Observer (quantum physics)
In quantum mechanics, "observation" is synonymous with quantum measurement and "observer" with a measurement apparatus and observable with what can be measured. Thus the quantum mechanical observer does not necessarily present or solve any problems over and above the (admittedly difficult) issue of measurement in quantum mechanics. The quantum mechanical observer is also intimately tied to the issue of observer effect.
A number of interpretations of quantum mechanics, notably "consciousness causes collapse", give the observer a special role, or place constraints on who or what can be an observer. For instance, Fritjof Capra writes:
"The crucial feature of atomic physics is that the human observer is not only necessary to observe the properties of an object, but is necessary even to define these properties. ... This can be illustrated with the simple case of a subatomic particle. When observing such a particle, one may choose to measure — among other quantities — the particle's position and its momentum" 
However, other authorities downplay any special role of human observers:
"Of course the introduction of the observer must not be misunderstood to imply that some kind of subjective features are to be brought into the description of nature. The observer has, rather, only the function of registering decisions, i.e., processes in space and time, and it does not matter whether the observer is an apparatus or a human being; but the registration, i.e., the transition from the "possible" to the "actual," is absolutely necessary here and cannot be omitted from the interpretation of quantum theory."
Critics of the special role of the observer also point out that observers can themselves be observed, leading to paradoxes such as that of Wigner's friend; and that it is not clear how much consciousness is required ("Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer - with a PhD?").