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Dark Matter Question

  1. Apr 4, 2014 #1
    I have been following stories about dark matter and have, I think, a layman's grasp of it, to whit: In 1933 the Swiss astrophysicist Fritz Zwicky, who studied clusters of galaxies while working at the California Institute of Technology, made an inference similar to one previously made by Carl Oort. Zwicky applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass. Zwicky estimated the cluster's total mass based on the motions of galaxies near its edge and compared that estimate to one based on the number of galaxies and total brightness of the cluster. He found that there was about 400 times more estimated mass than was visually observable. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some non-visible form of matter which would provide enough of the mass and gravity to hold the cluster together (adapted from Wikipedia).

    Now I was listening to a radio interview with Laurence Krauss, when he visited Sydney once, maybe 7 years ago. The subject of dark matter was broached, and the interviewer, a very scientifically-literate radio host called Spencer, casually opined that we might be 'sorrounded by' dark matter, even as we speak; that it might, as it were, interpenetrate the everyday domain of experience, but it exists in such a form that we can't detect it.

    Krauss appeared to assent to that - very casually, I thought - 'indeed, it might' - and the conversation moved on.

    But that idea has stayed with me ever since. What if, I thought, the actual extent of mass or matter, even right here in front of us, is actually some large percentage more that what we can actually measure? What if 'dark matter' is actually an imperceptible component of everything that exists around us? What if, then, the totality of matter that is detected by both instruments and sense, is less that what is really there?

    I mean, it is not at all established that dark matter hangs around in interstellar space like a large kind of blob of stuff. This kind of depiction indicated that it might not ever be detectable, even when you're sorrounded by it. So if it is actually all around us, it amounts to a kind of hidden dimension or component of everything that exists.

    I can imagine an Arthur Conan Doyle, or someone of that ilk, making something of it. But it doesn't sound very 'scientific'. It actually sounds more like science fiction to me.
  2. jcsd
  3. Apr 4, 2014 #2


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    Quo, DM seems not as adept at condensing and clumping as ordinary matter, so it tends to stay in diffuse clouds.

    the reason seems to be that when DM falls together it just keeps on going, less able to collide, get hot, and dissipate the excess energy by radiating it off.

    If that's true then in a solar system environment there is vastly more ordinary than dark, because ordinary has condensed and collected here and dark is only the same density as the diffuse clouds in and around Milky.

    OVERALL there's at least 5 or 6 times more dark than ordinary

    But if you look at the inner solar system, say a sphere 200 million km radius, that has a lot more ordinary in it than dark. It was understandable that Larry Krauss spoke casually about it.

    People who try to detect DM make calculations about how much there might be in our immediate locale and it's not very much, but they still try to detect it, so far without much success.
    Last edited: Apr 4, 2014
  4. Apr 5, 2014 #3
    But that still doesn't mean that you would ever be able to see, measure, weigh, or detect it. I have read that it is thought to be 'non-baryonic', meaning, 'not consisting of atoms'. Well, we have never found anything like that. So whether it is actually 'matter' in any meaningful sense is, I think, an open question.
  5. Apr 5, 2014 #4


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    Non-baryonic means it's not made of three quarks. And we've known such 'matter' for quite a long time: e.g., electrons and neutrinos(http://en.wikipedia.org/wiki/Lepton).

    It should be detectable, in so far as it bothers to interact with the detectors. And indeed there are experiments set up trying to detect it(http://cdms.berkeley.edu/).
    The point is, it's not expected to be abundant in the vicinity of the Solar system. As a weakly(or non?)-interacting particle, it tends to hang around in diffuse halos surrounding dense regions of ordinary matter.
  6. Apr 5, 2014 #5
    But electrons have never been generally observed apart from being in association with atoms. And neutrinos are only barely detectable.

