sentences:  It is important to realize that this does not imply that an observer actually takes note of what happens.It is sufficient to destroy the interference pattern, if the path information is accessible in principle from the experiment or even if it is dispersed in the environment and beyond any technical possibility to be recovered, but in principle still ‘‘out there.’’ The absence of any such information is "the essential criterion" for quantum interference to appear.For an equation describing a physical phenomenon, the superposition principle states that a combination of solutions to a linear equation is also a solution of it.