What is empty space, really?
We tend to think of a vacuum as empty space that contains nothing. That intuition is surprisingly accurate.
However, it requires a refinement to make it precise:
A vacuum is a state of utmost emptiness where nothing happens. Although this may sound oxymoronic, the term is often applied, for lack of a better alternative, to states considered the “emptiest” according to criteria that may not endure over time. Such states conflict with our definition, since they do not preserve emptiness. We therefore do not refer to them as vacua.
The reason for this pedantry is that the existence of a well-defined vacuum is essential to demonstrating the self-consistency of the theory. Theories without such a vacuum are like bottomless pits: they tend to violate fundamental principles, such as the requirement that the probabilities of all possible outcomes sum to unity.
It is important to keep in mind that quantum emptiness is different from a classical emptiness. Sometimes the vacuum is said to contain quantum fluctuations, corresponding to constant creation and destruction of particles. However, the vacuum does not exhibit dynamical fluctuations. Instead, it encodes a static pattern of quantum correlations, which manifest as probabilistic outcomes when measurements are performed. The apparent “fluctuations” are therefore not time-dependent processes, but measurement-dependent realizations of this underlying quantum structure.
Global vs. Local
There is an important distinction between what we call a vacuum in everyday life and the vacuum. The former refers to the local emptiness produced by removing everything from a finite region of space, whereas the latter refers to total emptiness throughout all of space, and indeed throughout time as we have defined it.
The vacuum as a reference point
Once the vacuum is identified, everything else in the theory becomes easier to describe.
Particles are objects placed into space. They are departures from the vacuum — disturbances relative to a state that would otherwise remain unchanged.
More than one state of emptiness
A theory can admit multiple vacua, degenerate in energy and admitting the same particle spectrum. It might even be physically indistinguishable whether the entire universe has settled into one or the other. This leads to the possibility of having tornados in emptiness, which are called topological defects. We will discuss this phenomenon another time.
Therefore, the vacuum is a state of lowest possible energy that does not evolve in time. It is the state that always has been and always will be, and every self-consistent theory of the universe must possess it.
See also: Theory vs. Speculation