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Examining the Expected Effects of Dark Matter On the Solar System 190

Posted by timothy
from the picks-up-loose-dirt-gets-out-tough-stains dept.
First time accepted submitter LiavK writes "Ethan Siegel recently wrote a great post for ScienceBlogs discussing the expected total mass of dark matter in the solar system. As far as we can tell, dark matter only interacts weakly, via gravity, both with itself and normal matter. So, it can't collide with itself, meaning that it has no way of getting hotter and radiating away energy and momentum. This means that it remains a diffuse mess, with a density that is ridiculously low, to the point where detecting its local effects is likely to remain... challenging for the foreseeable future."
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Examining the Expected Effects of Dark Matter On the Solar System

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  • The problem with dark matter observation in this case is that science is based on empirical observation. If you can't see it, can't measure it, and can't even draw inferences from what you can see and measure to detect something indirectly... it's not science. What this is saying is that the effects are so miniscule that there is no equipment presently capable of separating an actual effect or observation from systemic inaccuracy in the equipment itself. That is, you can't tell whether it's just random 'noi

    • by Ralph Spoilsport (673134) on Sunday August 11, 2013 @04:36PM (#44537467) Journal
      "hus, we have concluded there's a cat in the apartment... but nobody has actually ever seen the cat."

      Or, your significant other has some weird cat food fetish thing going on...
      Eeeek!
      :-)

    • by buchner.johannes (1139593) on Sunday August 11, 2013 @04:51PM (#44537543) Homepage Journal

      The problem with dark matter observation in this case is that science is based on empirical observation. If you can't see it, can't measure it, and can't even draw inferences from what you can see and measure to detect something indirectly... it's not science. What this is saying is that the effects are so miniscule that there is no equipment presently capable of separating an actual effect or observation from systemic inaccuracy in the equipment itself. That is, you can't tell whether it's just random 'noise' or an actual signal.

      But we do find it empirically. There is extra mass there, affecting other objects. We can detect it through it's gravitation, just not through light. It's a very strong signal, for example in the rotation velocity of galaxies. A lot of other science is, too, done without directly detecting the object of study, but through indirect effects and inference.

      Everyone would like to get rid of Dark Matter. But its effects are clearly there. And we need to explain it. It does not have to be particles, or a kind of matter we know. You can call it something else than Dark Matter if you don't like the name. Anyone is welcome to come up with explanations. But they have to be in agreement with the observations.

      • by tqk (413719)

        The problem with dark matter observation in this case is that science is based on empirical observation. If you can't see it, can't measure it, and can't even draw inferences from what you can see and measure ...

        But we do find it empirically. There is extra mass there, affecting other objects. We can detect it through it's gravitation, just not through light.

        Am I the only one astonished to learn that regular astronomers are finally twigging to the fact that a lot of stuff out there can't be seen or detected by what we've got to work with? Why hasn't it been in your face obvious to everyone that there's a lot of stuff that doesn't radiate in the visible spectrum, or strong enough in an altogether different part of the spectrum for us to have seen or been able to describe before now. Of course we're going to finally wonder why that galaxy is spinning oddly base

        • by samkass (174571) on Sunday August 11, 2013 @05:50PM (#44537879) Homepage Journal

          Why invent exotic matter when the right combination of dust could be the answer?

          Simply put, because baryonic matter (ie. dust) radiates. This article would be titled, "Why our instruments are sensitive enough to detect all that dust that's affecting galaxies and superclusters rotation" if it was dust.

          Here's a recent summary paper [arxiv.org] on the evidence for nonbaryonic dark matter. Dust has, alas, been hypothesized, tested, and rejected.

        • by dentin (2175)

          You're a fool if you think that regular astronomers are 'finally twigging' out about events that don't directly radiate in the visible spectrum. They are in fact painfully aware of the fact that they can only see the visible side effects of most events.

          As for your question regarding exotic matter and dust, the opinion for many decades was that dust was the answer. That opinion has been replaced with exotic matter over time, for extremely good reasons which you apparently don't yet understand.

          -dentin

          • Re: (Score:2, Informative)

            by stenvar (2789879)

            As for your question regarding exotic matter and dust, the opinion for many decades was that dust was the answer. That opinion has been replaced with exotic matter over time, for extremely good reasons which you apparently don't yet understand.

            "Extremely good reasons" is not the same as proof. And there are other possibilities, like large numbers of rogue planets, or multiple different mechanisms explaining different phenomena.

            Don't get me wrong: weakly interacting dark matter is plausible, but until there

            • by dentin (2175)

              So long as you agree that it is not just plausible speculation, but currently the most likely plausible explanation, then we're on the same page.

              Just because there are multiple plausible hypothesis, doesn't mean they're all equally probable.

