11 thoughts on “What is dark matter?

  1. Ah ha! I just made a connection.

    I took a university level astronomy course between junior and senior years in high school. We used a cool spectrographic image of a galaxy to measure its rotation and discovered a mystery: it (and by inference, all galaxies of similar type) rotates too fast and the relation of rotation speeds to radius is all wrong. (It’s too easy these days: http://en.wikipedia.org/wiki/Galaxy_rotation_problem )

    Dark matter would account for that. So while I understood the video, I had to have some additional knowledge which not everyone has. So as a purely introductory piece, it seems to me it glossed over some important bits. That said, I hope the video is successful at getting people to ask the next questions.

  2. At the very end is a vnice, very quick image / metaphor. An iceberg above the surface that only hints on a much larger story below the surface. A way for the visible to hint at the invisible. Love it!

  3. OK, this explains how we know there is dark matter, but it doesn’t actually answer the question of what dark matter actually is. I’ve got an astronomy buff friend who thinks dark matter is just interplanetary dark bodies and dust which doesn’t glow enough to be accounted for. That never seemed to make sense to me since if the iceberg analogy is right, then it seems to me that most of the sky would be obscured by the stuff. It’s not just that we don’t see dark matter, it’s that we can see other things in spite of it, right? Just wondering in case I ever get into a discussion with him about it again.

  4. Right. Dark matter does not interact with electromagnetic radiation electromagnetically. It does interact gravitationally. And that’s all it does. So while physicists don’t know what it is, they are piecing together limitations on what it can be based on how it behaves.

    I suppose it’s fair to say we’re at about the same stage of understanding dark matter as physicists were before the discovery of the neutron. It was postulated to exist, there was some disagreement about what it did, and no one had observed one.

  5. Inferred by observation? Inferred or observed? What observations? Didn’t Michelson and Morley falsify the ether in 1887?

  6. Timberwoof, physicists needed Special Relativity (1905) to displace Lorentz Ether theory (189?) which worked around Michelson-Morley.


    p.s. Jason, be careful what you wish for; Dark Matter theory can be viewed as a rejection of strong Materialism.

  7. Jason, I’m not accusing anyone of anything. *You* invoked/invited McCarthy who is, or plays in this blog, an anti-Materialist. Dark Matter is fodder for anti-Materialism. No big deal, and I do not have a dog in that fight (the philosophical implications of Physicalism/Materialism are not my strong suit.) Apologies if you thought I was aiming at you. I was just reacting to your comment. It was intended to be humor, but I know that doesn’t always work on the net.


  8. Depends on the explanatory goal. When friends ask I say it’s matter for which there is evidence via gravitational effects including lensing, but doesn’t give off or reflect radiation.

    It’s really up to your criterion for explain 🙂

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