Earlier this week, I wrote about an article that appeared in Nature, New Scientist and other places. The article — and especially the popular writeups — talked about a problem with dark matter and how MOND (MOdified Newtonian Dynamics) solves those problems.
And I’m livid about it. Another physicist/scienceblogger thinks my anger is misplaced, and left me the following in my comments section:
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.
First off, here’s the deal as far as the science goes. Whatever you choose as your theory of gravity, there are certain things it needs to explain.
1.) Earth-based phenomena. Falling, rolling, acceleration, etc., basically any motion that happens under the influence of gravity must be explicable by our theory of gravity. This includes the speed at which clocks run at different elevations and at different speeds. The only theory we’ve ever found that can do all of it? Einstein’s General Relativity. Well, and other people’s versions of General Relativity that don’t deviate too drastically from Einstein’s.
2.) The bending of starlight by matter and energy. From the apparent positions of stars during a solar eclipse to weak and strong gravitational lensing, the fact is that space gets bent by all types of energy, such as mass. By how much? And where? That’s something a theory of gravity will tell us. Einstein’s General Relativity gets this right, too, and it’s the only one that does.
3.) The orbits of massive bodies. From the planet Mercury to binary stars, calculating the orbits of objects in space is a vital test of any theory of gravity. Not only does Einstein’s General Relativity get this right, but even slight modifications to it (such as Brans-Dicke gravity) fail miserably.
There are plenty of other experiments and observations that confirm Einstein’s General Relativity as the correct theory of gravity. If we want for cosmology to work, too, we need to add dark matter to the theory. This one addition — dark matter — successfully allows us to explain a whole host of observations, including the large-scale structure in the Universe, the cosmic microwave background, the elemental abundances of hydrogen, helium, lithium and deuterium, the motion of galaxies within clusters, collisions between galaxy clusters, the observed expansion rate of the Universe, and the internal motions of spiral and elliptical galaxies.
So Einstein’s General Relativity and dark matter together explain what we see. Dark matter is the key addition that provides the extra gravitational force we need to make the Universe do what it does. Here’s what the total matter looks like — reconstructed — inside a galaxy cluster:
Yes, you’ll notice the big spikes where the galaxies are. But you’ll also notice this big, diffuse central blob that goes all the way to the outskirts. That’s what our observations tell us the dark matter looks like. You can read more about why dark matter works so well in parts I, II, III, 3.5, and IV of my series.
But out of all of these observations, one presents a small amount of difficulty for dark matter. When we simulate what the dark matter should look like at the center of individual spiral galaxies, we get an answer that’s slightly at odds with what we observe. Dark matter predicts the outer reaches of the rotation curve very well, but the very inner regions — especially of low-mass galaxies — look like they need a slight tweak. It’s a detail of dark matter theory that many people are trying to work out. An apt analogy is that the whole theory of dark matter is a beautiful boat, and these observations of the centers of spiral galaxies are like a broken oar. The boat’s in great shape, but the oar needs to be repaired.
Now, MOND comes along. MOND starts with Newton’s gravity — not Einstein’s General Relativity — and says that if we tweak Newton’s gravity, it can explain these rotation curves. It says it can do it better than General Relativity and Dark Matter together can do. And it’s right. It is, for reasons we do not fully understand, a fabulous oar.
But it’s a fabulous oar that does not fit our boat. It not only is incompatible with the cosmic microwave background, large-scale structure, galaxy cluster observations, colliding cluster observations, the elemental abundances, and the expansion history of the Universe, it’s also incompatible with the three basic tests of the theory of gravity outlined above. MOND is an oar with no boat to attach itself to. And the attempted modifications to General Relativity do not solve all (or even most of) the problems that dark matter does.
And nobody disputes these facts. The MOND people don’t, the dark matter people don’t, the simulators don’t, the observers don’t, the theorists don’t, nobody does.
And I don’t have a problem with pointing out a difficulty that dark matter has; I think that’s a good plan. But what ruffled my feathers was that the authors held up MOND like it was a reasonable alternative to dark matter. In fact, quoting from the New Scientist article:
Alternatively, they say our understanding of gravity may need modification to eliminate the need for dark matter entirely. Some existing theories, such as MOND (modified Newtonian dynamics) attempt to do just that, suggesting that gravity does not fade away as quickly as current theories predict.
And this is just contrary to fact. This idea that you can “eliminate the need for dark matter entirely” is not true. We’ve been trying. We continue to try. We cannot do it. The attempts all fail, and they fail spectacularly. While dark matter is unsettling in a lot of ways, the simple fact is that we cannot explain the things listed above (and there are a lot of them, I know) without dark matter, no matter what we’ve done to our theory of gravity. A functional boat with a broken oar is unequivocally better than a good oar with no boat. But is it evil to say otherwise?
This brings me back to Steinn‘s comment. 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?
We have an entire empty comment thread down here waiting for your input and your thoughts. What are you waiting for?!