              -dentin

          • by tqk (413719)

            You're a fool if you think that regular astronomers are 'finally twigging' out about events that don't directly radiate in the visible spectrum.

            I may indeed be a fool. I've also run across many a science program on teevee hosting a distinguished representer who gushes over this subject. "We can't see *a lot of stuff!* Who knew?!?" Go figure.

            • Re: (Score:3, Informative)

              by dentin (2175)

              [sarc]Naturally, everything you see on teevee is true and accurate, and all distinguished presenters are to be trusted, and all science program scripts are written for maximum accuracy and conveyance of relevant information. Why would we ever question something we saw on a tv program? Tune in for next week's "Ancient Aliens" for proof that the anti-TV conspiracy started in ancient Egypt![/sarc]

              People interested in real science don't get their science from TV. People interested in real science learn from

        • Re: (Score:2, Redundant)

          by ceoyoyo (59147)

          Because the right combination of dust can't be the answer. That was thought of, tested, and it failed to explain observations.

        • by Bengie (1121981) on Sunday August 11, 2013 @10:54PM (#44539193)
          You mean how this stuff is 100% transparent to all known frequencies from radio to gamma? Please, tell me what matter you know of that is 100% transparent to all forms of radiation. Enlighten us all knowing one.

          Yes, we know, 100% that is is transparent. There are HUGE spots in the sky where there is gravitational lensing affecting background galaxies, but no obstructions in front of the galaxies. Something is causing the gravity, but it is letting the background light through perfectly clearly, minus the lensing.

          We're not talking about small amounts of gravity either, whole galaxy masses worth. If you had a galaxy worth of gravitational lensing, you'd hope to find something causing it. Instead the background light comes through crystal clear, like nothing is there.. hmmmm...
          • by Zorpheus (857617)
            Black holes?
            • by Bengie (1121981)
              Blackholes tend to have mass around them, not off on their own, and they have a lot of lensing near them, then it drops off quickly, while the lensing they're observing is much more evenly distributed, like a lot of matter spread all over.
        • by tnk1 (899206)

          Well, you know that they do have radio and infrared telescopes, right? They already are checking other spectra. I'm not going to presume that they have it all covered, but I'd be very surprised to find it was just normal "dust" radiating in some very specific wavelength we just haven't checked yet. Let's not forget that while there is a wide spectrum of possibilities, you only radiate at certain wavelengths under specific circumstances such as being a product of high energy events. We've been looking at

          • by tqk (413719)

            Although current theories of Dark Matter are pretty vague and ether-like at this point, weakly interacting particles have been demonstrated in the past, like neutrinos. It's far from being implausible or wild guessing.

            I'm not saying that. It just seemed like extrapolating to new exotic particles wasn't called for if mere misunderstood dust might explain the phenomenon. I've since been informed that they indeed have thought of this and (rightly or wrongly) discarded it for various reasons.

            [The /. bottom page quote: "You can not get anything worthwhile done without raising a sweat. -- The First Law Of Thermodynamics]

      • by stenvar (2789879)

        But we do find it empirically. There is extra mass there, affecting other objects. We can detect it through it's gravitation, just not through light. It's a very strong signal, for example in the rotation velocity of galaxies.

        There are unexplained gravitational effects, but that's all we really know. The idea that they are due to a single mechanism based on weakly interacting dark matter requires additional assumptions, foremost the assumption that all these effects have a single common explanation.

        Weakly i

        • by sFurbo (1361249)
          How is "dark matter" not neutral? All it says is that it has mass (is matter) and doesn't interact electromagnetically, which is indeed what we observe. Modifying the laws of gravity doesn't seem to cut it, so I fail to see what further assumptions "dark matter" imposes.
          • by stenvar (2789879)

            All it says is that it has mass (is matter) and doesn't interact electromagnetically, which is indeed what we observe. Modifying the laws of gravity doesn't seem to cut it, so I fail to see what further assumptions "dark matter" imposes.

            It presumes that we are talking about a single phenomenon and that our interpretation of our measurements is correct and consistent. I think it's more likely that "dark matter" as a single phenomenon doesn't even exist. A combination of modifying the laws of gravity, correct

            • by tnk1 (899206)

              Although you're technically correct that there has historically been more than one cause of missing matter, we've managed to locate and calculate the amounts of extra planets or dust or brown dwarves and such already and found that while they do exist, they are in amounts that only explain something like 10% of the missing matter we needed to find when we started looking for "dark matter".

              So yes, MACHOs are part of the equation for the original dark matter problem, and no doubt things like neutrinos and oth

              • by stenvar (2789879)

                First of all, we can't put useful bounds on rogue planets at all; they are very hard to detect. Second, your reasoning still assumes that there is a single mechanism and that galaxies are largely all the same. We don't even know the rotation curve of our own galaxy very well; our local observations might simply not apply to most other galaxies. And different galaxies may have different kinds of dark matter.

