Hi all, I'm writing something that touches on issues of quantum measurement (though the main theme is something different), and I want to be sure I'm not saying anything wrong about this very tricky subject.
So I'm going to pose some thought-experiments to the physics experts on this list. I apologize if this is slightly off-topic but I think these issues are interesting and relevant enough that I don't feel like I'm exploiting the list improperly. The questions I have in mind pertain to the nature of quantum measurement in general, but for sake of concreteness I'm going to pose them in the context of a particular quantum phenomenon, the "quantum eraser." Specifically, I'll refer to the quantum eraser thought experiment summarized at http://grad.physics.sunysb.edu/~amarch/ though the same point could be made in regard to other similar (real or gedanken) experiments. The questions I have regard the replacement of the Coincidence Counter (from here on: CC) in the above experiment with a more complicated apparatus. What if we replace the CC with one of the following: 1) a carefully sealed, exquisitely well insulated box with a printer inside it. The printer is hooked up so that it prints, on paper, an exact record of everything that comes into the CC. Then, to "erase" the printed record, the whole box is melted, or annihilated using nuclear explosives, or whatever. 2) the same as 1, except the box has a person (let's call him Fred) inside who reads the printout. 3) a printer that is not encapsulated in a box. There is a person (Fred) standing near the printer who, when he takes each sheet of paper out of the printer, puts it in an envelope and seals the envelope, but without ever looking at the paper. Then, to erase the printed record, the envelopes are burned, or nuclearly annihilated, or whatever. 4) the same as 3, except that Fred looks at each sheet of paper before putting it in the envelope. But Fred never communicates anything about what he sees to Bob, who is the guy looking at the screen to see if there's a wavelike or particlelike output pattern on it. The erasure occurs by annihilating both Fred and the envelopes. 5) the same as 4, except that Fred and Bob are not individual people but the two brain-lobes of a person who has experienced a severance of the corpus callosum. Thus Fred cannot communicate information to Bob even though they are "in the same head." (OK, this one is proposed purely for entertainment value ;) 6) one can redo the experiments with Fred using a pigeon instead of a human, where the printer is replaced by an apparatus that flashes colored rectangles on a touch screen, and the animal is encouraged to pay attention to the screen by being forced to touch the rectangles to get food. What will the outcome be in these experiments? Has anyone performed experiments like these? (1, 3 and 6 are the onIes that could be humanely carried out, obviously.) My own understanding is that whether Fred, a pigeon or a printer is involved in the experiment should be basically irrelevant. That is, I don't think "registration in consciousness" (whatever that means) is the important thing, but rather registration in the sense of "statistical correlation with some macroscopic system effectively obeying classical probability theory." However, I realize that not everyone agrees with me on this; my reading of Penrose, for instance, is that he would predict a different outcome for 2 versus 1, because he believes that Fred's brain (via unspecified quantum gravity related effects) does something special to collapse the wave function, which the printer does not. Furthermore, my own understanding is that, so long as a) the information from the CC is not statistically correlated with the observer (Bob) who is observing the screen, prior to its "erasure," and b) the erasure is complete, i.e. there is really no way to reconstruct the information after the erasure is carried out then the situation should be just like in the original quantum eraser experiment. Regarding a, note that statistical correlation is transitive to a significant degree, so that if the information from the CC is correlated with some entity Y (such as Fred, or a printer) that is in turn correlated with Bob, then this means the information from the CC is correlated with Bob. The main open question I see regarding this "statistical correlation" interpretation is whether it's really possible to adequately annihilate a macroscopic entity like Fred or a printer, and whether it's really possible to adequately isolate two macroscopic entities (like Bob vs. the printer or Bob vs. the envelope or Bob vs. Fred). One could argue that there will always be subtle correlations between Bob and these other things (since e.g. both Bob and Fred almost surely contain atoms that once belonged to George Washington), and that these subtle correlations mean that in all these scenarios there actually is some statistical correlation between the screen and the contents of the CC, via transitive correlations such as CC --> printer --> George Washington --> Bob --> screen I don't like this argument because it seems to me that these correlations should be too small to make any difference, but I recognize it as an apparent mathematical loophole, according to which my variant scenarios may be considered as different from the original quantum eraser experiment. Comments? -- Ben Goertzel