When scientists calculate what will be observed in an experiment they manipulate wave functions. These do not describe properties being measured themselves but the probabilities of these properties. During the actual experiment properties will be manifest consistant with these results but specific values and outcomes cannot be predicted. Even whether wave or particle propeties will be manifest is determined by the choice of the experiement -- whether to observe waves or particles. Until the observation all possible properties and their values are said to have a superposed existence. In the observation of properties the wave functions are said to "collapse" -- to take on descrete values -- and the "real" world we are familiar with appears out of the mathematical fog. How and when this happens is the measurement problem.
Because quantum properties have only a quasi-existance until they are measured it seems logical to assume that the objects they compose must also. Since quantum properties do not exist until measured it therefore seems that nothing exists unless it is measured or observed. This can be extended not just to local reality but to the whole universe. It could be and has been argued that until humans evolved to observe it the universe did not exist as we know it. Or perhaps some extraterrestrial intelligence served this purpose or the mind of God holds it all in existance. This is clearly not a common sensical way to view things, but it is not unprecedented. The Empiricist philosopher Bishop George Berkeley (1685-1783) argued exactly the the same thing. And Bohr and Einstein disagreed on whether the moon continues to exist when we do not look at it (Bohr thought it was meaningless to say so).
(I remember reading a report of Bohr and Einstein discussing the existence of the moon but have been unable to locate the source. The best I can do at this time is refer you to p. 29 of Who Got Einstein's Office by Ed Regis, Addison-Wesley, 1987. He writes, "'We often discussed his notions of objective reality," said Einstein's biographer, Abraham Pais, who knew him at the Institute. 'I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it.'" Abraham Pais was a noted physicist himself. Also note the subtlety of the argument: Bohr did not say the moon did not exist when not observed, only that the question of its existence in that case was meaningless.)
Actually the idea that physical objects just aggregate the properties of their quantum constituants is sort of the classical principle of analogy working in reverse -- the whole is the sum of the parts. Since we already know that other classical principles just do not work in a QM world, maybe we should just assume that this one just doesn't either.
Nevertheless as the example of Schrodinger's cat shows we still have to worry about the measurement problem even if we can assume that the universe as a whole is stable. In the case of Schrodinger's cat the question is as much when as how the measurement occurs and the wave function collapses and reality is manifest. Unless you believe the cat assumes some superposition of life/death until the box opens -- which is absurd -- it seems impossible to avoid assuming that somerthing may have happened to that cat while we are not looking and that if that is the case we could forensically figure out exactly when it did after we opened the box.
You can always substitute lower (or higher) animals for Schrodinger's cat to show the absurdity of arguing that any intelligence inside the box is sufficient to perform a measurement and solve the paradox. And once you simplify the measurer to a mechanical device additional problems arise. Mechanical systems are just aggregates of smaller quantum systems and we know that we can calculate the quantum state of any such system very accurately. Once again until we look at the results of a mechanical recorder does it have any defined state? It begs the question to ask whether and when the experiment being so remotely observed in time has done the same.
But then aren't we just mechanical systems too -- albeit rather wet ones? If the mind is a function of the brain which seems almost certain given the clues being dug out by cognitive neuro-science these days then aren't we just large aggregates of quantum systems too? Certainly we are made of atoms and molecules perhaps with such fine structure that we ourselves may not be able to avoid the implications of QM for our own functioning (an idea suggested by Roger Penrose, though not confirmed with any certainty). So if a quantum system ultimately relies on a system that is ultimately a quantum system to resolve its state, wherein is the paradox?
Ultimately these questions remain: What exactly is an observation? What is a measurement? And perhaps even, Why is one needed?
For a growing number of scientists these are questions that are not bothersome because they are not considered. QM works. It predicts real physical properties and actions well. If we do not understand why that is of no consequence. We can still calculate and predict a result. That is all that is needed and all we need to know about the world. In this view QM is like a cookbook for producing the world. We don't know anything about the receipes except that they work and it is pointless to seek to know more. There either is no underlying reality to it or if there is it is somehow an embodiment of QM logic that is entirely different from what we call the real world.
.Return to Tom Jonard's Quantum Mechanics page.