      • by Shavano (2541114)
        The problems with dark matter are on the "what is it" side. We can tell that it exists. What we can't tell is whether it's an effect of some particle we already know about or something else. Maybe it's extremely decelerated (barely moving) neutrinos left over from the condensation of matter in the big bang. That's presuming neutrinos have mass. Because if they're just about anything else that is known to exist, they'd be easier to observe. Or maybe it's that gravity ain't what we think it is. Maybe i
        • by sFurbo (1361249)

          Or maybe it's that gravity ain't what we think it is. Maybe it's NOT QUITE inverse-square, but the difference only becomes observable at interstellar distances.

          That would fail to explain thing like the bullet cluster and the fluctuations in the cosmic microwave background.

      • Until dark matter can be directly, or indirectly but consistently detected (e.g. we can take a bunch of dark matter and move it around, if it doesn't move it is a property of that particular region of space, not something contained in it), dark matter stays as an abstraction that helps our formulas to explain, pardon, model gravitational interactions.

        That is, now you can either consider it an as yet undetected physical object, or the rationalization of an error, as you prefer, and orient your own research a

        • by Bengie (1121981)

          Until dark matter can be directly, or indirectly but consistently detected (e.g. we can take a bunch of dark matter and move it around, if it doesn't move it is a property of that particular region of space, not something contained in it), dark matter stays as an abstraction that helps our formulas to explain, pardon, model gravitational interactions.

          Same thing can be said about gravity.

        • Why is it so hard to imagine that there is a particle that interacts with gravity but not electromagnetically? That's really what this comes down to.
          Remember you only touch that key on the keyboard because of photon interactions.
          Are you happy with the existence of Neutrinos? These particles that barely interact with normal matter or do you think they are purely there to balance formulas too? (okay that's why they were originally there but not anymore)

          • I have no issues in imagining that, the problem is indeed the opposite. If there COULD be a particle, everything is ok. If there MUST be a particle, there is lack of imagination. Sometimes that is good, speeds up the search for an explanation. Sometimes that makes the explanation difficult or impossible to reach. With neutrinos it was good.

    • The problem with dark matter observation in this case is that science is based on empirical observation. If you can't see it, can't measure it, and can't even draw inferences from what you can see and measure to detect something indirectly... it's not science.

      But we can measure it, and we do draw inferences from what has been measured, and that's exactly what they're doing here - using the measured large-scale behaviour of galaxies - from which we infer the existence of dark matter - to predict what might happen on a smaller scale, like a solar system - a scale on which we are currently not in the position to do observations of sufficient accuracy to disprove the inference (theory).

      As I understand it, there's a big empty space in most of our theories and observations that says something should be filling it up

      I wouldn't refer to problems in cosmological theories as "big empty spaces." That

    • by ceoyoyo (59147) on Sunday August 11, 2013 @07:27PM (#44538459)

      Dark matter, in various forms, is the hypothesis(ses) that explain empirical observations. For the last couple of decades we've been at the stage of hypothesizing various kinds of dark matter and testing them to see if they fit. The one that fits best so far, and is thus the leading contender, is a new kind of subatomic particle that interacts weakly and is fairly heavy. The dark matter story is an excellent example of how science is supposed to work.

      • by mmell (832646)
        Best concise explanation I've seen in a while. Nice job.

        So we're basically talking about the modern version of WIMPS, tweaked to account for the WIMPS we haven't found yet.

        While we're on the subject ... Higgs particles, Higgs field - sounds a lot like what they used to say gravitons were like (cue the bad Futurama jokes here...).

        • Gravity and the Higgs are actually unrelated. http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-higgs-faq-2-0/ is worth reading, and some of the other articles on there go into quite a bit more detail.
    • by Livius (318358)

      There are many good reasons to think that the dark matter hypothesis is on the right track. It provides many predictions that are borne out by observations, with a minimum of extrapolation from the properties of regular matter.

      It's still a hypothesis.

      There is no contradiction. I don't understand why people feel a need to disagree about it.

    • by dbIII (701233) on Sunday August 11, 2013 @08:46PM (#44538785)

      If you can't see it, can't measure it

      You've got a bit mixed up here. The entire idea of dark matter is because we can measure something we can't see - there are gravitational effects but not electromagnetic ones that have been seen yet.
      It's more like stepping on a black cat in the dark. You've felt it underfoot for an instant and it's run off somewhere, so while you don't know what it is or where it is you do have empirical evidence that you've stood on something.

    • It's like a shy cat in an apartment. You won't see that cat again, and an exhaustive search of most of the rooms in the apartment comes up empty, but something keeps eating the cat food. Thus, we have concluded there's a cat in the apartment... but nobody has actually ever seen the cat.

