In my amateur attempts to understand quantum mechanics (QM) I realize I have usually either oversimplified or misconstrued the phenomenon known as decoherence. In my mind, I had pictured decoherence as an interaction of a quantum system with the environment that brought about a collapse of superposition states essentially the same way a measurement did in the Copenhagen interpretation of QM. After renewed attempts to understand this topic, (including reading summaries like this), I should say instead that decoherence is an entanglement of the system with its environment. This entanglement involves a sharing of the degrees of freedom of the original coherent system with its environment which leads to creation of a “mixture”. From the perspective of a potential observer, decoherence takes away the interference aspect of the quantum superposition, but it still doesn’t tell the potential observer that a definite outcome of a potential measurement on the mixture has been realized. So, decoherence may account for why quantum interference effects are not seen in our macroscopic world, but it doesn’t remove the quantum measurement effect/problem from the picture at all.Also, while many believe decoherence lends support to one or another interpretation of the measurement problem, it provides no decisive evidence for any of them.
Now having said this, I have read a couple of papers by one of the leaders in the field, Wojciech H. Zurek. He (along with collaborators) seems to be trying to take the program of decoherence farther in the direction of it becoming a replacement for the concept of measurement. As explained in papers such as this and most recently this (see also this brief summary in Nature), the idea is that multiple observers can sample the environment and gain knowledge of a preferred state of a system in question without direct measurement. Decoherence leads to a dissemination of redundant copies of information about the preferred state into the environment such that an observation of any fragment will do the trick. Thus further progress is made explaining why the objective-seeming classical world arises from quantum physics.
This is very good. To understand how a classical-looking world arises from quantum reality is an important project. But it still is a quantum reality.
Despite my mistakes in understanding decoherence, I think some of the issues I’ve focused on seem relatively untouched. It is still true that quantum interactions (whether an observing system is measuring another system or a fragment of the environment or whatever) have a very different nature than classical interactions. I think the issue is sometimes obscured by terminology. Is the interaction a measurement, an observation, or an information transfer? However you put it, there is more going on in these interactions than the classical causal picture of “A” effecting “B”. One or both systems need to have an additional property in order to have a quantum event. This “ability to measure” or “ability to receive information” property is an integral part of the picture. This is in step with Gregg Rosenberg’s postulate of a receptive property in nature discussed in my recent posts. And it continues to make sense to me that incorporating this important aspect of nature into one’s toolkit will help explain consciousness and also perhaps other complex phenomena which resist a reduction to classical mechanics.
The macroscopic world may look classical, but it doesn’t follow that macroscopic phenomena can be explained classically!