Unbeliever

I'm hoping someone out there can tell me: why it is that Shroedinger's cat wouldn't itself have constituted an observer, fully able to collapse the wave function long before the box was opened by some human scientist? I guess what I'm really asking is: what are the necessary characteristics for something to be considered an observer? Is self-awareness necessary? Can't a particle that has been struck, and therefore diverted from its original trajectory, by another particle, be said to in some sense have "observed" the other particle? Because it seems to me that the particle's wave function must have collapsed at the instant of contact. Someone help me out here,please?

Jesse

Except in some weird interpretations of QM, there's no requirement that "observations" be made by conscious observers--any physical system that in some sense records information about a quantum system will act like an observer (I think Feynman in one of his books mentioned an example of a charged particle that left a trail as it travelled through a crystal). I believe what matters is just whether a system is isolated or whether it is interacting with the outside world in such a way that information about its state is being recorded in some way. The Schrodinger's cat thought-experiment is unrealistic because in practical terms you can't completely isolate an object that large from the outside world--even small thermal interactions are enough to cause Decoherence. But if you did have some kind of shield that could completely isolate the cat, the experiment would work just as well if you had a human being in the box--whatever is in the box will behave as if it's in a superposition of states until the box is opened, in the sense that if you know the exact wavefunction of the system in the box at the moment it was sealed, you'll be able to calculate where to look for evidence of interference when the box is opened (interference which would not have been present if the system had not been completely isolated).

In a way it seems like there's something a little solipsistic about this though--if I'm in the box, then from my point of view I think it'd probably be like the entire outside universe was in a massive superposition until the box was opened, while I was in a definite state the whole time. I'm not really sure how the different interpretations of QM would deal this sort of "relativity" though...in practical terms it'd be even more impossible to calculate the wavefunction of the entire universe outside the box to figure out where to look for evidence of interference than it would be to calculate the wavefunction of a cat or a person inside a box.

Elvithriel

"Anyone who says they understand quantum mechanics, doesn't."

fando

I was never able to move from the math to a description in common terms and intuition, so good luck breaking your brain over this subject without math.

It will help immensely to study even the most basic concepts in Quantum Mechanics in the usual way taught to undergrads. Start with Young's Double Slit experiment and then move on to the classic thought experiment of a quantum particle in a one dimensional potential well, which is almost analogous to Schroedinger's cat experiment. That should be enough to grasp the nature of observation in the quantum world. I recommend Liboff's Introductory Quantum Mechanics. It's approachable to anyone with an year of college calculus, a familiarity with 'operators' and a motivation to learn.

Undercurrent

In the "many minds" interpretation of wavefunction collapse, the wavefunction never really collapses, but the mind of the observer becomes entangled with it. In cheezy faux-equations, the state changes from:

(|live cat> + |dead cat>)|uncertain observer>

to:

|live cat>|observer seeing live cat> + |dead cat>|observer seeing dead cat>

In words, the cat, before observation is in a superposition of live and dead states, while the observer is in a single "don't know about the cat" state. After the observation, the observer is also in a superposition of states, one of which observed a dead cat, and if it made the observation again, would always see the dead cat, the other observed the live cat, and if it checked again, it would always see the dead cat.

And if the observer went off to tell someone else what he saw, the person he told would get into an entagled state based on his observation of what the observer said.

The good news with that, is that we get rid of that messy non-unitary wavefunction collapse thingee. The bad news is it gets very confusing.

SteveD

Shroedingers cat is usually misrepresented as an actually interpretation of QM. He actually used the example to illustrate that the indeterminacy of the wave function at an atomic scale should not be extended to macroscopic objects.

Here are some excerpts from his paper which can be found here

Schroedingers Cat

Steve

Jesse

Well, I would agree that it is a mistake to equate "treating an isolated system as being in a superposition of states in order to calculate probabilities" with the system being genuinely "blurry" in some sense while you're not observing it. We don't know what systems are "really" doing when they're not being observed--that's the question that different interpretations of QM try to answer. But the point is that if a system is not being observed, then when you do measure it the probabilities of different outcomes will be different than what they would be if the system had been observed all along. This is the meaning of "interference". For example, in the double slit-experiment, the particle's position when it hits the screen is equally well-defined whether you were watching which slit it went through or not, it's just that the probability that the particle lands on different areas of the screen will be different depending on whether you measured which slit it went through. And if you didn't measure it, you calculate the probability of it landing on different points on the screen using a model involving a "superposition" of all possible paths through the slits, but it's an open question whether the particle actually "blurred out" and went through both slits at once or if it had a definite path.

