APS News

Letters

Demystifying the Schroedinger Cat

It is time to stop all the nonsense about the Schroedinger cat [APS NEWS, March 2002]. If the experiment is carried out and one waits a long time before the cage is opened and finds a dead cat, one also finds all the information to determine exactly when the cat died. Although we cannot predict beforehand exactly when the cat died, the cat is never in a coherent quantum state with an amplitude for being alive, an amplitude for being dead and a definite relative phase between the two amplitudes. There is no definite relative phase between the state of the live cat and the state of the dead cat and no interference can be observed between the two states.

In precise quantum-mechanical language, one can say that the state of an isolated radioactive nucleus is described by a wave function with a definite phase between the initial nuclear state and the final state where an alpha particle has been emitted. But as soon as there is any interaction, like having the emitted alpha particle break a box of cyanide, the quantum mechanical description of the system must also include the box of cyanide and the interactions involved in its breaking by the alpha particle. The state of the nucleus is no longer described by a wave function, but by a density matrix in which all the degrees of freedom of the cyanide box have been averaged out and all relative phases between the initial and final states of the nucleus have been randomized.

In this context the Schroedinger cat experiment is not really different from a classical chaos experiment in which a ball is moving chaotically in a box and has a certain probability of finding a hole where it can get out and break the box of cyanide.

The essential feature of quantum mechanics is not that a particle can be in two states and we don't know which. Ignorance is not quantum mechanics. The crucial difference between a classical description with ignorance and a quantum description is the existence of probability amplitudes and observable relative phases. There are no observable relative phases in the Schroedinger cat experiment. The fact that the observer who hasn't looked does not know whether the cat is alive or dead at a given time is simple ignorance, not quantum mechanics.

Harry J. Lipkin
Rehovot,Israel

g-2 Experiment is Rock Solid

I was surprised to read in the letter by Burton Richter ("Crotchety but Saintly", April 4, 2002) that he considered the g-2 experiment at BNL an example of a misplaced big claim. This experiment received a lot of attention when it indicated a 2.6 sigma deviation from the Standard Model.

However, when the theorists looked at their prediction more closely they discovered a sign error in their computer program, and the net result was a 1.6 sigma deviation. It should be noted that the experimental results, spearheaded for about 20 years by that master of precision experiments, Vernon Hughes, remained rock solid. Further data will come out this year. Perhaps Richter's argument was that everybody should have waited until the experiment reached its lowest error.

Alas, DOE plans eliminate any further running for this experiment after this year. So what are the experimenters to do: Should they act crotchety or saintly?

Peter Paul
Brookhaven National Laboratory

Polygraph Should be Judged Objectively

I read Don Prosnitz' back page article [APS NEWS, April 2002] expecting imminently to come across his ideas for improving a key element in crime detection- credibility assessment-but was disappointed not to find it. Perhaps that is because the present best instrument, the polygraph, is in such disfavor in the liberal community despite its durability in police and security investigations. If that is the case, then we should be giving high priority to re-examining some of our value judgements.

One pertinent judgment is that the polygraph invades privacy and has a witness testify against himself. True, but if the trade-off is substantial enhancement of national security, perhaps we can no longer afford to be too choosy.

In any case we should approach the polygraph honestly and judge its worth objectively. A decade ago physicists questioned "The Scientific Validity of the Polygraph"-a disingenuous question, like asking when did you stop beating your wife. Obviously, the proper question is what is the best method of Credibility Assessment, and let the chips fall where they may. It is time we did just that.

Lawrence Cranberg
Austin, Texas


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