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.
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Hey Chad, have you seen this:
http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/ear…
Where are Aaron and Nathan when we really need a good string theory cage match!
I'm not really happy with this explanation at all. It's not the angular velocity that is constant in the "galaxy rotation curves", it's the orbital velocity. It does not even on average spin as a solid disk. (Not that I understand the dynamics of spiral arms, that always seemed strange to me. Maybe I should read up on that some day.)
Also, with a general audience without physics background, does it really make sense to say that it's "non-baryonic"? It just sounds mystical.
Jamie: I haven't read the paper, but I've seen it talked about by Peter Woit and Sabine Hossenfelder. I'm not likely to get enough out of the paper to add anything substantive to the discussion, which is why I haven't posted about it.
That's a fair critique. I'm new to this whole video thing and working on it was an eye-opener. It was hard to cast around for ways to explain the concepts using found objects. As for "non-baryonic", the producer wanted to include a smidgen of jargon for dramatic effect.
George
There is something annoyingly reminiscent of ID about dark matter. Effect observed, mysterious hand postulated. It doesn't sound like science, yet.
"....the secret is to bang the rocks together, guys."
if there is a "dark energy in 60 seconds" it can save me 59 minutes of talking in a guest HS lecture tomorrow.
Thanks for the feedback on this... I think one thing we're still figuring out is our audience. Is this specific enough for people who know little about the subject? Is it too abstract for people who know more?
Anyway, we just turned it into a vodcast, so anyone who's curious can subscribe through iTunes and see if we do any better with future episodes, which will be on quantum computing, synthetic biology and evo-devo:
http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewPodcast?id=268784…
Consider the following: Dark energy is simply non-reflective. Since the light we measure and see is reflected. Dark energy does not reflect other energy. Dark energy is the velocity component of a traveling light wave.