According to quantum field theory, the empty space around and within an atom, often referred to as vacuum or vacuum state, is not actually "empty" in the classical sense. It is filled with various quantum fields that fluctuate even in the absence of particles.
These quantum fields give rise to virtual particles and their corresponding anti-particles, continuously popping in and out of existence. This phenomenon is known as vacuum fluctuations. These fluctuations contribute to the energy density of the vacuum, often referred to as vacuum energy or zero-point energy.
The concept of vacuum energy implies that even in the absence of any particles, there is still an inherent energy associated with the vacuum state. However, it is important to note that the vacuum energy is not directly observable or measurable in the traditional sense.
The reason for this is that when physicists calculate the total energy of a system, they usually consider the difference in energy between different states, rather than the absolute value. In the case of vacuum energy, this difference can be large, but the absolute value itself is not directly measurable or significant in most practical contexts.
It's worth mentioning that the vacuum energy density has implications in theoretical physics, particularly in cosmology and the study of the universe's large-scale properties. However, in the context of individual atoms or everyday observations, the vacuum energy density does not have a direct measurable impact.