Wednesday, December 19, 2012

I sometimes wonder if astronomers actually talk to each other

Several years ago I came across the "Principle of Mediocrity" in a book by astrophysicist Alexander Valenkin, who argued that even when we have only one example of a case we should assume that this case is generally not exceptional, based on the principle of mediocrity.

Here is his premise as he states in an article for Edge:
If we randomly picked observers in the universe, their observed values of X would be in the predicted interval 95% of the time. Unfortunately, we cannot perform this experiment, because all regions with different values of X are beyond our horizon. We can only measure X in our local region. What we can do, though, is to think of ourselves as having been randomly picked. We are just one in the multitude of civilizations scattered throughout the universe. We have no reason to believe a priori that the value of X in our region is very large or small, or otherwise very special compared with the values measured by other observers. Hence, we can predict, at 95% confidence level, that our measurement will yield a value in the specified range. The assumption of being unexceptional is important in this approach; I called it "the principle of mediocrity". 
If you (like me) would like that in a bit more digestible form, try the explanation from Wikipedia, which dumbs it down sufficiently to be comprehensible even after a beer:
The mediocrity principle is the philosophical notion that "if an item is drawn at random from one of several sets or categories, it's likelier to come from the most numerous category than from any one of the less numerous categories" (Kukla 2009). The principle has been taken to suggest that there is nothing very unusual about the evolution of our solar system, the Earth, humans, or any one nation. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the place of humanity. The idea is to assume mediocrity, rather than starting with the assumption that a phenomenon is special, privileged or exceptional. 
Notice that section I put in bold--the mediocrity principle predicts that "there is nothing very unusual about the evolution of our solar system."

This has always bothered me as a working intellectual tool, because it is effectively the equivalent of suggesting that you can know the direction of a line when you only have one point.

And now one member of the team that just found a potentially habitable Earthlike planet orbiting Tau Ceti more or less blows the mediocrity principle out of the water:
Over 800 planets have been discovered orbiting other worlds, but planets in orbit around the nearest Sun-like stars are particularly valuable. Steve Vogt from University of California Santa Cruz said: "This discovery is in keeping with our emerging view that virtually every star has planets, and that the galaxy must have many such potentially habitable Earth-sized planets. They are everywhere, even right next door! We are now beginning to understand that Nature seems to overwhelmingly prefer systems that have a multiple planets with orbits of less than one hundred days. This is quite unlike our own solar system where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up."
So while I am very aware of Dr. Valenkin's accomplishments and his erudition, we now have one major case in which his principle of mediocrity has apparently been proven to be entirely incorrect.  This should call into question the viability of assuming that other "island universes" will be governed by similar physical laws, but it probably won't.

1 comment:

Hube said...

Plateau will be awaiting us at Tau Ceti.