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I think you're basically right, but I'd encourage you to feel comfortable with it as a normal part of the discovery process. I would phrase it as "There's some

I think you're basically right, but I'd encourage you to feel comfortable with it as a normal part of the discovery process. I would phrase it as "There's something happening that we see only indirectly—by its diffuse gravitational influence. Let's give it a name." That's happened many times before in science. For example, the measurable perturbation of Mercury's orbit due to special relativity—it was measured before it was explained, and there were multiple competing explanations. Or, long before that, the causes of disease. What we believe now (invisibly small germs) was laughed at.

thaumasiotes 9y ago
I agree that this is a normal part of the discovery process. But I don't agree that it constitutes a theory of dark matter, any more than the observation of the precession of Mercury constitutes a theory of general relativity. I was contrasting the claim from the article, "there is no theory that predicts galaxies without dark matter" with the implied claim that there is a theory that predicts galaxies with dark matter. If the extent of our theory is "we conclude that the galaxies we've observed are permeated by a substance which is massless[1] but nevertheless generates gravitational fields, because their gravity is all messed up", that is not a theory that predicts the nonexistence of galaxies that lack this substance and display normal gravity, nor is it a theory that predicts how a galaxy would acquire any of this weird substance. If it doesn't do those things, what's surprising about the quote I highlighted? If the dark matter concept is purely observational -- "I'm going to call the weird gravity of this galaxy 'dark matter'" -- it cannot possibly conflict with any other observations. As far as this theory is concerned, it does predict the existence of galaxies without weird gravity, because we observe those, and that is how we've defined "predictions" as generated by this theory. If that sounds stupid to you -- and it does to me -- either we shouldn't be talking about "predictions" or there's something better qualified to be called a "theory". Is there? [1] I'm using "massless" to refer to the property called "collisionless" in the wikipedia article. Physicists may define mass as the property by which gravitational fields are generated, but I think the ability to collide with other masses is more fundamental to the basic concept.
- neolefty 9y ago
  Haha, I'm no physicist, so take anything I say with a grain of salt. Also, I don't think I can address everything you wrote, so apologies for picking and choosing. I'll say what I can: > the ability to collide ... is more fundamental That's exactly the funny thing—when physicists say "dark matter" they're saying matter that attracts but doesn't collide. They can only detect it indirectly. I think they normally want to agree with you that matter interacts with light and other matter. "Dark matter" would not be their first choice of explanations. > the observation of the precession of Mercury constitutes a theory of general relativity I'm going to get pedantic here and separate observation from theory, because I think it helps talk about it. Mercury was observed to precess oddly, and nobody knew why. For quite a while, the theory was that an unseen planet was doing it. So I'd phrase it only slightly differently: > the observation of the precession of Mercury constitutes evidence for a theory of general relativity I draw the distinction because it's central to the debate about dark matter. Weird gravity isn't a theory at all—it's an observation. And it's one physicists pretty much agree on—galaxies are definitely acting weird, with respect to their visible mass. One possible theory to explain it is dark matter. Another is MOND. Any of them could be true—it's kind of probabilistic. For some physicists this new observation is shifting the probabilities in favor of dark matter, but it's obviously still flawed because we can't really explain dark because we have no positive confirmation of its existence outside of gravity. We'd love to find a WIMP, for example, but we haven't.

thaumasiotes 9y ago

I agree that this is a normal part of the discovery process. But I don't agree that it constitutes a theory of dark matter, any more than the observation of the precession of Mercury constitutes a theory of general relativity. I was contrasting the claim from the article, "there is no theory that predicts galaxies without dark matter" with the implied claim that there is a theory that predicts galaxies with dark matter. If the extent of our theory is "we conclude that the galaxies we've observed are permeated by a substance which is massless[1] but nevertheless generates gravitational fields, because their gravity is all messed up", that is not a theory that predicts the nonexistence of galaxies that lack this substance and display normal gravity, nor is it a theory that predicts how a galaxy would acquire any of this weird substance. If it doesn't do those things, what's surprising about the quote I highlighted? If the dark matter concept is purely observational -- "I'm going to call the weird gravity of this galaxy 'dark matter'" -- it cannot possibly conflict with any other observations. As far as this theory is concerned, it does predict the existence of galaxies without weird gravity, because we observe those, and that is how we've defined "predictions" as generated by this theory. If that sounds stupid to you -- and it does to me -- either we shouldn't be talking about "predictions" or there's something better qualified to be called a "theory". Is there? [1] I'm using "massless" to refer to the property called "collisionless" in the wikipedia article. Physicists may define mass as the property by which gravitational fields are generated, but I think the ability to collide with other masses is more fundamental to the basic concept.

neolefty 9y ago
Haha, I'm no physicist, so take anything I say with a grain of salt. Also, I don't think I can address everything you wrote, so apologies for picking and choosing. I'll say what I can: > the ability to collide ... is more fundamental That's exactly the funny thing—when physicists say "dark matter" they're saying matter that attracts but doesn't collide. They can only detect it indirectly. I think they normally want to agree with you that matter interacts with light and other matter. "Dark matter" would not be their first choice of explanations. > the observation of the precession of Mercury constitutes a theory of general relativity I'm going to get pedantic here and separate observation from theory, because I think it helps talk about it. Mercury was observed to precess oddly, and nobody knew why. For quite a while, the theory was that an unseen planet was doing it. So I'd phrase it only slightly differently: > the observation of the precession of Mercury constitutes evidence for a theory of general relativity I draw the distinction because it's central to the debate about dark matter. Weird gravity isn't a theory at all—it's an observation. And it's one physicists pretty much agree on—galaxies are definitely acting weird, with respect to their visible mass. One possible theory to explain it is dark matter. Another is MOND. Any of them could be true—it's kind of probabilistic. For some physicists this new observation is shifting the probabilities in favor of dark matter, but it's obviously still flawed because we can't really explain dark because we have no positive confirmation of its existence outside of gravity. We'd love to find a WIMP, for example, but we haven't.