On my favorite mailing list, it was written:
> Anyway, if you think someone who lives and breathes some field is missing > some obvious point, they're probably right and you're probably wrong.
Generally I think this is true. However, I hereby submit the story of Eric and the Quantum Experts as a cautionary tale for all bright children.
Once, long ago in the 1970s, there was a bright young sprout named Eric who was exposed to the Schrödinger’s Cat thought experiment. There was explained to him the standard account of what happens.
Eric’s reaction was that the standard account seemed like obvious nonsense. His initial objection was that the account seemed to make mystifying, ungrounded assumptions about “observation”. Eric had learned about operational definitions from Alfred Korzybski and C.S. Peirce and Bertrand Russell, and he asked: What is operationally special about opening the box?
Eric pestered physics-literate people about this, and read some books. It did not take him long to discover that essentially the same objection had been raised (in a slightly disguised form) by a physicist named Wigner, who – alas – launched from it to land at an interpretation that seemed even crazier and less grounded than the standard one.
Try as he might, Eric could elicit no sense from what the “experts” had to say on the matter, just a load of snottiness about how you have to understand the math. However, Eric was, at the time, in training to become a theoretical mathematician and knew that sort of bullshit when he smelled it.
Eventually Eric managed to corner an unusually bright and lucid physicist who said, essentially: “There’s nothing special about observation. You ‘observe’ a quantum system whenever you bounce a photon off it. The key thing about Schrodinger’s box is that it’s a closed system until the experimenter opens it.”
This greatly relieved Eric, because it disposed of all the mystifying nonsense about human observers and consciousness that had somehow accreted around the physics. Eric, you see, was also an experimental mystic – a student (though not follower) of Zen and a third-degree Wiccan of anti-religious type – and he knew that kind of bullshit by its smell, too.
However, operationally equating “observation” with “any interaction between previously separate wave functions” did not solve Eric’s problem. It simply moved the problem to a different level.
Eric’s new question was: “OK, then. Why don’t the walls of the box observe the cat? It’s like, emitting thermal radiation, yes? Why doesn’t the cat’s hindquarters observe the cat’s forequarters?”
The question Eric was really getting at is this: If “observation” is some mystically special moment not captured in wave-function interactions, we’re not doing science any more but miracles; we might as well collapse into an occasionalist theology in which God makes every sparrow fall by observing its wave function. Game over.
To be doing science – that is to construct confirmable causal accounts with predictive value – we must assume that “observation” is not special. But under this assumption, Eric sees no reason to believe that “mixed states” (e.g. quantum superpositions of multiple classical states) ever persist for more than time epsilon even in small ensembles of particles. Even Schrödinger’s bacterium would be way too large to ever be in a mixed state, let alone his cat!
Either way, there appears to Eric to be a big fucking hole in the Schrödinger’s Box thought experiment. A dumb, obvious hole. It amazes Eric – it completely confounds and gobsmacks him – that physicists do not seem to get this.
When he presses the issue, the response is essentially “Shut up, kid. It’s a thought experiment; you’re not supposed to ask these questions.” Either that, or the perennial favorite “You have to understand the math.” Eric has become a programmer rather than a mathematician-in-training at this point in the story, but the smell of that bullshit has changed not at all.
Eric is deeply frustrated. Eventually, reluctantly, he concludes that there must in fact be some flaw in his reasoning invisible to him. It seems beyond the bounds of plausibility that every quantum physicist in the world is wrong and he is right. They have Nobel prizes: all he has is dreams and some interesting friends and a one-room walkup on Sansom Street. It is about 1978 or 1979.
Many years pass. Eric becomes rather successful in his field; in fact, in the late 1990s he develops something of a reputation for asking simple but devastating questions that can up-end entire disciplines. A certain measure of fame duly follows upon this. But he still has no answer to his Schrödinger’s-cat question, and occasionally it still bothers him.
Then, one day shortly after the century changes, Eric is reading a science magazine and stumbles over an account of decoherence theory:
The effect of decoherence on density matrices is essentially the decay
or rapid vanishing of the off-diagonal elements of the partial trace
of the joint system’s density matrix, i.e. the trace, with respect to
any environmental basis, of the density matrix of the combined system
and its environment. The decoherence irreversibly converts the
“averaged” or “environmentally traced over” density matrix from a
pure state to a reduced mixture; it is this that gives the appearance
of wavefunction collapse. Again this is called
“environmentally-induced-superselection”, or einselection. The
advantage of taking the partial trace is that this procedure is
indifferent to the environmental basis chosen.
Eric reads something quite similar to this in his pre-Wikipedia paper source, his jaw drops open, and he realizes “Holy leaping fuck, I was right all along!”. That phrase “indifferent to the environmental basis chosen” means, exactly, that it doesn’t matter whether you choose an account in which the walls of the box observe the cat or in which the cat’s hindquarters observe the cat’s forequarters; the off-diagonal elements of the matrix still vanish rapidly, the mixed state doesn’t last for more than time epsilon.
Upon further investigation, Eric learns that the groundbreaking work on this theory was done in the early 1980s, shortly after young-sprout-Eric had given up on the question in frustration. It will not actually reach popular accounts until Penrose’s 2004 book Road to Reality, a few years after Eric’s moment of jaw-drop.
Eric realizes that if he’d had a bit more courage and self-discipline, and moved from mathematics into physics rather than programming, he would have been rather likely to have invented decoherence theory himself and become a physicist renowned for kicking the props out from under the Copenhagen Interpretation. Which would have been, all things considered, much niftier and more fundamental than becoming a hacker renowned for kicking the props out from under closed source.
What is our lesson for today, children?
If you think you have spotted something fundamental that all the experts missed, don’t ignore it. Because, after all, you might be right.