Extraordinary Claims and Universality

"Thoreau," guest-posting at Unqualified Offerings, has a nice post commenting on a Physics Today article about the use of language in science, by Helen Quinn. The article is pretty standard stuff for anyone following the "culture wars" debates here-- use of the word "belief" to describe scientific conclusions causes confusion, and attempts to put science on the same level as faith. She calls for scientists and science writers to use "Scientific evidence supports the conclusion that" instead of "scientists believe."

Thoreau rightly points out that this issue is somewhat overblown, in that the people who claim to be confused by the use of "believe" or "theory" in a scientific context are generally not arguing in good faith to begin with. I think Thoreau underestimates the degree to which the ambiguous language can be used by people arguing in bad faith to create confusion in the general public, but I think he's got a point.

My bigger problem with the article is that I think Quinn badly overreaches, particularly when she attempts to argue that the common people lack an understanding of the scientific principle of universality:

[N]onscientist listeners find no mystery in the fact that the laws of gravity are the same in Paris and Melbourne, but they hesitate to extrapolate from that to the entire universe. Stranger yet, when they read that scientists discovered a planet orbiting a distant star, they accept that news. The distant planet was not seen either--it was inferred from the motion of the star and the laws of gravity. However, language is loose enough that the report might even say that "scientists have seen," or more likely "scientists have discovered," the planet. Apply the laws of gravity to discover something as mysterious and hazy as a cloud of diffuse dark matter, matter that cannot be seen, with properties different from anything we have seen, and the report and its acceptance are quite different!

I think the problem here is not that the people Quinn talks to on planes have a hard time with the idea that the laws of physics are universal. Rather, the problem is that Quinn has chosen an absolutely terrible example to make her point.

She's trying to argue that people don't accept the existence of dark matter because they aren't willing to apply the laws of physics in a universal way, but she's forgetting that dark matter was a hard sell to scientists. Even today, when the theory is well-established, you find papers published in refereed journals in which professional scientists argue in favor of modifying the theory of gravity at large distances, rather than accept the idea of "matter that cannot be seen, with properties different from anything we have seen."

People in planes are willing to accept that scientists have discovered planets around other stars because we have planets around our star. We've all seen the pictures, and the existence of planets elsewhere is not a terribly surprising claim. In fact, it would be far more surprising if scientists claimed that there aren't planets around other stars.

Dark matter is a completely different thing. It's matter that supposedly accounts for most of the mass in the universe, but it's both invisible to us and made of something other than the protons and neutrons that make up ordinary matter. That's an extraordinary claim, and requires extraordinary evidence to back it up. And a lot of people-- even highly educated people with scientific credentials-- have a hard time buying into the idea that 90% of the mass in the universe is made up of something that we've never detected or predicted theoretically, just on the basis of indirect observations of galaxy rotation curves and the like.

I realize that the scientific consensus is now pretty much settled on the issue, but that doesn't mean that it's percolated out to the general public to a significant enough degree that people will generally accept it. And, in fact, I don't see anything particularly wrong with people having a healthy skepticism about extraordinary claims of this nature-- if anything, I'd be more troubled by having people just blindly accept whatever we say is the scientific consensus.

The fact is, there hasn't been any really dramatic and convincing demonstration of the existence of dark matter, of the sort that is sure to convince the man on the street. The scientific consensus on the issue has been built up from a large number of indirect observations-- galaxy rotation curves, the motion of galaxy clusters, cosmological models of the WMAP power spectrum-- that are highly technical and difficult to explain to non-scientists. The Bullet Cluster results from last year may do the trick-- I find them pretty dramatic and convincing-- but they're too recent to have had much effect yet.

The fact that people accept the detection of planets around other stars, but are dubious about dark matter does not say anything about the ability of non-scientists to think universally. All it really tells us is that dark matter is a difficult thing to believe in, and we already knew that.