In quantum theory, specifically in the framework of quantum field theory, it is indeed possible for virtual particle-antiparticle pairs to briefly appear and annihilate in a vacuum. This phenomenon is known as vacuum fluctuations or quantum fluctuations.
According to Heisenberg's uncertainty principle, there is a fundamental limit to how precisely certain pairs of physical quantities, such as energy and time, or position and momentum, can be simultaneously known. This uncertainty allows for temporary violations of energy conservation, enabling the creation of particle-antiparticle pairs from the vacuum for very short durations.
In the case of electron-positron pairs, these fluctuations can occur, but the probability of their appearance and subsequent annihilation depends on the energy scale and duration involved. In empty space or a flat vacuum, the likelihood of observing such fluctuations is low because they are typically associated with high-energy scales or short time intervals.
Quantum field theory provides a mathematical framework to describe these vacuum fluctuations and calculate probabilities associated with particle creation and annihilation processes. However, it's important to note that these fluctuations are not directly observable and are generally treated as virtual processes within calculations rather than actual measurable events.
Furthermore, the concept of a "litre of flat vacuum" is an idealized notion, as the vacuum itself is a complex quantum field with intricate properties. The magnitude of vacuum fluctuations is more prominent in regions with higher energy densities or strong fields.