Philosophy in Astronomy: Unique vs. Ordinary

As with any science, philosophical notions have played an important role in the development of astronomy. It seems to me that one philosophical notion that has had a tremendous influence on astronomy is the idea that Earth is (or is not) a unique place in the Universe. There is no denying that Earth is special (to us) in that it is the planet from which all of our astronomical observations have been made (well, nearly all, and those that weren't made from Earth were made from relatively nearby). But is Earth truly unique in the Universe?

In classical astronomy Earth occupied a singular location in the Universe. In Aristotle's cosmology Earth was located at the center of the Universe (which was finite and spherical and therefore had a very well-defined center). As pointed out in a recent Physics Today article, Aristotle didn't think that the center of the Universe was wherever Earth was, but rather that Earth was at the center because all matter fell toward the center and therefore Earth (which was nothing more than a collection of all the matter in the Universe) had to be located there. In a way it is hard to say whether or not Earth really occupied a unique place in Aristotle's cosmology because all the matter in the Universe was part of Earth. Everything else was celestial aether and not base matter at all. Earth was unique because it was everything, in a sense. This idea certainly came to take on philosophical (and later theological) dimensions, but initially it was based on sound observation. All celestial objects can be clearly seen to rotate about Earth, and any attempt to move matter away from Earth just results in that matter falling back again (they couldn't achieve escape velocity in ancient Greece). So it fit the data to consider Earth as the center of it all. Nevertheless, as I said, the concept that Earth occupies the center of the Universe ultimately became a philosophical and theological principle.

The Copernican revolutions changed all this, but in small steps. Copernicus moved Earth away from the center of the Universe, but put the Sun in its place. Earth's place was no longer unique, but it was still one of only a handful of planets orbiting the Sun which occupied the center of the (still spherical and finite) Universe. Even Kepler (who was willing to consider that there might be life on some of the other planets) still retained the Sun at the center of the Universe and Earth as one of the few privileged planets to orbit it. It was really only after Newton (when there was a physical mechanism for the planet's orbital motion about the Sun, rather than a geometric explanation) that it became easy to think of the Sun as one of many Suns and Earth as one of a potentially very large number of planets. It was no longer necessary that either Earth or Sun be at a unique geometric location.

Contemporary astronomy has come to embrace the notion that Earth and Sun are not unique, but are wholly ordinary. Indeed, astronomers become suspicious of any evidence that seems to indicate that Earth or Sun are special. For the most part these suspicions appear to be justified. I've been studying the history of galactic astronomy in the early 20th Century and this issue played an important role. For many years it was thought that the Sun was located very near the center of our galaxy (although the concept of a galaxy was not entirely clear at the time) because statistical studies of stellar distances seemed to place us at the center of all the stars we could observe. It turned out later that this was because the absorption of starlight by interstellar dust limited the distance to which the telescopes of the time could penetrate. In fact, all of the stars were observed were just a small part of the Milky Way galaxy. At the time, though, nobody thought there was much interstellar absorption and the data putting the Sun at the center of the galaxy seemed rock solid. Still, it was viewed with some concern because it seemed to give the Sun a special location. When Shapley studied the distribution of globular clusters and found that the center of the clusters (which was presumably also the center of the galaxy) was far from the Sun, he considered it a triumph on the scale of Copernicus displacing Earth from the center of the Universe.

Even with the Sun dislodged from the center of the galaxy, astronomers still struggled against the notion of a unique location. Some astronomers (Shapley included) thought that our galaxy was the only galaxy, and that the so-called "spiral nebulae" were just objects within our enormous galaxy. Even when Hubble's observation of Cepheids in Andromeda showed that Andromeda was a separate star system from the Milky Way galaxy, it was still thought that the Milky Way was vastly larger than any other galaxy including Andromeda. If the spiral nebulae were "island Universes" then the Milky Way was a continent. This was also viewed with suspicion by some astronomers who thought that the Milky Way must surely be very similar to at least the larger and more prominent spiral nebulae (like Andromeda). Later revisions to the diameter of the MIlky Way and the distance (and thus the diameter) of Andromeda showed that in fact Andromeda is a bit larger than our Milky Way, so in fact our galaxy is an ordinary galaxy and not even the biggest in the Local Group.

