4 ms·

I'm not a physicist but both dark energy and dark matter have a definite whiff of 'luminiferous aether' imho. Of course they're just placeholders for things we

I'm not a physicist but both dark energy and dark matter have a definite whiff of 'luminiferous aether' imho. Of course they're just placeholders for things we don't understand, but my guess is that they are not any form of energy or matter at all, but a misunderstanding of the geometry of space or something similar.

devoutsalsa 2y ago
Totally! All these ideas are just a placeholder to make a note of something we can observe until we have a better theory. Luminiferous aether was invoked to explain the ability of the apparently wave-based light to propagate through empty space. [1] Eventually we found evidence that contradicted that idea. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. [2] We are searching for direct evidence, but haven’t found any yet. [1] https://en.wikipedia.org/wiki/Luminiferous_aether https://en.wikipedia.org/wiki/Luminiferous_aether [2] https://en.wikipedia.org/wiki/Dark_matter https://en.wikipedia.org/wiki/Dark_matter
- the__alchemist 2y ago
  This applies here too: See my comment above; most cosmologists are confident (And/or focus most work on) Cold Dark Matter, e.g. in an ellipsoidal halo around galaxies. I am not making a qualitative judgement, but dark matter is not used in cosmology papers as a placeholder.
  - danparsonson 2y ago
    It's a placeholder in the sense that we don't know specifically what it is though, right? No cosmologist uses "matter" when they mean "protons" for example.
    - the__alchemist 2y ago
      Yes; it's still reasonably broad. (MACHOs? WIMPs?) But this is still, in terms of all possible explanations for observations re rotation curves, and the Bullet Cluster etc, well defined. It includes a lot of possibilities, but also excludes.
      - uoaei 2y ago
        Unreasonably narrow, rather! We have not convinced anyone, much less done the proper science, to stick to our guns so long that it is truly matter we are hunting to explain this phenomenon.
    - elashri 2y ago
      > No cosmologist uses "matter" when they mean "protons" for example. They will call that "baryonic matter".
      - danparsonson 2y ago
        No, they'd call them protons; baryons are a class of particles, of which protons are a member.
        elashri 2y ago
        This is what a particle physicist use the term for. But for cosmologist on the question of matter composition they will use the term in different way. I was replying to exactly the quote from your reply.
    - uoaei 2y ago
      The point is, it's already a theory to even suggest it's "matter" and not any other explanation.
      - devoutsalsa 2y ago
        It certainly doesn't matter yet. I'll see myself out.
    - floxy 2y ago
      >No cosmologist uses "matter" when they mean "protons" for example. Don't astronomers call elements like neon metals? Maybe cosmologists don't? https://en.wikipedia.org/wiki/Metallicity https://en.wikipedia.org/wiki/Metallicity
ggambetta 2y ago
I've been saying this for at least 6 years [0]. Got downvoted to oblivion back then, glad to see opinions are changing. [0] https://news.ycombinator.com/item?id=21279144 https://news.ycombinator.com/item?id=21279144
lutorm 2y ago
"Dark matter" is just a description that refers to matter which interacts gravitationally but not, or at least very weakly, in other ways. It doesn't imply anything about the nature of such matter. We know that there are several types of dark matter, specifically neutrinos and brown dwarfs, although it is now established that they don't make up anything near the density required by the cosmological models. Primordial black holes, which may exist, would be a dark matter. Some particle physics models, like supersymmetry, also naturally predict that there will be a massive particle that would behave like dark matter. So I don't think the analogy with the aether is very good, because that really was pulled out of thin air (pun intended). Given how successful general relativity is, it's perfectly rational to interpret the observations as probing mass distributions while assuming GR will continue to hold. There are of course people also assuming that GR is wrong and attempting to explain the observations without dark matter, but my impression is that they struggle to come up with self-consistent theories that fit all the data, although I'm not very familiar with that work. What other people have been doing over the past 40 years is attempting to devise tests for these various dark matter candidates. We know, for example, through lensing observations, that MACHOs/brown dwarfs don't exist in the required numbers and that the neutrino mass seems too low. The problem, of course, is that there are only so many ways to try to observe matter that is truly dark. I agree, though, that, in the end, it may be that dark matter will be an untestable hypothesis, just like quantum gravity or whatever.

