> But, in principle, the wave-function of the sun after you throw a book that has a coffee stain into it will be different from the wave-function of the sun aft

> But, in principle, the wave-function of the sun after you throw a book that has a coffee stain into it will be different from the wave-function of the sun after you throw the same book without the coffee stain. That's different from saying that it can be measured in principle. And a having or not a coffee stain is different from the previous example. A coffee stain of a different shape would be closer. > And that difference, in turn, could be detected through measurement, as it will affect the probabilities of a measurement of the sun. I'm not sure how to interpret that. You just said that "It's perhaps not possible even in principle to actually measure it". [edit: maybe you mean that some kind of « statistical trace » remains, but we surely agree that’s very different from being able to identify the words in a particular book.] Anyway, the sun is not an isolated system so it cannot be described with a wave-function. The "wave-function of the universe" - if we assume that such a thing exists which evolves unitarily since the beginning of time - would contain the "information" about the universe in which you threw a book with a coffee stain and about the universe in which you threw a stainless book and about the universes in which there is no Sun to throw things into.