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.