I should probably stick to doing only one audience-participation thing at a time (there are more Top Eleven posts on the way), but it’s a busy week for me at work, and I’m not really going to have time to post a lot of long articles, so there will be a few “talk among yourselves” entries over the next few days, in hopes of generating some interesting content without a whole lot of typing on my part.
Back when I posted my request for “Great Experiments” in other sciences, Kate remarked that another good topic would be something along the lines of “Most annoying misconception about your field.” Given the number of biologists in blogdom, “Most annoying misconception that isn’t ‘Evolution is only a theory'” might be better, but however you choose to phrase it, I’d like to hear what people think.
I was reminded of this today in class, as I was lecturing about the Heisenberg Uncertainty Principle, which is probably the source of my personal least favorite misconception. The Heisenberg Uncertainty Principle, as any fule kno, is prominently displayed in the banner graphic for this blog, and also discussed in one of my very first blog posts, back on Steelypips. In its best-known form, it states that the product of the uncertainty in the position of a particle and the uncertainty in the momentum of the same particle is greater than or equal to h-bar divided by two. Or, put more simply, that it is impossble to know both the position and the momentum of an object to arbitrary precision.
It’s a deceptively simple equation, but a really difficult idea to grasp, and it’s probably been mangled and misused more than any other concept in physics. The most flagrant abuse I think I’ve seen was a SF novel that use uncertainty as the basis for a star drive– by defining the position really well, they magically attained really high velocity.
Of course, that’s too stupid a misconception to really get worked up over. The more annoying misuse is a subtler one, that re-casts the Uncertainty Principle as a statement that measurement necessarily changes the state of a system. That’s not a bad thing to keep in mind in and of itself, but it’s not what the Uncertainty Principle is about.
The problem with that take on the Uncertainty Principle is that it implictly assumes a sort of local realism. That is, it allows you to imagine that the object in question actually has a perfectly well defined position, and a perfectly well defined momentum, and your inability to measure both is simply a matter of screwing up one when you try to measure the other. An attempt to measure the position necessarily introduces some uncertainty in the momentum that wasn’t there before, and so on.
(There’s a standard textbook illustration that develops the Uncertainty Principle along these lines. I’ve talked about it in past versions of this class, but I dropped it from my notes this year, because I think it creates more confusion than anything else.)
The actual quantum situation is much stranger than that. In reality, neither the position nor the momentum are perfectly defined at all. There are no “real” values that you’re simply unable to measure– what Heinsenberg was getting at is that given the quantum nature of the world, the very idea of a perfectly defined position or velocity is physically unreasonable. Asking what the “real” position and velocity are is not a sensible question. At the most fundamental level, it’s simply impossible to perfectly define those quantities.
It’s a subtle point, but it’s one that gets on my nerves, largely because it’s usually deployed as justification for some sort of fuzzy relativism. It’s one of the strangest and coolest points of quantum theory, and I hate seeing it used in an attempt to claim some sort of hard-science basis for an abstract point of literary theory.
Anyway, that’s my pet peeve about popular conceptions of modern physics. I’d love to hear what gets under the skin of people in other fields.