Tuesday, April 19, 2005

Jeffrey Mishlove interviews Murray Gell-Mann

The Quantum and the Quasi-Classical: Jeffrey Mishlove interviews Murray Gell-Mann (1998)
(William James Bookstore)
MISHLOVE: The paradox here, if I understand it, is that in quantum theory the probabilistic event is sort of viewed as a probability function, or sometimes I’ve even heard the term a probability cloud. It’s as if both true and false are occurring at the same time.

GELL-MANN: Yes, and that’s what I think is very misleading, and my colleagues think is very misleading. In the Schrödinger cat story, for example, the part I told is very reasonable and simply illustrates that a probabilistic quantum event can be coupled to some classical change in the heavy, macroscopic objects around us. That’s fine. But the other thing people say is, “Well, suppose the cat is in a box, and the quantum event occurs, but you don’t know which way it went, and the cat is dead if it went one way and alive if it went the other, and so until you open the box and see, well, the cat is in some sort of funny quantum-mechanical, coherent mixture of being dead and being alive. That’s very strange and paradoxical and weird, and so on.” It isn’t really true.

MISHLOVE: That was the point Schrödinger tried to make.

GELL-MANN: Well, I don’t know exactly what he was after, but it’s a point that people have belabored after Schrödinger, and I think it’s not really a very good way to look at it, because a live cat certainly is in interaction with its environment. It’s not isolated.

MISHLOVE: That’s right.

GELL-MANN: Even the dead cat is in interaction with its environment. It’s decaying, emanating various chemicals. The live cat of course is breathing and in contact with its environment. Even if the cat is in a box, the box is in contact with the environment. It’s being hit by photons from elsewhere in the universe. It’s radiating a certain number of photons because it’s not at absolute zero; if it were at absolute zero it would certainly not contain a live cat. And so on and so forth. Therefore, whatever it is that we’re talking about, it’s in interaction with other things, and those other things are being averaged over and integrated over and not seen. And under those conditions, the two situations, alive and dead, decohere, as we say. There is no interference between them; they are simply alternatives—just like the alternatives at the race track when either one horse wins or another horse wins; there’s nothing mysterious or peculiar about it. And when you open the box it’s no different from the experience that you may actually have of going to the airport and accepting a cat box and not knowing whether the poor animal is alive or dead until you open the box. It’s exactly the same. The two situations are on different branches of history. They are not coherent with each other because of the interaction with the rest of the world that’s averaged over.

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