- #106
Ken G
Gold Member
- 4,921
- 552
That's only partially correct, and highly misleading at best. When you couple an electron to a classical system, the electron may be part of a classical system, but it's still an electron, so projecting onto the electron subspace is still projecting onto a quantum system. And when you project a decohered pure-state closed system wavefunction onto an electron subspace, you get a mixed state, as I have been stressing. The electron subspace was originally described as a pure wavefunction (let's say), but after the decohering interaction it is now in a mixed state. That is very non-unitary! But it's not a problem for quantum mechanics because it is not a closed system, it is just a subsystem. Projections are not supposed to be unitary, and an electron does not become a "classical object" just because it is coupled to a classical system, but we do lose the ability to give the electron a wavefunction until we take the next step of perceiving a unique outcome for the electron, and then manually asserting a new pure state for it (which is also a non-unitary step). These are all things happening to the quantum subspace, the electron.BruceW said:No, an electron in quantum state superposition will not undergo a non-unitary process. Only classical objects can undergo a non-unitary process.
Here you mean a measurement in terms of a full collapse to a single new state, and to that I can only ask you: what evidence do you have that any classical object can do that, without a perception to register that final state? Certainly you can draw no evidence from the formal structure of quantum mechanics, which has no such provision. So you must draw from experience of scientists to assert it-- and that is not a classical system free of perception. If we simply watch closely where the various issues crop up, it does indeed crop up when there is an "experiential agent" to register the single outcome, and as I said above, a mindless machine would never have any need or reason to register a single outcome rather than a probability distribution of single outcomes-- just as quantum mechanics predicts they would.From this method, you don't have to set up the system so that a person is making the measurement. Any classical object can 'make a measurement'.
Well this is certainly the fundamental disconnect. You are simply asserting we can do that, but you cannot really give any good reason why we should think that is a reasonable thing to do. We cannot test it, we agree on that, so it becomes a matter of convention more than anything else. I am merely pointing out that there is simply no justification for your assertion, whereas the justification for mine is that it is what quantum mechanics formal theory predicts. I grant you that the predictions of a formal theory are not the same thing as the reality, but at least it is some justification. There's no perfect solution-- we either stick in the non-unitary step in an ad hoc way within the theory, or we say we are leaving the theory when a perceptive conscious agent enters. But I suppose it is not experimentally answerable, so must be classified as an issue of personal taste.But generally, we can choose the non-unitary process to happen at the lab equipment, instead of at the person.
Yes, I think that's a fair characterization, though I'd put it in the way I did above.Ken G's opinion (as far as I understand) is that since we only need to find predictions for what happens from a human's perspective, we can choose our definition of 'measurement' such that only a human can make a measurement.
Well, you certainly wouldn't want to do that, it would be terrible scientific epistemology. We rely on several key concepts in science that this program would not support: in particular, objectivity. The whole theory of relativity is predicated on a symmetry principle among observers, for example. So this would be a bad epistemology. The one I'm talking about suffers none of those problems, because it not only allows for other conscious agents, it even allows for hypothetical conscious agents, to help us form a useful scientific language about what is happening in classical systems. All I'm doing is pointing to where the shell went in the shell game of designing such a language.Ultimately, you could go one step further and say that the only perspective I am interested in is my perspective.
Yes, this is a way to frame it where we are in agreement. It is useful to find the common ground, and recognize then that our only disagreement lies in what is the most logical way to identify what the ability to make a measurement actually entails. It seems like a smaller disagreement that way. I think my earlier discussion with GO1 followed a similar course, but that was a lot of thread ago!So when Ken G was saying 'conscious', in my words, I would say 'the perspectives which we are interested in finding predictions for'. Or more precisely, 'classical objects which we define to be able to make measurements'.