Physics aims to describe the state the universe actually finds itself in. But in doing so, we work within the language we understand the universe to be speaking to us with. And that language contains more than what is realized. It contains not only what is, but also what could, in principle, have been.
A theory does not merely describe the world we happen to observe. It also defines other possible states, in all the combinations allowed by its rules.
That may sound abstract, but it is not unusual. A well-formulated legal system does not only deal with events that actually took place. It must also be able to speak about hypothetical situations, even cases that never happened anywhere. Something similar is true in physics. The laws must be able to describe not only the state that is realized, but also the other states that would be possible under those same laws.
This matters because the universe we see is complicated.
If physics were forced to speak only about this one realized arrangement, it would be much harder to see the basic rules clearly. We understand those rules by also considering simpler states.
Among them there is usually one especially important state: the state of utmost emptiness.
The universe we actually see is clearly not in that state. And yet the theory still needs it.
That is why physics is allowed to speak not only about the actual universe, but also about idealized states such as a single particle in otherwise empty space, or even something stranger: oneself alone in that emptiness. This is not the familiar kind of solitude one may experience within our world, aware of everything one is separated from and could in principle rejoin. It is solitude against a background in which, by assumption, there is nothing else to return to.
The universe does not seem to inhabit such states globally. But the language must still be able to describe them.
This is not philosophical decoration. It is how the theory becomes usable.
Some of these idealized states can be approximated well enough in the laboratory. By isolating a few particles and letting them interact under conditions that come close to emptiness, we can study their behavior in a cleaner form than the universe usually offers. In this way, we learn about states that are never realized globally, which in turn reveal the rules by which the universe operates.
That is one of the reasons we can learn about the universe as a whole from local experiments. We do not recreate the whole cosmos. We approximate simpler states that the theory allows, and from them extract the rules.
There is therefore a difference between a state being allowed by the laws and a state being realized in history.
A simple example is the coexistence of humans and dinosaurs. As far as the laws of nature are concerned, this does not sound absurd. One could imagine arranging matter atom by atom into such a world. And yet the actual history of the Earth did not realize that possibility. The universe removed dinosaurs long before it produced us.
That does not make such a state meaningless. It remains part of the space of possibilities defined by the laws. And that larger space helps us understand what was actually chosen.
A physical theory is therefore not just a portrait of the world as it is.
It is also a map of the states the world could be in, together with the rules that relate them. The actual universe is one realized state, or one realized history, inside that larger space.
This may seem abstract, but it is a necessary step toward understanding both where the universe came from and where it may be headed.
See also:
What is the quantum structure of the universe?
What is a theory in physics?