What is this business about observers? What are the other quantum theories without observers? How do they compare to each other? Why should we care?
Standard textbook quantum mechanics has observers at the very heart of the theory. It discusses what happens when a measurement is performed. This is fine as a phenomenological prescription, but it is meaningless for a fundamental theory, a theory which is to explain the existence of those experiments.
John Bell in “Quantum mechanics for cosmologists” (Speakable and unspeakable in quantum mechanics) eloquently and directly presents the problem:
It would seem that the theory is exclusively concerned with ‘results of measurements’ and has nothing to say about anything else. When the ‘system’ in question is the whole world where is the ‘measurer’ to be found? Inside, rather than outside, presumably. What exactly qualifies some subsystems to play this role? Was the world 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 more highly qualified measurer — with a Ph.D.? If the theory is to apply to anything but idealized laboratory operations, are we not obliged to admit that more or less ‘measurement-like’ processes are going on more or less all the time more or less everywhere? Is there ever then a moment when there is no jumping and the Schrödinger equation applies?
Bohmian mechanics is one theory that satisfies the need to avoid measurement and observers in the formulation of the theory. But there are others.
Ontology
What is the theory about? Decide this before equations and deductions are made. What is the ontology of Bohmian mechanics?
GRW and Spontaneous Collapse
Can we modify the wave evolution so that collapse is occurring independently of measurement? What is the ontology of such a theory?
Everett and Many Worlds
Is it possible not to collapse the wave function and to avoid using particles with positions? What is the ontology of such a theory?
Decoherent Histories
How much can we do with focusing on theories as information? What is the ontology of such a theory?
OtheR QTWo
Other quantum theories without observers may exist. What are they?