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Thursday, February 09, 2006

Dark Matters

Last week I was reading a book on recent advances in astronomy. It seems that astronomers detected that stars in the outer arms of spiral galaxies are moving "too fast" -- in fact, way too fast -- and should just proceed on their own into space instead of orbiting the galaxy, when their velocity is compared with the amount of matter currently known to be in such galaxies.

Here is a classic case of Popperian falsification. The current theory of gravity is falsified, and "bold conjectures" should be forwarded for its replacement. So is that what scientists did? Not at all! Instead, they saved the current theory with an unfalsifiable, ad hoc assumption: galaxies are full of dark matter, which, by its very definition, escapes detection! (That doesn't mean that one day we might not detect it, but it does mean that failures to detect it will never "refute" the theory -- it can always be maintained by assuming the stuff is just really, really hard to detect.)

And what's more, the above is good science, and following the Popperian formula would be bad science. The current theory of gravity is so well confirmed that it would be silly to abandon it without taking a good, long time trying to save it. Now, if in 30 or 40 years, no one has found the dark matter, then it might be time to consider looking at gravity.

3 comments:

  1. Imagine that, Karl Popper didn't completely crystallize the entire enterprise of modern science in one word. He must be a moron. (Did you just falsify his theory, Gene?)

    But seriously, I think you need to be careful. I think the history of the Big Bang theory is useful here. As I understand it, there were all sorts of initial ad hoc attempts to explain Hubble's discoveries of galactic speeds etc. Einstein had his cosmological constant, and some other guy proposed that matter gets created out of nothing (I can't remember the term). But after a while without confirming evidence, they dropped those other ad hoc things and decided the universe had a finite history.

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  2. Come on Gene ! I'm hardly an expert but ....

    1. Scientists responded to the observations you describe in two ways. Most responded by proposing modifications to the (then) existing model, which incorporated our current theories of gravity.

    2. The new revised model was not arbitrarily ad hoc because there was nothing arbitrary about retaining the current theory of gravity - since to jettison it would have created a whole host of other problems. When to modify and when to jettison is a matter of judgment, not epistemology.

    3. The new standard model's predictions of dark matter and dark energy are not only testable, but it is my understanding they have actually been tested by the observations of the WMAP mission!

    4. Some other scientists did indeed conjecture that our understanding of gravity was wrong. Most notably Motti Milgrom in Israel. I don't know what tests his theory has been put to; but I think that the WMAP findings probably refute it.

    Anyway, all perfectly Popperian.

    Julius

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