Observers in quantum mechanics at the moment of observation

In quantum mechanics, the measurer (subject) plays important roles influencing the state of an object under experimental observation. That implies the necessity of inclusion of the subject (observers) at the moment of observation in the final theory. What is “the observer” that makes the state of object jump into one of the eigenstates? By observing the observer at the moment of observation phenomenologically, we find that there exists a state of observers which is common, at the moment of observation, to all observers and which may be described by physics terminologies such as vacuum and ground state in extended sense to other fields of science. This state of observers satisfies the requirements of the universality and reproducibility which are necessary for any object studied in physics. Taking the entangled two neutral kaons as an example, we show how it could be included in quantum mechanics.