Chapter 11 – Edition 1

Some notes on Chapter 11: Schrodinger’s Controversial Cat

We’re hardly the first to modify Schrodinger’s story to fit a different background. The original had only one box and a radioactive atom with a fifty percent probability of decaying and firing the Geiger counter, opening the cyanide, and killing the cat.

We strongly emphasize in class that whether the cat is seen to be dead or alive is absolutely random, and cannot be influenced by the observer (or anything else).

When talking about Schrodinger’s cat in class, we address quantum pseudo-science. RE-address once again, actually. A good majority of students coming into our conceptual physics course have heard something about quantum mechanics, including some quantum nonsense. It’s not their fault that they’re unclear about separating the real quantum mystery from the quantum nonsense. Most conceptual physics books “safely” avoid a discussion of the quantum mysteries and thus abandon this territory to the purveyors of pseudo-science. (It’s the way some biology books avoid presenting evolution as established science or any open treatment of sex.)  Our book emphasizes that, while opening the box puts the cat in either the living or dead state, the opener has no control over which one. Best-selling books, and the movie “Bleep,” imply otherwise.  One best seller says, the “secret” of quantum theory tells how proper thinking can bring about wealth and happiness. We note it might equally well “bring about” poverty and misery, or anything in between.

Quantum mechanics texts in my student days made little, if any, mention of the Schrodinger cat story.  And they certainly hardly treated the issue involved. It was never mentioned in class. Today, the best-selling, upper-division-level quantum text (Griffiths) has a large, alert cat pictured on the front cover, and on the back it is dead.  (One colleague claimed that the publisher was appealing to students’ “misguided” love of mysticism.  But instructors, not students, chose the text.  We’re gratified that modern instructors consider the mysteries of quantum mechanics worth addressing.)

It is in this chapter that we most explicitly make the point that quantum theory has observation not only creating the reality observed, but the appropriate history to go with it (hungry cat or rigor mortis cat, pages 119 and 123). History creation was implicit in the box-pairs experiment, but it was less dramatic .  Realizing that quantum theory implies history-creation is important to counter the argument that it was somehow the mechanical process of opening the boxes differently (sequentially or simultaneously) in the two parts of the experiment that brought about the different results–even if there is no physical explanation for such an effect. Or the result is sometimes erroneously brushed aside by claiming the opening “collapsed the wavefunction.” This latter argument misses the point that the experiment is quantum-theory neutral.  The creation of history is not easily brushed aside.

Something we hardly discuss–it’s just too tricky–is “peeking,” partial observation. We say “any peek’s a look,” but that’s in some sense an overstatement. Initially, all states consistent with the cat being in the box are needed to fully describe the situation. If we now, for example, send a photon into the box, as described on page 119, and, say, we see it come through, demonstrating that the cat was not standing up. Not necessarily dead, just not standing up. That peek, in the Copenhagen interpretation, collapsed the wavefunction into a superposition of all states consistent with that observation. That is, for example, a superposition of states in which the cat was killed by the atom coming into its box and triggering the release of cyanide and of states in which the atom went into the other box, and the cat is just lying down. Each component of this superposition contains a vast number of states. As more information is gained, say with more photons, more states are excluded. All this can also be described with interpretations other than Copenhagen.

After we present the counter argument to Schrodinger’s cat story, it’s amusing to try to start a debate in class. We feel particularly successful in presenting the story with some neutrality if there is a substantial fraction of the class on each side.  Typically, in a conceptual physics class, the majority side with Schrodinger.  When I last very briefly discussed this in a senior quantum physics class, it was about three to one against Schrodinger.