November 25, 2008 by Craig
Although I don’t have much real by way of reliable comparisons, I think I attended a pretty decent science program in high school. Our teacher was actually involved in the scientific community (instead of being a teacher who has science thrust upon him) and made an effort to introduce real scientific principles into the classroom. (He also shares a name with a very famous writer so he’s completely un-Googleable.) From what I’ve read, most other students are not so lucky.
Eventually, he taught us about the Heisenberg uncertainty principle. Of course, he taught that it was the fact that you can’t know both the position and the momentum (speed) of a particle at any given time. He explained this in terms of trying to measure electrons; to do so, you needed to bounce photons off of them, and this changed both their position and momentum. This fundamental fact meant that particles didn’t really have a precise position in the classical sense, and so we had to think of particles as being somewhat random/probabilistic at quantum levels.
To me, this particular argument made sense, but it didn’t seem to explain why this was an all-encompassing principle of physics. Was there really no other way to measure electrons? What about other particles? Isn’t this just a problem with our technology & process? Why does it have to be a fundamental property of particles? Surely they have both a definite position & momentum, even if we can’t measure it.
Unfortunately, I didn’t ask these questions at the time. Looking back on it, Mr. Adams probably wouldn’t have been able to provide satisfactory answers anyway. None of the other students raised similar questions, and I have to assume that most of them didn’t consider them in the first place. Over the years, I’ve seen similar explanations from a wide variety of sources.
Then, on one of my random walks down Wikipedia, I happened to come across an description for the Heisenberg Uncertainty Principle that not explained its fundamentality but also explained why the common answer I’d been told was completely incorrect.
In quantum mechanics, the particle is described by a wave. The position is where the wave is concentrated and the momentum, a measure of the velocity, is the wavelength…. The only kind of wave with a definite position is concentrated at one point, and such a wave has an indefinite wavelength. Conversely, the only kind of wave with a definite wavelength is an infinite regular periodic oscillation over all space, which has no definite position. So in quantum mechanics, there are no states which describe a particle with both a definite position and a definite momentum. The narrower the probability distribution is for the position, the wider it is in momentum.
To understand the HUP, you first need to ditch the idea that particles are discrete balls of stuff, and instead think of a wave in motion. Think back to the Slinky wave experiments you may have done in school (if you haven’t, this YouTube clip is a good substitute). What’s important is the change in distance between the coils (which make up the abstract entity known as the “wave”), not the coils themselves. You can’t point to the wave as being “at” any particular position in the slinky; you have to measure it over a given distance. You can start with the entire length of slinky, and reduce that down to smaller and smaller (ie: more “precise”) chunks, but there’s no discrete part where you can say “here is where the wave is”. The wave is defined by its wavelength, which in turn is directly related to its momentum (speed). But you can only measure momentum by measuring over a distance. Thus, as you gain a more precise measure of momentum, your measure of position necessarily has to be less precise.
This is not the same thing as saying “measuring one value will necessarily change the other.” This is better described as “the description of either quantity on its own is meaningless; are really two opposing aspects of the same property”. In fact, the common explanation of the HUP is really an explanation of a completely separate issue: the Observer Effect. Both Wikipedia articles on the HUP and the observer effect talk about how the two are often conflated in the minds of the layperson (including those who instruct others). Furthermore, the Observer Effect article explains how it’s not a fact specific to weird quantum particles (which is another explanation I see on a regular basis) but in fact is a class of issues that applies to many separate circumstances.
The misexplanation surrounding the Heisenberg Uncertainty Principle and the Observer Effect is one of many examples of how laypeople and popular culture can get the wrong idea about natural / scientific issue. Another common one is lift: the force that keeps airplanes in the air. More than likely you’ve seen an explanation of how lift works that involves the air over a wing flowing faster than the air under it. This is wrong. There’s lots of other examples too.
Most people won’t ever need to know the details of how these principles work. But that then raises the question: why are we spending time teaching them in the first place? If it’s important to have a base of not-immediately-useful knowledge, then why do we accept incorrect knowledge to fill that void?