Posted by Over at Starts with a Bang, Ethan Siegel expressed exasperation that Nature and New Scientist are paying attention to (and lending too much credibility to) an astronomical theory Ethan views as a non-starter, Modified Netwonian Dynamics (or MOND):
[W]hy is Nature making a big deal out of a paper like this? Why are magazines like New Scientist declaring that there are cracks in dark matter theories?
Because someone (my guess is HongSheng Zhao, one of the authors of this paper who’s fond of press releases and modifying gravity) is pimping this piece of evidence like it tells us something. Guess what? Galaxy rotation curves are the only thing MOND has ever been good for! MOND is lousy for everything else, and dark matter — which is good for everything else — is good for this too!
So thanks to a number of people for bringing these to my attention, because the record needs to be set straight. Dark matter: still fine. MOND: still horribly insufficient. Now, maybe we can get the editors and referees of journals like this to not only do quality control on the data, but also on the reasonableness of the conclusions drawn.
In a comment on that post, Steinn took issue with Ethan’s characterization of MOND:
Ethan – this is not a creationism debate.
Hong Sheng is a top dynamicist and he knows perfectly well what the issues are. The whole point of science at this level is to test models and propose falsifiable alternatives.
MOND may be wrong, but it is not evil.
Cold Dark Matter is a likelier hypothesis, by far, but it has some serious problems in detail, and the underlying microphysics is essentially unknown and plagued with poorly motivated speculation.
MOND has always approached the issue from a different perspective: that you start with What You See Is What You Get, and then look for minimal modifications to account for the discrepancies. It is a phenomenological model, and makes little attempt to be a fundamental theory of anything. Observers tend to like it because it gives direct comparison with data and is rapidly testable.
I think Leslie Sage knew what he was doing when he published this paper.
In a subsequent post, Ethan responded to Steinn:
Yes, Steinn, it is evil to present MOND as though it is a viable alternative to dark matter.
It is evil to spread information about science based only on some tiny fraction of the available data, especially when the entire data set overwhelmingly favors dark matter and crushes MOND so as to render it untenable. It isn’t evil in the same way that creationism is evil, but it is evil in the same way that pushing the steady-state-model over the Big Bang is evil.
It’s a lie based on an unfair, incomplete argument. It’s a discredited theory attacking the most valid model we have at — arguably — its only weak point. Or, to use a favorite term of mine, it is willfully ignorant to claim that MOND is reasonable in any sort of way as an alternative to dark matter. It’s possibly worse than that, because it’s selectively willful ignorance in this case.
And then I look at the effect it has. It undermines public understanding of dark matter, gravity, and the Universe, by presenting an unfeasible alternative as though it’s perfectly valid. And it isn’t perfectly valid. It isn’t even close. It has nothing to do with how good their results as scientists are; it has everything to do with the invalid, untrue, knowledge-undermining conclusions that the public receives.
And yes, I find that incredibly evil. Do you?
I have no strong views on MOND or Cold Dark Matter, but given that my professional focus includes the methodology of science and issues of ethics in science, I find this back and forth really interesting.
Ethan is saying that it is evil to present as a viable alternative to a reasonably successful model a competitor that:
- does worse as far as predicting or explaining the majority of the data
- focuses on only the fraction of the available data where the successful model runs into trouble
- misleads the public about its strengths and weaknesses compared to the successful model
Steinn responds by claiming that the scientific attention being paid to MOND is indicative that science is working the way it’s supposed to work. Scientists are supposed to test models, whether against data we already have or new data coming in. (There may be other tests, too, that consider things like internal consistency, fit with other models, and such.) Scientists are also supposed to propose falsifiable alternatives to the models they’ve been using. As Steinn writes in a follow-up post:
Scientists must test models – that does not mean celebrating their successes, it means finding where they fails.
Among other things, this means is that judging which of two competing hypotheses is likelier is a different matter from working out the problems in each of the hypotheses. The latter assessment will include things like:
- the hypotheses’ fit with the available data
- how well the hypotheses explain the data (and, in the places where they don’t give good explanations, where we encounter the bad predictions or hand-waving)
- where data that we don’t have and haven’t yet figured out how to get (like the relevant underlying microphysics) might support or undermine a given hypothesis
- the plausibility or iffiness of the premises grounding each model (and whether there are direct or indirect ways to expose any of these premises to empirical testing)
Now, if you have a model that’s working pretty well right now, you might wonder why you’d bother messing around with an alternative model. One good reason is that exploring alternative models may help you find ways to make your best model even better. Considering the places where your best model has the hardest time accounting for the phenomena is important. Considering other models that approach those tough spots differently may give you better insight to the phenomena.
And here, it’s worth underlining the point that scientists are looking for models that are useful. Models abstract away details from the phenomena they are modeling to simplify them and make them tractable. This abstracting away of details usually leaves us with a simpler picture of at least some of the crucial entities and interactions. This makes it easier for us to understand the phenomena, but it doesn’t mean that what we’ve abstracted away is unimportant — as it may well be if we’re interested in understanding other features of the phenomena.
One of the things this means is that multiple models may help us grasp more of the whole thing than any single model could. This would mean that pursuing more models is likely to be more useful than only working with the single model that seems to be the best of the available options (assuming there’s some set of criteria that would let us objectively rank all the competing models). As Steinn points out, this is hardly news to physicists, especially given that two wildly successful theories, general relativity and quantum field theory, are inconsistent with each other and cannot both be true. Taking both of these models of reality seriously hardly ever results in claims that you’re being evil.
So, as long as the scientists looking at competing models like Cold Dark Matter and MOND are being as objective as they can about the strengths and weaknesses about each model, I don’t think we ought to count the pursuit of either as evil.
Still, I feel the pull of Ethan’s worry once we consider what’s going on beyond the community of physicists and how the news of MOND is filtering to the broader community. (I don’t know how much of the publicity about MOND or about criticism of Cold Dark Matter is actually making its way to non-scientists, but let’s assume there is some penetration here.)
To the extent that members of the public who are not practicing astrophysicists might be paying attention to the battle between Cold Dark Matter and MOND, there may be reason to worry if that public doesn’t understand the difference between models (which scientists view as worth exploring and using) and scientific theories that aim to tell a literally true story about the world. Without understanding this difference, the non-scientist might conclude that renewed interest in MOND could only mean that evidence has come to light to overturn Cold Dark Matter once and for all (or at least that MOND explains new empirical evidence at least as well as Cold Dark Matter does).
If a segment the public is paying attention to recent work on MOND and Cold Dark Matter, this seems like an opportunity for the scientists working on these models to talk to them about scientific methodology. In particular, they might explain why pursuing a less likely hypothesis might be fruitful and how such exploration might ass add to our understanding of the phenomena over what we could get from the most likely hypothesis alone. Indeed, this is also a great opportunity for physicists to communicate to the public about how it is that scientists can reasonably work with competing (and even contradictory) theories without having their heads explode.
When the public is watching on the sidelines of a scientific dispute, it strikes me as more ethical to give the public the necessary context to help them see what is — and what is not — at stake in the dispute. I’m not sure that failing to provide this context is evil, but it is an opportunity missed.
October 19, 2009
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