In each of these cases observations that seemed to indicate that Earth or the Sun or the Milky Way were unique ended up being erroneous and in fact all three appear to be ordinary members of their respective classes. The assumption of ordinariness was becoming firmly entrenched by the time Hubble carried out his study of the redshifts of spiral nebulae. The data clearly indicated that nearly all galaxies were moving away from the Milky Way with speeds that increased with their distance from the Milky Way. On the surface this would again seem to indicate a special location, and thus a unique status, for the Milky Way. But as far as I can tell astronomers never even considered this possibility. This may be due to the fact that General Relativity was already on hand to provide an explanation that did not assume a special location for the Milky Way (in fact, from any point in the Universe the same phenomenon could be observed). One wonders, though, how this data would have been interpreted had Einstein (nor Hilbert nor Poincare, etc.) not come up with GR. Hubble speculates a bit on this in his book The Realm of the Nebulae.

In reflecting on this history what stands out is the distinction between specialness and uniqueness. As I said above, there is no doubt that Earth (and the Sun and the Milky Way) is special, because it is where we are. There is always something special about the observer's location when interpreting data taken by that observer. In many cases that "specialness" may look like "uniqueness", but there is a subtle difference between the two. Special means special only from our point of view. Unique means special in a grander, more objective, more Universal sense. The history of astronomy is riddled with instances of specialness being confused with uniqueness. In light of that history astronomers have adopted as (I would say) a philosophical principle the idea that there is nothing unique about our location (Earth, Sun, or Milky Way). We now build theories based on the assumption that Earth is a typical inner planet (who knows?), the Sun is a typical G star (it seems to be), and the Milky Way is a typical galaxy (it seems to be a typical spiral). The validity of these assumptions is rarely questioned. Astronomers have been burned to many times in the past.

This assumption of non-uniqueness seems entirely reasonable to me, but there is some danger of it becoming too dogmatic. It is possible that some aspects of our location might be unique, or at least very rare. In fact, some proponents of the Strong Anthropic Cosmological Principle argue that we are in a unique Universe, perhaps one that is specially designed to produce intelligent life. Again, most astronomers (and physicists - including myself) view this idea with suspicion. But we must take care to not be closed to the idea of uniqueness, or we will be no better than the classical astronomers who closed themselves to the idea of ordinariness.

Kuhn's "Copernican Revolution" and Incommensurability

It's been ages since my last post. I hit a point in the semester where I was sufficiently far behind so as to preclude any thoughts of essay-writing for this blog. But now the holidays have arrived and I have a backlog of topics to write about. Fortunately my reading did not halt when my blogging did...

In the time since my last post I finished reading Thomas Kuhn's "The Copernican Revolution." It's an incredibly good read for anyone interested in intellectual history, and particularly the history of astronomy. I was very motivated to read it because I will be teaching astronomy starting next Fall, and I intend to teach a course developed by a colleague that focuses on the Copernican Revolution. I was also interested in the book because I had heard that Kuhn's work on the Copernican Revolution had ultimately led him to the conclusions about the nature of science that he presents in his "The Structure of Scientific Revolutions." In particular I was interested to see the origins of his idea of incommensurability (the idea that there is no logical way to decide between two competing paradigms because each paradigm has different standards of evidence and makes different fundamental assumptions that cannot be questioned within the paradigm).

What struck me most about Kuhn's presentation of Aristotelian cosmology was how sensible it ancient science was. Sure, I know that most of it has now been discredited. But Kuhn did a great job of showing how well the Ptolemaic/Aristotelian system explained much of what was "known" at the time (some of what was "known" turned out to be wrong as well, but they couldn't anticipate that then). There was also a great deal of internal consistency in ancient science, and in fact it was this internal consistency that produced much of the scientific resistance to Copernicus' proposals. Making Earth a planet did not just change astronomy, but it also had an impact that would be felt through all of physics as well as in other areas. If ancient science had been a collection of ad hoc ideas then there would have been little resistance to Copernicus since his ideas would have impacted only the highly specialized area of mathematical astronomy (in which Copernicus was a recognized leader). I was also impressed by how far medieval science advanced beyond the ideas of Aristotle. In particular, Oresme and Buridan were on the verge of the concept of momentum and something like Newton's Second Law. Kuhn also points out that Descartes was the first to clearly formulate a Law of Inertia. This makes the work of Galileo and Newton somewhat less revolutionary than I had thought (though still incredibly revolutionary).

