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Quantum Copenhagenists will tell you if there is a stop sign, it only manifests itself by the act of observation. When no one is looking, there is nothing there. This is like the cognitive process of a baby before learning object constancy. But if nothing is there, how does the stop sign know how to appear at the same spot when observed later? Why can't we look, see a red stop sign, look away, look again and see a green stop sign, look away, see the stop sign has moved, look away and look and find the stop sign has gone? Copenhagenists will tell you it is the mind of the observer which keeps track.

What about change blindness then? Huge changes in perception can be made without being noticed if the observer looks away for a while. The mind does not always keep track of such things. But if it doesn't consciously register, maybe such changes are going on all the time? How can we tell? Only when the location of the stop sign is registered in short term memory can we notice that it has changed. Might we as well claim we create reality according to our beliefs? When something is registered in short term memory, the mind keeps track and manifests according to its subjective beliefs. If it is not registered? Might the laws of physics only hold when we keep note, and be blatantly violated when we don't notice? Can we have glitches where we see a black cat cross our path from one side to another, and then see the same black cat cross again from the same initial direction without seeing it walk back, but only if we never registered the first crossing in our memory in the first place?

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The laws of physics, according to quantum mechanics, would assign definite probabilities - incredibly small ones, such that these events would not be expected to occur once in a googolplex universes - to the "glitches" of which you speak. That's one difference between quantum mechanics and philosophical skepticism. –  Mitchell Porter Jun 4 '12 at 10:34
    
But of course, qualitative and philosophical discussion of QM is surrounded by a miasma of bad or badly motivated interpretive ideas (even when misconceptions about the technical workings of QM aren't present in such a discussion, and they usually are)... –  Mitchell Porter Jun 4 '12 at 10:41
    
The professional physicists tend to insulate themselves from this by adoption of some dogma which allows them to get on with applying QM. Perhaps the best recommendation I can make is a mixture of "positivism" and "realism". Positivism means, pay attention to the testable parts of the theory; realism means, reality does exist and truth takes a definite form, whether or not you know what it is. –  Mitchell Porter Jun 4 '12 at 10:45
    
Whoa! SE allows two people to have exactly the same username. Seeing that this question was posted an hour ago by not me was weird. This is me, physics.stackexchange.com/users/1588/peter-morgan, this is the OP, physics.stackexchange.com/users/9644/peter-morgan –  Peter Morgan Jun 4 '12 at 11:04
    
I am strongly tempted to close this as a invitation to discus not a question, but I'm willing to be proved wrong. –  dmckee Jun 4 '12 at 14:09

2 Answers 2

The OP's question presupposes Copenhagen idealism. It denies the existence of an objective reality out there in place of the mind of the observer. Sadly, some philosophers like Daniel Dennett have demonstrated there is far less in the human brain than once thought. The human brain itself uses the world out there as a representation of itself. The visual field of a human is mostly limited to its fovea and is very small. Only the constant saccades of the eye and the use of the external world as a representation of itself hides this fact. If the existence of the external world is denied, then where is the representation stored? Not in the brain itself.

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Copenhagen idealism doesn't deny or assume the existence of objective reality, it just denies the ability to assign position and momentum to the external objects without a measurement. This is not a crazy stance, it just says that we shouldn't think of quantum mechanics as some sort of probability on classical information, rather as a new way of generating information when you do a measurement. The Dennett stuff is both dubious and of no significance-- less in the human brain compared to what? What model is there other than Dennett's? Copenhagen doesn't store the world in the brain. –  Ron Maimon Jun 18 '12 at 6:11

You describe a common misconception about quantum mechanics. In particular:

Quantum Copenhagenists will tell you if there is a stop sign, it only manifests itself by the act of observation. When no one is looking, there is nothing there.

is not the case. Sticking with basic quantum mechanics for simplicity (but the arguments apply to all of QM) we describe a physical system by a wavefunction. There is nothing approximate or random about a wavefunction. It describes the physical system exactly and it's evolution is time is exactly described by it's Hamiltonian. The wavefunction is as real and predictable as for example the solar system: actually it's more predictable since the solar system is chaotic on long time scales.

Where you get apparent randomness is when the system described by the wavefunction interacts with something else. This is what is implied by the term "observation" as observing the system necessarily means interacting with it. In the original Copenhagen interpretation the wavefunction was described as "collapsing" in a random way. However we now understand the apparent collapse to to be the result of the wavefunction interacting with a much large system with a huge number of degrees of freedom. The interaction is described by decoherence, and it is important to emphasise that this is not a random process.

To use a cliche much beloved of popular science TV programmes, if you drop your cup of coffee on the floor the way it smashes is apparently random. However the randomness is only apparent. If we had very precise knowledge of the coffee cup and the floor we could in principle predict exactly how the cup would shatter. In practice we could never have that much information and the smash looks random. The same is true when a wavefunction interacts with it's environment. It's just the complexity of the system it's interacting with that makes the process appear random.

So to return to your original question, or the title at least, you (and you are far from alone in this :-) have misunderstood what is meant by "observation" and "looking" at the system. The only way a wavefunction is affected by looking at it is by interacting with whatever system you're calling the "observer", and this interaction is perfectly described by QM and is not random. The moon cannot usefully be described by a wavefunction because it's decoherence time is vanishingly short, but you can be confident it does not disappear when you're not looking skywards.

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"The same is true when a wavefunction interacts with it's environment. It's just the complexity of the system it's interacting with that makes the process appear random." this sounds like a hidden variable interpretation of quantum mechanics. –  Larry Harson Jun 4 '12 at 11:47
    
No, it is most emphatically not a hidden variable interpretation. To use my coffee cup analogy, is there a hidden variable that specifies how the cup is going to shatter when it hits the ground? –  John Rennie Jun 4 '12 at 13:49
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Hidden variables - yes, such as the vast number of imperfections in its structure that will lead to irregular large scale ruptures, the point at which it makes contact with the ground, how the shock wave propagates through the cup etc. –  Larry Harson Jun 4 '12 at 20:55
    
Take the EPR experiment: the hidden variables idea says that one particle is spin up and the spin down from the moment they're emitted, but we don't know which. Decoherence says the spin is not determined until the moment of interaction with the environment, and different environments would give different results. So there is no hidden spin variable. But, the interaction with the environment is deterministic. If we knew everything about the particles and the environment we could predict which what result the spin measurement would give. This is not hidden variables as I understand the term. –  John Rennie Jun 5 '12 at 6:13

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