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> "Dark matter is a form of matter..." is a definitive statement, but you can't point to a single example of a direct detection of dark matter. Are these not a
by ailideex 7y ago
> "Dark matter is a form of matter..." is a definitive statement, but you can't point to a single example of a direct detection of dark matter.
Are these not all cases of direct detection: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
- rpedela 7y agoDirect detection would be detecting/trapping the dark matter particle(s). Those observations are indirect and it may turn out that there is another explanation other than dark matter such as we are using GR equations incorrectly as suggested in the post.
- snowwrestler 7y agoWhat kind of detecting/trapping? We have to be specific. If we hypothesize a kind of matter that does not interact electromagnetically (only gravitationally), then it's kind of nonsensical to say that only direct electromagnetic interactions with such matter will count to confirm its existence.
- rpedela 7y agoWell that is the crux isn't it? How do we directly detect gravitational particles?
- snowwrestler 7y agoMaybe we don't. The evidence for general relativity is all indirect--we can't directly detect space-time. But plenty of people seem to think it's a solid theory anyway.
- deleted 7y ago[deleted]
- simonh 7y agoThere is a significant difference in that GR made numerous very testable claims about all sorts of phenomena, which would not be at all expected without GR to explain them. It also depends what is consider direct detection, which I accept is a matter of opinion. For example I'd consider comparing the times on atomic clocks inside and outside a gravity well to be direct measurement of time dilation.
- simonh 7y agoThis is exactly why the hypothesis is weak. At the moment it postulates a particle that only ever manifests in such a way that it explains gravitational behaviour at a microscopic scale, in certain observations, and has no other detectable interactions or properties whatsoever. There’s no theory of how it forms, no theory of how it might be created or annihilated, no evidence for it in any particle physics experiments. We just have this one class of equations that produce unexpected results, so we put in an extra term and say were done. That’s just not good enough to say anything definitive about what it is. Physicists know that’s not good enough, and that it’s a major problem. I hope we do make progress, do come up with further experimental evidence or ideas for experiments we can test. For dark matter I think that’s actually likely, but I’m not so convinced about dark energy, there the evidence is a lot thinner.
- ailideex 7y agoHow do you distinguish direct as opposed to indirect? I mean, to me it directly observes something which is matter and non-luminous - and satisfies the technical definition of dark matter.
- rpedela 7y agoRotation and velocity hint that something is amiss but doesn't directly measure dark matter. Gravitational lensing is a direct measurement of matter but doesn't tell us much about its composition. If using the most general definition of "dark matter", lensing would be a direct measurement. But if "dark matter" implies non-baryonic matter, the leading hypothesis, then gravitational lensing is indirect because it is hinting something is amiss just like the rotation and velocity evidence. Now if there was a predictive theory of non-baryonic matter and that predicted gravitational lensing with specific characteristics, that would be direct evidence. Just like GR predicted black holes and we searched and found them.