Well, I think we need Orac on this one, but I will try one more thing.
Consider a helium atom. It consists of a nucleus with 2 electrons in 'orbit' around it. I emphasize orbit because of course it isn't strictly speaking an orbit, but we can use that for this discussion. An electron has a spin, which can be either up or down. However, there cannot be 2 electrons in the same state, so one of the 2 electrons in a helium atom must be up and the other down. But we have no way of knowing which is up and which is down. In fact electron spin is indeterminate, it could be either up or down, until we measure it, or until it in some way interacts with another particle and decoheres. I will talk in terms of measurement. So we measure the spin of one of the 2 electrons, and now know whether it is up or down, and we know that the other one is the opposite. But we look away for a second, then go back and measure the spin of the same electron again. But when we quit looking at it, it becomes indeterminate again. So we we look back at it and we don't know which way it is spinning. When we measure it again, we may find it in either of the 2 spin states, at random. So we can't have full decoherence, because indeterminacy is always there. Observing an indeterminate state doesn't lock it in, when we look away it becomes indeterminate again.
And I think that is how it works and is the answer to your question.
Bill Gill
