Stop beating a dead cat

[Lucifuge Rofocale]

For the non-versed about Schroedinger's Cat, there is an interesting article here

From the paper:

From the amount of ink that has been spilled on this subject, one might assume that Schroedinger had devoted a whole book or at least a lengthy paper to the thought experiment when he introduced it. In fact, his mention of it is hardly more than an aside. In a general paper on quantum mechanics, he discusses and rejects the interpretation that a single quantum is somehow phyiscally "spread out" or "blurred" among the different parts of a superposition (for example, that the electron in the double slit experiment somehow makes itself into a cloud and manages to go through both slits). He then emphasizes this as follows:

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrochloric acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The [wave] function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.
It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.

Schroedinger's thought experiment has suffered much abuse at the hands of poorly informed writers. In many treatments, for example, it is taken for granted that a real cat placed in a box like the one described would be in a quantum superposition state until someone opens the box and looks at it. This is not true, and it confuses the lesson to be learned from the thought experiment. Unfortunately, Schroedinger was writing in the earliest days of quantum mechanics, before much attention had been given to the distinction between superposition and mixture states. Let's look first at what actually happens to Schroedinger's cat, and then return to the questions raised by the thought experiment.

So What's the Problem?
It might seem as if the process of decoherence can dispel these questions of the role of the observer in quantum mechanics. Decoherence through amplification is, after all, a purely physical process. No consciousness is required to make it happen. Our choice to look in the box has no effect on the subsequent behavior of the system; it only effects our state of knowledge about it.

In the Copenhagen Interpretation, all unobserved states (superpositions and mixtures both) are viewed very much alike from a philosophical standpoint. Because no observation has yet been made on them, they are not considered objectively real (since objectivity implies that different persons have a shared experience of the phenomenon, and before observation there is no experience to share!). Some of these unobserved states (the coherent ones) have interference terms in their descriptions, others do not. Either way, our description incorporates the incompleteness of our knowledge--knowledge that can only become complete through actual observation. With this philosophical orientation, one would indeed say that the cat is neither alive nor dead as a matter of objective reality until an observation is made.

From a philosophical orientation that is more metaphysical and less epistemological, there is no great difference between a pre-observation mixture and a post-observation single state. From this perspective, the objective reality of the cat's condition does not change on opening the box. If the cat is objectively dead, it became so the instant the flask broke, not at some later time when the box was opened. These two philosophical viewpoints are equally consistent with the facts; there is no experiment that can distinguish between them. It is more a matter of preference than of science.

The transition from superposition to mixture, and hence the transition for quantum weirdness to classical normalness, can indeed happen long before any human observes the system. In fact, it usually does. But this fact alone is not enough to completely separate the process of conscious observation from the behavior of quantum systems. Although we can have mixtures without observation, we still cannot have superpositions with observation. We can only directly observe states that are no longer in superposition. The coherence can be lost long before the observation, or it can be lost at the moment of observation, but it cannot be lost after the observation. Whenever we look we see something--something definite, the system in a well-defined state. In classical physics, we are free to assume that the system was in that state all along, before we bothered to look. In quantum mechanics, this assumption is sometimes possible (if we are observing a mixture), but soemtimes not (if we are observing a state that was in superposition).

Observation on a mixture state has no physical consequences, and so we are free to endulge either an epistemological or a metaphysical view of reality when it comes to mixtures; it makes no difference. But if you follow the metaphysical perspective, you enter a nightmare world when you try to extend your picture of reality to encompass the superposition states too. A dead cat and a live cat do not interfere with each other, but an electron going through slit 1 and an electron going through slit 2 do. Accounting for the interference and maintaing a picture of a single, localized, objectively real electron traversing the experimental apparatus is impossible. It is this impossibility that is demonstrated and put into experimentally testable terms by Bell's inequality.

In no case, however, should one think of consciousness as physically producing a change in the state of a system, either forcing a superposition into a mixture or forcing a mixture into a single state. Observing the cat does not kill the cat. It is very unfortunate that many popular writers on quantum mechanics have given this impression of the role of the observer. It is inconsistent with both the epistemological philosophy of the Copenhagen Interpretation and the metaphysical philosophy of hidden-variable approaches such as Bohm's.