Dear Editor,
I always enjoy reading “This Month in Physics History” and the January installment on Hubble’s discoveries was no exception. However, I would like to point out a few minor errors in that piece. Most astronomers in the early 20’s favored the theory that spiral nebulae were “island universes” and in fact believed the Milky Way to be much smaller than we now know it to be. Shapley and a few others favored the idea of a much larger Milky Way which contained the spiral nebulae, but Shapley’s letters indicate that he knew he was in the minority on this issue. Also, it was Henry Norris Russell who presented (on behalf of Hubble) the data on Cepheids in Andromea at the AAS meeting in January 1925. Most importantly, it is untrue that “Hubble didn’t discuss the implications of what he had found” in his 1929 PNAS paper. In the final paragraph of that paper he says “the velocity-distance relation may represent the de Sitter effect”, referring to the model of the Universe presented by Willem de Sitter in 1917. This model was originally interpreted as a static model, but did predict a redshift that increased with distance because of scattering and an apparent slowing down of distant atomic vibrations. So in 1929 Hubble did not interpret his data as indicating an expanding Universe, but rather as supporting de Sitter’s static model. It was only later realized that de Sitter’s model was equivalent via a coordinate transformation to expanding models such as that proposed by Georges Lemaitre in 1927 (Lemaitre’s model was unknown to Hubble and most astronomers until 1930). A detailed account of this history is given in Robert W. Smith’s The Expanding Universe (Cambridge U Press, 1982).
Now let me add a few comments:
My pointing out the second error may be me just being picky. It WAS Hubble's data on Cephieds in Andromeda that was presented at the AAS meeting, even if it Russell presented it for him. The piece in APS news implied that Hubble presented it himself, but the wording could be interpreted to fit the facts (though I doubt many readers would interpret it that way). The other errors are more problematic in that they serve to glorify Hubble at the expense of historical accuracy. I seriously doubt that this was the conscious intent of the person who wrote the piece (or the APS News editors), but there it is. Most astronomers were already convinced that there were other galaxies long before Hubble's Cepheid discovery. That discovery, though, put the nail in the coffin. It was a MAJOR discovery, but ultimately what it indicated was what most astronomers thought already. It did when over the few dissenters, some of whom were very important astronomers like Harlow Shapley. The discovery of Cepheids in Andromeda was of immense importance because up to that point the evidence for the extra-galactic nature of the spiral nebulae was circumstantial and conflicting. Hubble found the smoking gun, and subsequently got rid of the conflicting evidence by dismantling Adrian van Maanen's work on the rotation of spiral nebulae (and interesting story in its own right).
It is the third error that I found most surprising. Hubble clearly proposes in his 1929 paper that the velocity-distance relation could be evidence that favored de Sitter's model of the Universe (which was a static model). Hubble did not at that time think that he had found evidence for an expanding Universe. In fact, Hubble continued to resist the idea of a non-static Universe for years. I'm guessing that this is where the statement in the APS News article came from. In later years Hubble did refuse to comment on the interpretation of the velocity-distance relation. But this was after de Sitter's model had been invalidated (mainly because the mean density of the Universe was too high for his model to be relevant) and new non-static models (actually old models that nobody had paid attention to, like Lemaitre's and Friedmann's) had become the focus of the discussion. Hubble apparently did not believe the the redshifts he observed were genuine Doppler shifts, due to actual recessional motion. He did not withhold his opinion because he thought interpretation should be left to others (after all, he was quite ready to support de Sitter's model and in fact his work was likely an attempt to test that model directly). But when the only options up for discussion were expanding models he did not want to side with any of them.
Again, the importance of Hubble's (and Humason's) work on the velocity-distance relation can hardly be overstated. We NOW recognize it as a crucial piece of evidence for the expansion of the Universe. But it was not recognized as such in 1929 (certainly not by Hubble). I don't intend to fault Hubble for this - after all, he was an observational astronomer and an incredibly good one. And in 1929 astronomers were essentially unaware of the existence of expanding models like Lemaitre's. Given what he had to work with, Hubble made a reasonable suggestion that his data supported de Sitter's model. This turned out to be wrong and from that point on Hubble was reluctant to throw his support behind any particular model. All of this is entirely reasonable behavior on his part. But let's not try to hide the fact that Hubble backed the wrong horse.
The errors in the APS News piece were innocent enough. But unfortunately I suspect that such errors are made in many similar cases. They serve to produce an alternate history of science in which our greatest scientists made no mistakes. But this dehumanizes them and makes their accomplishments seem out of reach. Even the greats stumble on occasion. And the achievements of the greats are inevitably built on the work of many who came before (even Einstein was preceded by Lorentz, Fitzgerald, Poincare, etc.). A more accurate history of science might actually be more interesting and might help us to see that science really is, of necessity, a community enterprise. Even the great ones need others to lay the groundwork, catch their few mistakes, and follow up on the leads they leave open. We certainly wouldn't want incorrect physics in such a publication - let's try to keep incorrect history out as well.
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