    And it *must* be abundant somewhere, seeing as it outweighs 'normal matter' by some vast amount. The point is, if you flew a probe 'out there' where we supposed it must be, you still not might be able to shovel a bit of it into a lander and look at it through instruments, because it won't be what physics, before about five minutes ago, defined as 'matter'.
    Last edited: Apr 5, 2014
  7. Apr 5, 2014 #6
    We've detected neturinos, which are non-baryonic matter and very weakly interacting.

    There are also baryonic candidates for dark matter, specifically large astronomical objects that emit very little radiation eg. black holes.

    It's believed that these only account for a small fraction of the the dark matter though.

    The word "matter" in dark matter refers to the fact that it must have rest mass.

    The solar wind contains free electrons, though electrons aren't candidates for dark matter.

    It doesn't need to be abundant anywhere, there's a lot of space out there, it's thought to be much more evenly distributed than the matter that we see.

    In the same way that you can't shovel neutrinos because they're weakly interacting, you can't shovel dark matter.

    A primary candiate for dark matter is the axion. You can read about attemps to detect it here:
    Last edited: Apr 5, 2014
  8. Apr 5, 2014 #7


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    I fail to see the problem. Of course you can't see an electron with a naked eye. But you can infer its existence from countless clues the nature leaves you. From electricity and light to chemistry and nuclear processes.
    Same with neutrinos, same goes for dark matter. You only need to construct an experiment that detects a something that could not be produced by anything else.

    By the way, 'matter' is not a well defined word in physics. It's more of a shorthand for various elementary particles.
  9. Apr 5, 2014 #8
    The others have already answered the essential questions. However I would like to expand a bit on DM's weakly interactive nature.

    there are 4 known forces see chart below for their strength and theory of interactions

    Force...................Strength......................Theory........... ........Mediator

    Strong.................10................................Chromodynamics ....Gluon
    Weak...................10-13........................ Flavordynamics......W and Z

    DM has no interactions with the electromagnetic, therefore it has no charge, it isn't known to interact with the strong force so it has no color(color interactions). If its found to interact with other weakly interactive particles such as the neutrino then it has a flavor interaction.(electroweak). we already know its influenced by gravity so is subject to geometrodynamic interactions. I included the relative strengths of each force. Neutrinos share the same characteristics, however not enough abundance to account for dark matter.

    By 1950, then, there was compelling theoretical evidence for the existence of neutrinos, but there was still no direct experimental verification. A skeptic might have argued that the neutrino was nothing but a bookkeeping device-a purely hypothetical particle whose only function was to rescue the conservation laws. It left no tracks, it didn't decay; in fact, no one had ever seen a neutrino do anything. The reason for this is that neutrinos interact extraordinarily weakly"
    Neutrinos and dark matter both carry similar weakly interactive characteristics, both are affected by gravity. Neutrinos are so weakly interactive that it can penetrate a 1000 light years of lead without an interaction.
    Neutrinos were later proved to exist by experimentation at the Savannah river nuclear power station by Cowan and Reines in the later 50's ( Cowan-Reines reaction) and also by the( Davis reaction).

    You can see by this that at one time neutrinos were just as mysterious as dark matter, thought by many to be a nonexistent mathematical construct. Many today still wonder if this is true for dark matter, however their is far too much indirect evidence for its existence to be taken for a book-keeping mathematical construct.
    I recall reading an arxiv review but can no longer locate it that even MOND requires dark matter for some of its metrics to work properly lol which sort of defeats the purpose of MOND. Wish I could locate that pentagonal review of both LCDM vs MOND. Had some very useful info, as it explained problems still facing both models. Ah well.
  10. Apr 5, 2014 #9
    The problem is that dark matter might not be something 'out there'. It might not have any particular location, it might not be in any particular place. As it is indetectable, it seems quite possible that it might be distributed evenly throughout the Universe. If that were the case, it means the matter we can detect with senses and instruments only comprises a small percentage of what actually exists. And that has some interesting philosophical implications, doesn't it?