The case of a truly isolated macroscopic system, if such a thing were possible, would be similar. If you knew the maximum possible information allowable by the uncertainty principle about the system before it became isolated (sealed in the box), you could then calculate the evolution of its wavefunction, and use this to make probabilistic predictions about its state when it was measured (the box was opened) later on. If the macroscopic system had a "memory" of some kind--say it's a person sealed in the impenetrable box--then its memory would be in a definite state when the box was opened, so it would remember a single clear history while it was sealed up, not some kind of blurry superposition. But the quantum effects would show up in your precise calculation of the probabilities of where each particle would be (or each particle's momentum or spin or whatever) when you opened the box--you might see subtle correlations that you would not have gotten if the system had been measured all along, a much more complicated version of the strange probabilities you get in the simple case of a two-particle entangled system like the EPR experiment which I discussed on this thread. There'd be a variety of ways to interpret this, like a "many minds" interpretation where the person in the box split up into various versions of himself which "interfered" with each other (influenced each other's probabilities).

Keeping a macroscopic system like a cat or a person completely isolated, or being able to measure the maximum possible information about its state in order to calculate its exact wavefunction, is something that's impossible for now. If we ever develop large quantum computers, something like this might become possible though--perhaps we could simulate an isolated artificial intelligence on such a computer, and since it's a computer program we could have very precise knowledge of its initial state. David Deutsch, a many-worlds advocate and one of the pioneers of quantum computing theory, actually suggested that an experiment like this, involving an artificial intelligence running on a computer and kept isolated for some time so that it would show interference, could provide evidence for the many-worlds theory. I guess his idea was that showing interference would be evidence that the artificial intelligence was being influenced by its counterparts in other worlds (one paper where he discusses this idea is here--the part about the artificial intelligence is on p. 16).

Ernest Sparks

People forget that Irwin Schrödinger (1887-1961) posed his ugly thought experiment to counter the prevailing probability interpretation of the wave function, which interpretation he strongly disliked.

Peter Soderqvist

TO JESSE

Soderqvist1: If you write down a quantum mechanical wave equation for the whole universe, both the cat-box, and the rest of the universe are in a superposition of states, what causes the state vector reduction when everything in your equation is in an Unitary Evolution? This paradox can be solved by john Neumann's hypothesis, and Eugene Wigner 's immaterial mind hypothesis. John von Neumann consider everything is in a superposition of states until consciousness enter the scene, and collapse the quantum wave function, because a Geiger counter connected to a quantum system (cat-box] ends up in a superposition of states too, and so on with further adding with measuring apparatuses in a infinite regress, this is called Neumann's infinite chain! Consciousness is the collapser of wave function according to Neumann! When gödel introduced his incompleteness theorem I Königsberg, Germany, on September 7 1930, Neumann was first to see the implications of Godel's theorem, (I know this because I have read the book by John L. Casti and Werner DePauli, Gödel a Life of Logic). I think the reason was that both deals with infinite regress, and the human minds' ability to end the chain! Goswami is a specialist regarding the Quantum measurement! I have posted links to gödel here in the philosophy forum!
http://www.iidb.org/vbb/showthread.p...494#post963494

Emphasis in bold type by me!

University of St Andrews in Scotland School of Mathematics and Statistics
These statements contradict two of our basic notions. We are rejecting realism, which tells us that a quantity has a value, to put things more grandly -- the physical world has an existence, independent of the actions of any observer. We are also rejecting determinism, the belief that, if we have a complete knowledge of the state of the system. In fact, though, Niels Bohr and Werner Heisenberg were convinced that one should not aim at realism. They were therefore pleased when John von Neumann proved a theorem claiming to show rigorously that it is impossible to add hidden variables to the structure of quantum theory. This was to be very generally accepted for over thirty years. Bell had showed rigorously that one could not have local realistic theories of quantum theory. Henry Stapp called this result the most profound discovery of science.
http://www-groups.dcs.st-and.ac.uk/~...Bell_John.html

Professor Amit Goswami at the University of Oregon!
The interpretational difficulties of quantum mechanics can be solved with the hypothesis (von Neumann, 1955; Wigner, 1962) that consciousness collapses the quantum wave function. The paradoxes raised against this hypothesis have now all been satisfactorily solved (Bass, 1971; Blood, 1993; Goswami, 1989, 1993; Stapp, 1993). There is, however, one question that continues to be raised: Is consciousness absolutely necessary for interpreting quantum mechanics? Can we find other alternatives to collapse and consciousness as the collapser?
http://www.swcp.com/~hswift/swc/Summ...oswami9901.htm

Peter Soderqvist

I have no reference to the whole quote to post, but it can be found in john Gribbin 's book in Search for Schrodinger 's Cat, that Schrodinger has said after his visit in Bohr's home in Copenhagen something like; if I had known that I have to accept quantum jumps, in quantum physics, I would never have taken part of it in the first place! But he consider like Amit Goswami that consciousness is the ground of all being, that can be confirmed if you click on my home side and read his little book; what is life? endnote! He has also said in his lecture mind and matter that he has proved that with use of Einstein's theory of relativity, that the mind is indestructible by time! Mind and matter is included in later edition of What is Life? on Amazon com!

Quotations by Erwin Schrodinger
[On quantum mechanics ] I don't like it, and I'm sorry I ever had anything to do with it.
http://www-groups.dcs.st-and.ac.uk/~...hrodinger.html