1. Re: NYTimes: quantum theory w/o observers (meekerdb) There's a good paper by Schlosshauer and Camilleri that discusses the historical debate over the Heisenberg cut and how the question has mostly been answered by the idea of decoherence.
http://arxiv.org/pdf/0804.1609v1.pdf
I say 'mostly' because pushing the Heisenberg cut all the way to include conscious observers, as in Everett's interpretation of QM, is still a little controversial because we lack a good theory of consciousness. You won't learn anything useful from the NYT discussion (Callendar's essay is good, but it's short and assumes a lot of background). The best current ideas are decoherence and Everett's many worlds. Read Schlosshauer's review papers:
http://arxiv.org/pdf/0804.1609v1.pdf
or for more depth
http://arxiv.org/pdf/quant-ph/0312059.pdf
Brent Meeker
--ok. First of all, the "decoherence program" seemed to me inadequate. This is that every physical system is contained in an "environment" and since it interacts with environment it effectively is being "measured" (no real measurement is needed though, that concept is claimed really not to exist). So for example, why do you not experience being in a superposition of living in Paris and Tokyo? Because some cosmic ray, or air molecule, or whatever (environment) interacts with you in Tokyo only, thus "measuring" the fact you are in Tokyo or not. Sounds great... except why wasn't the cosmic ray in a superposition of being in Paris & Tokyo? This doesn't solve the problem of why the world is not weird; it merely pretends to solve the problem by already assuming the answer. ("It's turtles all the way down.") I once wrote a short paper on "why quantum computers won't work" whose argument rested on 4 axioms about the nature of decoherence, which may or may not be true. Read it: "Argument against quantum computers (or against certain decoherence models)" http://rangevoting.org/WarrenSmithPages/homepage/whywontwork2.ps Recently Scott Aaronson read this paper and pointed out things are not as simple as my paper made it appear... for example, some of the axioms are not really well defined. (What is the "location" of a "qubit"?) Second. What if the physical system is "the whole universe"? I have an idea about that, but it is only very slightly mentioned in the papers Meeker cited. It is this. There is a part of the universe we can perceive, and there is a part we cannot. For example, we have a hard time perceiving dark matter and gravitons. Nevertheless, there are interactions between parts A and B. So my proposal is, since we can only see part A, part B acts as the external environment from our point of view. Now what is needed to make this work, is that almost always (or at least, "often"), when parts A and B interact, they never do so again. Similarly to, if you interact with that cosmic ray, it flies off into outer space, never to interact with you again. This causes it to carry away "measured information" permanently. If you could keep interacting over & over again with that cosmic ray, it'd get entangled with your position, and hence would no longer be "measuring" your position in the sense of Von Neumann measurement. (There is an interpretation of Von Neumann measurement involving randomization of the phase angles of off-diagonal terms in density matrix; this randoms need to be independent.) So with the cosmic ray, the problem is, the whole universe might get entangled with your position, so the whole damn universe would not be capable of "measuring" it, so the whole argument would become doubtful. That's why the cosmic ray explanation is inadequate. Now, to return to my "parts A & B" explanation, I have proven, that gravitons after they are created almost never interact with anything. Ever. For the rest of time. See this paper by me: "From time-irreversibility in gravity to measurement in quantum mechanics" http://rangevoting.org/WarrenSmithPages/homepage/absorber.pdf http://rangevoting.org/WarrenSmithPages/homepage/absorber.ps "Almost never" means the interactions occur for less than any fixed positive constant fraction of gravitons. "Anything" means matter. My point is this. The gravitons form a second, shadow universe, or external environment, for the parts of the universe we can see. There is no issue that this external environment might get entangled with our part and hence unable to "measure" our part, because of my "almost never interacts again" theorem -- the first time our part interacts with a graviton, it is a virgin, and that graviton (now entangled), never comes back to haunt us (re-interact, non-independent randomness). So effectively via gravitons, there is an inexhaustible resource of new, non-entangled, random qubits in part B of the universe, which is able to keep decohering our part A. If you examine the numbers, this effect alone is large enough (even though gravity is an extremely weak interaction) to explain why the world appears classical to a human observer. So that is my theory.