporary and cannot be directly observed or harnessed for useful work.
The concept of vacuum energy has implications in various areas of physics, including quantum field theory, cosmology, and the study of the Casimir effect, among others. It is worth noting that the precise nature and magnitude of vacuum energy are still subjects of ongoing research and investigation in theoretical physics.
To summarize, the vacuum does have an energy associated with it due to the fluctuations of quantum fields. However, this energy is not directly accessible or usable in the conventional sense, and its effects are primarily relevant within the framework of quantum field theory and related fields of study.