Via email, a plug for the newish site 60 Second Science, which is a project from Scientific American built around a podcast featuring one-minute explanations of, well, science.
The email was specifically highlighting their new project, a set of video podcasts going by the name of Instant Egghead. The first video (also the only one so far) is a one-minute explanation of how we know there’s dark matter using items found in editor George Musser’s office.
It’s a well-done video, and a nice explanation of galaxy rotation curves using coffee, crumbs, and a CD. I do have one quibble about the explanation (which I’ll put below the fold), but I like the concept, and this will probably be worth keeping an eye on.
The quibble: He talks about the effect of dark matter as making the galaxy behave like a solid object. This is correct in that the rotation curves astronomers measure for distant galaxies look more like the rotation curves for a solid object (with points on the outside moving faster than points near the center) than a system of bodies orbiting a central mass (like our solar system, in which the more distant planets orbit more slowly than the inner ones). The problem is that it gives the impression that dark matter is somehow “locking” things together and making them rotate as one.
That’s not the case, of course. Dark matter interacts with ordinary matter extremely weakly if at all– the only interaction that we know of between dark matter and ordinary matter is gravitational. The faster-than-expected rotation comes about because there is more mass inside the orbit of the outer stars in a galaxy than we would expect from the visible matter alone. The disk of the galaxy isn’t a rigid body in any meaningful sense– the stars are all free to move in any direction, constrained only by the gravitational attraction of the matter (both visible and dark) in the rest of the galaxy.
As lies-to-children go, though, this isn’t a terrible one, and it’s a nice visual demonstration of the different effects.