You said "empty set" (? ), an empty collection means that there are no elements in this collection. An empty set is a subset of any set and a proper subset of any non-empty set.

You can understand the vacuum state as an empty set. There is no substance in a vacuum. If matter is regarded as a set element, vacuum is an empty set of matter set.

In mathematics, such as equations, every single equation with practical significance must have a solution, but if it is combined into an equation, if there are no * * * elements in the solution set, then the equation has no solution, that is, the solution set is empty.