    The fact that 'matter' is not well-defined in physics is part of the same issue. A century ago it might have been thought that the nature of matter was on the verge of being defined. It seems rather less so now, doesn't it? Like, the more you discover about it, the stranger it seems.
  11. Apr 5, 2014 #10
    Sounds like your argument is based on a personal philosophy... I feel for you, as I once felt the same. That being said I decided that my best course of action was to study the reasons why dark matter is necessary.

    Although at first I struggled with the concept, going so far as to study any alternative metrics
    such as MOND. However the more I studied, the more I came to realize the necessity of the dark sector.

    Now I find no reason as to why dark matter cannot exist. Its properties are similar to other weakly interactive particles. As a result it has a low density. However due to the sheer volume of space it is the largest gravitational contributer. Eventually we will understand its properties well enough that we will be able to remove the term "dark".

    Not every particle is easily found. Also not every particle is part of the standard model of particles. The Higgs boson is another particle that met with scorn and denial. We know now that it does indeed exist
  12. Apr 5, 2014 #11
    If you read the snippet in the OP about the interview with Lawrence Krauss, you will see where the idea came from. To recap - there was a discussion in a radio interview; the interviewer suggested the idea that because dark matter can't be detected by instruments it might actually exist all around us right now, to which Krauss said something like 'may be', or 'indeed'.

    That is what prompted this thread. The responses seem to be, it's 'out there somewhere' - lurking in interstellar space. Out of sight, out of mind, but useful for making the equations balance. But if it is something completely different to all the matter that comprises the periodic table, then what is saying it has to exist in a location? It's not as if you could send a probe up to where it is, and pick up a piece of it.

    So if it is something that interpenetrates the domain of normal matter, but is not detectable, then it might not be 'out there' at all; it might be 'right here'. It's just that we never see it and can't measure it.

    In other words, the matter that physics deal with is only a small percentage of what actually exists; but the remainder of what is really there is imperceptible.

    And what makes you think that we will be able to retain the description 'matter'?
    Last edited: Apr 5, 2014
  13. Apr 5, 2014 #12
    Ah gotcha. All I can say on that is that it may be possible.
  14. Apr 5, 2014 #13
    thanks! That is why I find it an intriguing idea.

    I'm working on a story draft - a kind of science fiction idea, but more like 'science drama', and this idea comes into it. I just wanted to see what the reactions were to it. (That is why I mentioned Conan Doyle in the OP.)
  15. Apr 5, 2014 #14


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    But it is NOT "indetectable" and it IS in a particular location, albeit both statements are in the aggregate not about individual "particles", which it may or may not be. It is detected very clearly by a number of things, most obviously its gravitational affects on galaxies and its role in the creation of Einstein Rings. These detections place it in halos around galaxies, at the very least.

    Since you clearly KNOW all that, I'm not clear about what your argument is.
  16. Apr 5, 2014 #15
    Right. So given the appropriate apparatus and a way to get to that location, you could go and get a sample of it and bring it back to the lab for analysis?
  17. Apr 5, 2014 #16


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    As has already been pointed out, we haven't yet figured out how to do that but our not having figure out how to do it does NOT, as you seem to believe, mean that it is not theoretically possible to do it. I may well BE impossible, but that's not proven, and even if it is that's just another indication that it doesn't interact with normal matter, which we do know how to make shovels out of.
  18. Apr 5, 2014 #17
    It hasn't been detected. It's existence has been inferred on the basis of calculations.
  19. Apr 5, 2014 #18


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    Time for this thread to be moved to the Science Fiction and Fantasy forum, where it belongs.
  20. Apr 5, 2014 #19


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    +1 on that.
  21. Apr 5, 2014 #20
    And what about that statement belongs in 'science fiction and fantasy'. It is an indisputable fact that nothing corresponding to 'dark matter' has been detected. Its presence has been inferred on the basis of calculations.

    But I have nothing against this thread being moved to that forum. I am working on a sic-fi story.
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