devoutsalsa 2y ago

Totally! All these ideas are just a placeholder to make a note of something we can observe until we have a better theory. Luminiferous aether was invoked to explain the ability of the apparently wave-based light to propagate through empty space. [1] Eventually we found evidence that contradicted that idea. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. [2] We are searching for direct evidence, but haven’t found any yet. [1] https://en.wikipedia.org/wiki/Luminiferous_aether https://en.wikipedia.org/wiki/Luminiferous_aether [2] https://en.wikipedia.org/wiki/Dark_matter https://en.wikipedia.org/wiki/Dark_matter

the__alchemist 2y ago
This applies here too: See my comment above; most cosmologists are confident (And/or focus most work on) Cold Dark Matter, e.g. in an ellipsoidal halo around galaxies. I am not making a qualitative judgement, but dark matter is not used in cosmology papers as a placeholder.
- danparsonson 2y ago
  It's a placeholder in the sense that we don't know specifically what it is though, right? No cosmologist uses "matter" when they mean "protons" for example.
  - the__alchemist 2y ago
    Yes; it's still reasonably broad. (MACHOs? WIMPs?) But this is still, in terms of all possible explanations for observations re rotation curves, and the Bullet Cluster etc, well defined. It includes a lot of possibilities, but also excludes.
    - uoaei 2y ago
      Unreasonably narrow, rather! We have not convinced anyone, much less done the proper science, to stick to our guns so long that it is truly matter we are hunting to explain this phenomenon.
  - elashri 2y ago
    > No cosmologist uses "matter" when they mean "protons" for example. They will call that "baryonic matter".
    - danparsonson 2y ago
      No, they'd call them protons; baryons are a class of particles, of which protons are a member.
      - elashri 2y ago
        This is what a particle physicist use the term for. But for cosmologist on the question of matter composition they will use the term in different way. I was replying to exactly the quote from your reply.
  - uoaei 2y ago
    The point is, it's already a theory to even suggest it's "matter" and not any other explanation.
    - devoutsalsa 2y ago
      It certainly doesn't matter yet. I'll see myself out.
  - floxy 2y ago
    >No cosmologist uses "matter" when they mean "protons" for example. Don't astronomers call elements like neon metals? Maybe cosmologists don't? https://en.wikipedia.org/wiki/Metallicity https://en.wikipedia.org/wiki/Metallicity

ggambetta 2y ago

I've been saying this for at least 6 years [0]. Got downvoted to oblivion back then, glad to see opinions are changing. [0] https://news.ycombinator.com/item?id=21279144 https://news.ycombinator.com/item?id=21279144

lutorm 2y ago

"Dark matter" is just a description that refers to matter which interacts gravitationally but not, or at least very weakly, in other ways. It doesn't imply anything about the nature of such matter. We know that there are several types of dark matter, specifically neutrinos and brown dwarfs, although it is now established that they don't make up anything near the density required by the cosmological models. Primordial black holes, which may exist, would be a dark matter. Some particle physics models, like supersymmetry, also naturally predict that there will be a massive particle that would behave like dark matter. So I don't think the analogy with the aether is very good, because that really was pulled out of thin air (pun intended). Given how successful general relativity is, it's perfectly rational to interpret the observations as probing mass distributions while assuming GR will continue to hold. There are of course people also assuming that GR is wrong and attempting to explain the observations without dark matter, but my impression is that they struggle to come up with self-consistent theories that fit all the data, although I'm not very familiar with that work. What other people have been doing over the past 40 years is attempting to devise tests for these various dark matter candidates. We know, for example, through lensing observations, that MACHOs/brown dwarfs don't exist in the required numbers and that the neutrino mass seems too low. The problem, of course, is that there are only so many ways to try to observe matter that is truly dark. I agree, though, that, in the end, it may be that dark matter will be an untestable hypothesis, just like quantum gravity or whatever.