Can observers be particles? Generally Quantum mechanics divides a system what is to be observed and an observer. This is generally taken to be some human being. But why restrict it to such? Why not a particle?
Is there a good physical reason or philosophical reason for this to dismissed as not sustainable?
I'm thinking here specifically of the Copenhagen interpretation, or of its modern incarnation, consistent histories. I understand that decoherence in consistent histories completely replaces the idea of the wave collapse in the Copenhagen Interpretation.
Essentially, the idea of observed system and an observer is supplemented with an environment, which on the face of it seems entirely natural. The idea of decoherence comes from statistical physics.
I'm suggesting that a particle that acts like an 'observer' needs to 'know' what state the observed system is in to 'know' how to react to it. Decoherence resolves the superposition of states in the observed system to a probability mixture.
 A: This depends on the placement of the Heisenberg cut. The placement of Heisenberg cut is arbitrary (but certain choices would be called different interpretations of quantum mechanics). Everything beyond the Heisenberg cut (on the side of Heisenberg cut opposite to the observed system) can work as the observer.
A: Whether a particle can be an observer or not is partially a question of which interpretation of QM is being considered and partially a semantic question (how we define “observer”).
The viewpoint you describe in the question is closest to Rovelli’s relational quantum mechanics.
A: Since this question has come up to the front again, I think it involves the navel gazing of people discussing philosophy of quantum mechanics.
In particle physics , all the experimental results on which the standard model is validated happens with "particles in --- particles out" , to get the crossections and decays. The primary interaction is the observer. Certainly it is not the data banks where the interaction's measurements are held, nor the physicists that gather the thousands and millions of interactions to check the quantum mechanical probability distributions for crossections and decays.
In the  interaction region there are only particles and fields,  no human observers observe each event as it happens, it is the motion of the incoming particles, protons on protons,  that creates the interactions at LHC.
A: I think that in QM one usually refers to the observer as a human, since is a human who reads the result on the screen of the experimental device but, fundamentally, for anything more.
When a human observe a quantum mechanical system, it does through fields and particles, not with his mind. The fact that a human observe spin up or down in a Stern-Gerlach experiment is indeed the fact that the electron interacts with a magnetic field and then it passes through a screen. Where is the human?
Of course, there exist a philosophical question: if the world exists if there isn't anybody looking at it. I suppose the answer could depend on who answers the question, but of course I don's think that human beings have any special role in the universe and, of course, not in the physical laws. So i'd say definitely no, QM doesn't need humans.
