[RobFromGa]

It's not that the cat IS both alive and dead, it's that we can't know which is true. We can only know the probability of the event being true. Only by actual observtaion is the actual state know and then it becomes one or the other.

from whatis.com:

Here's Schrödinger's (theoretical) experiment: We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we cannot know, the cat is both dead and alive according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that it can never be known what the outcome would have been if it were not observed.

[Mind-numbed Robot]

...it can never be known what the outcome would have been if it were not observed.

The old tree in the forest thingy. Science ain't so hard. :-)

[Physicist]

It's not that the cat IS both alive and dead, it's that we can't know which is true. We can only know the probability of the event being true. Only by actual observtaion is the actual state know and then it becomes one or the other.

That's not correct. The two states are simultaneously "real" at the 50% level. Quantum superposition is a statement about reality, and not only about our knowledge of reality.

Einstein shared your misconception, and insisted that the two would be experimentally indistinguishable in any case. After his death, however, it was discovered that he was wrong: indeterminate states behave differently from merely undetermined states. The latter will obey Bell's Inequality when you compare correlated measurements; the former will violate it.

The experimental fact is that entangled states yield measurements that violate Bell's Inequality.

[Petronski]

That's the clearest explanation of Schrödinger's cat I have ever read.

Thanks.

[TChris]

This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that it can never be known what the outcome would have been if it were not observed.

I still have a problem with the generalization that an obervation cannot be made without affecting the outcome. I would accept that in some cases, even most cases, that an observation would affect the outcome, but absolutes seem to be dangerous territory.