Question I once asked a physics Q&A site, "Is physical reality possible without an observer?" They told me it was to broad of a question, and I got no answer. But from a philosophical or metaphysical stand point, is there an answer? I'm aware of the Copenhagen Interpretation of QM, and it's requirement of observation and measurement for existence of macroscopic structure to be. If, in philosophy, an observer is needed, then how did galaxies, stars, planets, and the Universe itself, come into being? Also, can other organisms other than human beings, make a measurement, and thereby effect their ambient reality? Sun Flowers, for instance, follow the moving sun as it traverses across the sky, via heliotropism. Can this be considered making a measurement?

Accepted:
July 10, 2016

Comments

You're quite right to be

You're quite right to be skeptical of the idea that physical reality needs observers to exist. But very few people who work in foundations of quantum mechanics would disagree. Bohr never claimed otherwise. What he insisted is that in describing quantum experiments, we have no choice but to appeal to classical concepts. There's room to argue about what that means, but nothing I've ever read by Bohr suggests that measurements are necessary for the very existence of physical systems.

The same is even more clearly true of Bohmian "pilot-wave" interpretations. Bohmians claim that there really are particles that really have positions and trajectories---whether or not anyone makes measurements. It's just that the details of these trajectories aren't accessible to us.

Everettian ("many-worlds") accounts of quantum theory likewise don't hold that the existence of physical reality depends on measurement. It's true that "branching" in Everettian quantum mechanics has a connection with measurement, but this is only because measurements are a special case of processes that produce what's called decoherence. And decoherence is a physical phenomenon that occurs when systems lose quantum coherence through multiple interactions with the environment. (Put another way: decoherence is a matter of "entanglement" getting dissipated into the environment. )

Collapse interpretations may seem to give measurement a special place, but the most important ones don't really do that. Rather, they state physical conditions in which the wave function is likely to collapse. Measurements fulfill the conditions, but so do other interactions. Very roughly, as the number of particles in a system increases, the more likely collapses become on standard collapse theories. But the equations governing the probability of collapse don't depend on the notion of measurement.

Like you, I think it would be very peculiar to believe that galaxies, stars, etc. couldn't have existed without observers. But contrary to what you sometimes hear, almost everyone who works in foundations of quantum theory would agree!