Friday, October 8, 2010

Leave me alone or I'm going home!: The Heisenberg uncertainty principle in evolution and epidemiology

To a non-physicist, the gist of the Heisenberg uncertainty principle, or the observer effect phenomenon associated with it, is that studying an object changes the object.  You want to know the position and movement of a subatomic particle, say, but to find that out you have to study it with energy, like a light beam, which allows you to identify the position by seeing how the collision with your measurement beam occurs. But that alters the target particle's movement so you can't know both.

Similar kinds of issues apply to modern-day epidemiology.  We referred to this yesterday in a comment about the effect of maternal drinking during pregnancy affecting the future of their offspring.  A Heisenberg analogy for epidemiological studies goes like this:  if by studying something that has many contributing risk factors, the persons being studied change their behaviors, and thus their exposures because they know the results of the study, you can no longer estimate what the exposure risks will be.

Often, the change in behavior is of a magnitude that it's a major effect relative to the signal that's being studied.  If you stop eating pork because a study says that eating pork gives you a somewhat increased risk of warts (it doesn't--this is just a made-up example!), then the effects of pork-eating will be changed by virtue of the exposure change and the knowledge that this is being studied.  If this happens often enough--as it does with our 24/7 many-channel news reports--then tracking risks or measuring them becomes very problematic, except for the really major risk factors (like smoking and lung cancer) which are robust to small changes.  The science and the scientist become part of the phenomenon, not the external observer that they need to be to do the science.  This leads to many of the serious challenges to modern epidemiology, behavior, education, political, economic, etc. studies, including those of genetic causation.  And since trivial risk factors are mis-reported in the news as big ones, the signal-noise ratio is even less favorable to clear-cut conclusions.

There is a kind of Heisenberg analog in evolutionary terms, too.  If relative fitness--reproductive chances--are affected by both genome and ecologic contexts, and the differences are small, then what happens tomorrow to a given genetic variant, is highly dependent on all sorts of environmental or other genotype changes. A given variant won't have the same relative effect tomorrow as it did today, and since evolutionary models are about relative fitness, the evolutionary landscape changes.

This becomes Heisenberg-like not because it's about observer-interference effects in this case, but because the context changes can be as great or much greater than the net fitness advantage of a genetic variant.  This means fate-prediction is difficult, and in this case the observer analog has to do with the screening-efficacy of natural selection.  Changes in the frequency of an allele can change its net fitness effect.  When fitness (like electron positions?) is not just contextual but essentially probabilistic, something that affects position (current relative fitness) affects evolutionary trajectory.  That's one reason evolution is  essentially unpredictable, except under unusually strong conditions, and in that sense not deterministic as it is viewed in the usual Darwinian concept, especially as put forward by those not versed in evolutionary theory--and that includes many biologists and all the blathersphere that invoke Darwin or natural selection in making pronouncements about society.


Steve Bates said...

I am uncomfortable with your apparent understanding of the Uncertainty Principle. As I understand it (and I'm no expert, but I did just read books by Leonard Susskind, Lisa Randall and Steven Weinberg, so it's fresh in my mind), uncertainty is not a consequence of measurement, but a property of the system itself as manifested in measurements of certain pairs of quantities. The classic pair we all know is position-and-momentum. The Uncertainty Principle is a statement that the product of the uncertainty in position and the uncertainty in momentum is (in suitably compatible units) Planck's constant, one of the truly fundamental properties of our universe. The Uncertainty Principle's limit is on what's knowable, not on whether, say, better equipment could measure the pair more accurately, or whether the measurement of one quantity changes the other quantity. It's nowhere nearly as intuitive as that. :-) Wikipedia has a fairly well-written summary.

Ken Weiss said...

Well, I'm no expert either but I have always been led to understand (and I think the tail end of the Wiki page you linked says this in a way, too) that the measurer's effect is roughly as we said,which doesn't mean that things are not inherently uncertain on their own. This I think is what the Copenhagen Interpretation was about.

Anyway, I certainly will be happy to stand corrected.

But the point we were trying to make by that analogy is still relevant, in that if what we do affects what we're trying to study, we are after a moving target, moving in ways we are partly responsible for but to an unknown extent.

At worst, I'd say it's an analogy that may limp but the issues are important.....

Steve Bates said...

I agree that it's hard to talk about uncertainty without talking about measurement; I do think it's something more fundamental than that, though, that measurement uncertainty is just a symptom of a deeper phenomenon. And I agree that the consequence of your analogy stands on its own, whatever the specifics of the Uncertainty Principle. There's no need for you to stand corrected.

Hey, these days, I can't be too critical of things that limp, being such a thing myself. :-)

snoring solutions said...

The classic pair we all know is position-and-momentum. The Uncertainty Principle is a statement that the product of the uncertainty in position and the uncertainty in momentum is (in suitably compatible units) Planck's constant, one of the truly fundamental properties of our universe.