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I don't think its correct to say "Dark Matter is the popular name for a variable in some models of how fast galaxies of different sizes rotate". The properties

I don't think its correct to say "Dark Matter is the popular name for a variable in some models of how fast galaxies of different sizes rotate". The properties of Dark Matter are pretty tightly constrained, not just by galactic rotation properties, but also by CMB observations, gravitational lensing, galactic structure observations, big-bang nucleosynthesis populations and direct detection experiments. Obviously what one considers a "plainly observable thing" is up for debate. But the effects of dark matter are seen in a very wide range of astrophysical phenomena. Its not true that its just some fudge factor stuck in to account for galactic velocity curves.

guscost 12y ago
Would you care to enumerate these properties?
- batbomb 12y ago
  There are four Fundamental Forces: Strong, Weak, Electromagnetic, and Gravity. In general, Dark Matter does not interact electromagnetically, and therefore does not emit nor absorb light. We can't see it. That's the defining property. There may be a few flavors of dark matter, each with it's own additional properties. However, Dark Matter probably interacts through gravity and the weak force: http://www.lsst.org/files/img/xxnyt.jpg http://www.lsst.org/files/img/xxnyt.jpg http://en.wikipedia.org/wiki/Weakly_interacting_massive_particles http://en.wikipedia.org/wiki/Weakly_interacting_massive_part...
  - guscost 12y ago
    So perhaps we can say (to a layman) that the properties of Dark Matter are: - It interacts via Gravity - It interacts via the Weak Nuclear Force But has any experiment ever observed something that fits this description? The galaxy rotation and gravitational lensing stuff must only indicate a gravity interaction, and as far as I can tell the attempts to observe WIMPs in isolation have not yet succeeded[0]. It looks like there might be some information gained from this PICASSO experiment mentioned in the wiki, but I would be grateful for some help decoding this one: > The obtained limits restrict recent interpretations of the DAMA/LIBRA annual modulation effect in terms of spin dependent interactions. Anyway, it seems like PR nonsense when folks ignore Popper and list "confirmations" that tend to support the SUSY model. I was careful not to use the phrase "fudge factor" because that's unfair, but there is a wide gulf between a fudge factor and a well-defined observable thing. [0] http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm http://news.discovery.com/space/lhc-discovery-maims-supersym...
tjradcliffe 12y ago
It is true that it's one of several potential alternative explanations, though, and laypeople or some subset of them seem to have a lot of trouble understanding why we focus on dark matter in preference to the alternatives. Some of the alternatives are crazy, of course, and laypeople are also apt to ask, "Why can't you just..." and then express some suggestion that either makes no sense, or is known to be maximally implausible. Some of the confusion around dark matter is sincere, but there does seem to be a bit of a fringe community who think that physicists just like it, and are ignoring viable alternatives, which is laughable: if anyone could come up with a convincing alternative we'd be all over it. There is simply no downside.
- nitrogen 12y ago
  ...[people] think that [scientists] just like it, and are ignoring viable alternatives... I think this happens all too often with various sciences (biology, climatology, physics). I speculate it's because people don't think about the very long history that produced the current consensus in each domain; rather they only see the consensus as it currently exists, and assume it must have been placed that way by some design. If these things come up in casual conversation, as an interested layperson I try to emphasize that science is a process that has been proven to work better than other processes for gaining knowledge, and the current consensus in a scientific field is only a snapshot of that process.

guscost 12y ago

Would you care to enumerate these properties?

batbomb 12y ago
There are four Fundamental Forces: Strong, Weak, Electromagnetic, and Gravity. In general, Dark Matter does not interact electromagnetically, and therefore does not emit nor absorb light. We can't see it. That's the defining property. There may be a few flavors of dark matter, each with it's own additional properties. However, Dark Matter probably interacts through gravity and the weak force: http://www.lsst.org/files/img/xxnyt.jpg http://www.lsst.org/files/img/xxnyt.jpg http://en.wikipedia.org/wiki/Weakly_interacting_massive_particles http://en.wikipedia.org/wiki/Weakly_interacting_massive_part...
- guscost 12y ago
  So perhaps we can say (to a layman) that the properties of Dark Matter are: - It interacts via Gravity - It interacts via the Weak Nuclear Force But has any experiment ever observed something that fits this description? The galaxy rotation and gravitational lensing stuff must only indicate a gravity interaction, and as far as I can tell the attempts to observe WIMPs in isolation have not yet succeeded[0]. It looks like there might be some information gained from this PICASSO experiment mentioned in the wiki, but I would be grateful for some help decoding this one: > The obtained limits restrict recent interpretations of the DAMA/LIBRA annual modulation effect in terms of spin dependent interactions. Anyway, it seems like PR nonsense when folks ignore Popper and list "confirmations" that tend to support the SUSY model. I was careful not to use the phrase "fudge factor" because that's unfair, but there is a wide gulf between a fudge factor and a well-defined observable thing. [0] http://news.discovery.com/space/lhc-discovery-maims-supersymmetry-again-130724.htm http://news.discovery.com/space/lhc-discovery-maims-supersym...

tjradcliffe 12y ago

It is true that it's one of several potential alternative explanations, though, and laypeople or some subset of them seem to have a lot of trouble understanding why we focus on dark matter in preference to the alternatives. Some of the alternatives are crazy, of course, and laypeople are also apt to ask, "Why can't you just..." and then express some suggestion that either makes no sense, or is known to be maximally implausible. Some of the confusion around dark matter is sincere, but there does seem to be a bit of a fringe community who think that physicists just like it, and are ignoring viable alternatives, which is laughable: if anyone could come up with a convincing alternative we'd be all over it. There is simply no downside.

nitrogen 12y ago
...[people] think that [scientists] just like it, and are ignoring viable alternatives... I think this happens all too often with various sciences (biology, climatology, physics). I speculate it's because people don't think about the very long history that produced the current consensus in each domain; rather they only see the consensus as it currently exists, and assume it must have been placed that way by some design. If these things come up in casual conversation, as an interested layperson I try to emphasize that science is a process that has been proven to work better than other processes for gaining knowledge, and the current consensus in a scientific field is only a snapshot of that process.