Overall I just can't see where Kuhn got the idea of incommensurability from. It just doesn't seem to be there in this book. He goes to great lengths to point out that Copernicus himself was a die-hard Aristotelian in almost all of his thinking except the planetary nature of Earth. Tycho Brahe was of a similar frame of mind. Kepler was not Aristotelian, and his general approach was quite different from that of most of his contemporaries. But Kepler was just one of the first to ride the wave of neoPlatonism. Kunh readily admits that Kepler's explanation of planetary motion would have won over professional astronomers without any additional evidence. His predictions were simply more accurate than those of anyone else, and this was what counted for professional astronomers. Note that this was a common piece of evidence that both geocentrists and heliocentrists could agree on. There is no incommensurability there. Granted, Kepler's work was unlikely to win over the general populace to the heliocentric model. But that is a process that lies beyond the realms of science itself.

I've always heard of one example of incommensurability being the refusal of anti-Copernicans to admit telescopic evidence as valid. This is a disagreement over what constitutes valid evidence, but it is a scientifically legitimate disagreement. Galileo was the first to use a telescope for astronomy, and the science of optics was new on the scene. It is no surprise that some scientists viewed telescopes with suspicion. It was an as yet unproven technology. If those same scientists had lived long enough to see telescopes and other optical devices in common use they doubtless would have conceded that Galileo's evidence was valid. This is not a matter of incommensurable paradigms, but rather an appropriate cautiousness with regard to a completely new technology. Frankly, there were a wide variety of scientific reasons for rejecting Copernicus' system. For one thing, it wasn't any better than Ptolemy's, as Kuhn points out. For another, it required the dismantling of virtually all the physics that was known at the time. It turns out this was a good thing because that physics was wrong, but it was surely reasonable for Copernicus' contemporaries to hesitate to throw away what they knew of physics for something that would bring them little or no gain. Copernicus himself knew his theory had major problems and expected it to be criticized (which is why he resisted publishing it until his death). There was a lot that needed to be worked out before the benefits of the Copernican idea could be reaped.

Perhaps a genuine incommensurability lies in how various astronomers judged Copernicus' theory. To those with an empirical, Aristotelian viewpoint it could only be deemed a failure or at best a "nice try." To those with a more Platonic perspective (like Kepler) the theory had much to credit it. It was conceptually more economical than Ptolemy's system, even though this conceptual economy had to be covered over with ad hoc additions to make the predictions match the level of accuracy of the Ptolemaic system. But this difference in perspective does not represent an incommensurability between two scientific paradigms. Rather, it seems to be a possible incommensurability between individual scientists who may place different value on different types of evidence. Differences between individual scientists have been around as long as science has. Kuhn is claiming something much larger in "Structure" then that sometimes scientists disagree with each other.

I wonder if a similar examination of a smaller-scale scientific revolution would have led Kuhn away from the idea of incommensurability. The Copernican revolution involved many philosophical the theological issues in addition to the scientific issues. Copernicus' idea ultimately overthrew a worldview that had dominated Western thought for millenia. The revolution itself spanned a long period of time (from Copernicus to Newton) and it came at a time when great technical advances were made (though this may be typical of any important scientific revolution). As Kuhn points out, the backlash against Copernicus' ideas was driven in part by the fundamentalism of the new Protestant faith and the need for the Catholic Church to find a target to attack in order to show that it was not lax about biblical authority. The examination of a similar revolution that did not have all of these complicating factors might not lead to the idea of incommensurability. An example that comes to mind (because I've been studying it recently) is the revolution that saw our Sun moved from the center of the Universe to out near the edge of one spiral galaxy among billions. There were issues of evidence here as well, particularly in regard to the Cepheid variable period-luminosity relation and van Maanen's measures of the rotation of spiral nebulae, and as a result astronomers disagreed on some major points (such as whether spiral nebulae were inside or outside our galaxy). Ultimately, though, a consensus was reached and the main players on both sides of the debate came to the same conclusions in the end. No incommensurability there, it seems.