      Well, at this point, the cat food is being eaten, there is cat hair everywhere, the litter box is getting filled, and if somebody meows they will hear another meow come back in return from inside the wall. People can argue that it's not a cat, but whatever it is, it acts like a shy cat nobody has ever seen and any other explanations are a lot stranger than a cat that nobody has ever seen. Same thing for dark matter, either there is matter that only interacts via gravity, or there is something else out there

  • My bet is that the need for dark matter will disappear when relativistic effects are properly taken into account.

    There seems to be the belief among astrophysicists that general relativity can be safely ignored when speeds are low. I'm not so sure.

    Anyone that can integrate knows large values can be obtained when summing even the smallest values. Perhaps billions of otherwise ignorable relativistic effects become a large effect when acting together.

    • Re: (Score:2, Informative)

      by Anonymous Coward

      Rebuttal: Bullet cluster.

    • by wonkey_monkey (2592601) on Sunday August 11, 2013 @05:22PM (#44537703) Homepage

      My bet is that the need for dark matter will disappear when relativistic effects are properly taken into account.

      And I bet that at some point during the last few decades of thousands of observations, theories, and calculations by thousands of astronomers, physicists, and mathematicians (some with Nobel prizes, no less), someone would have already thought of this if it was an issue.

      • by mc6809e (214243)

        And I bet that at some point during the last few decades of thousands of observations, theories, and calculations by thousands of astronomers, physicists, and mathematicians (some with Nobel prizes, no less), someone would have already thought of this if it was an issue.

        They're not gods (and there's no Nobel prize for mathematics).

        And there has been some movement towards using relativity instead of dark matter to explain galactic rotation curves:

        General Relativity Resolves Galactic Rotation Without Exotic D [arxiv.org]

        • That paper's from 2005, but as they're still talking about dark matter I'm guessing it didn't satisfy many scientists.

          They're not gods

          No, but I think it's reasonable to assume they're a bit ahead of the average Slashdot poster in this department.

          (and there's no Nobel prize for mathematics).

          Two ways out of that one:

          a) as it was meant to be read:

          by thousands of [ astronomers, physicists, and mathematicians ] (some with Nobel prizes, no less)

          b) I didn't say what they won the Nobel prizes for. Mathematicians have won Nobel prizes.

        • by Bengie (1121981)
          That still does not explain empty space having detectable mass in the form of gravitational lensing.
      • by Chemisor (97276)

        > someone would have already thought of this if it was an issue.

        Then please tell me who has already thought of explaining the expansion of the universe by considering the matter-to-energy conversion occuring within stars and realizing that the disappearing matter reduces space curvature, expanding it. Accelerating star formation and total power output would thus produce accelerating expansion of the universe. Do try to find any astrophysicists who has done these calculations. I'd be very interested to re

        • Then please tell me who has already thought of explaining the expansion of the universe by considering the matter-to-energy conversion occuring within stars and realizing that the disappearing matter reduces space curvature, expanding it.

          Well, you have, so why don't you do the calculations, write a paper, and win a Nobel prize? From my point of view (that of not being an astrophysicist and only have an interested reader's grasp of the subject) you're the one making the claim, so it's on you to find the evidence.

          Besides which, I was under the impression that energy warps space just as mass does (though perhaps you're referring to mass/energy being lost to the space around the star by radiation).

          I was also not aware that if you remove mas

        • Then please tell me who has already thought of explaining the expansion of the universe by considering the matter-to-energy conversion occuring within stars and realizing that the disappearing matter reduces space curvature, expanding it. Accelerating star formation and total power output would thus produce accelerating expansion of the universe. Do try to find any astrophysicists who has done these calculations. I'd be very interested to read their papers.

          E=MC2 is an equivalence, not a transformation.

        • You're making a classic mistake there. When matter gets converted into energy gravitation doesn't care. It cares about the total mass-energy. Which doesn't change. So while the emitted photons do not have mass they do have energy which of course has a mass equivalent.
          In a closed system* even if there are nuclear reactions taking place the mass-energy of the system does not change.
          *to actually do this you'd have to contain all the mass and photons and neutrinos, which we don't know how to do, but the point s

  • It can't collide with itself? Good to know that the Majorana versus Dirac particle question is settled then. Oh, wait...

    Of course, I didn't bother to RTFA...
  • by blincoln (592401) on Sunday August 11, 2013 @06:04PM (#44537977) Homepage Journal

    This is probably a dumb question, but I've been wondering about it for something like a decade, and I never see it referenced (even to debunk it) in legitimate science discussions.

    A mysterious effect which looks like matter, but is invisible except for its gravitational effect. A second mysterious effect which causes the rate-of-expansion of the universe to increase.

    I grow more and more skeptical of string theory and its relations every year, but the first of those definitely sounds to me like matter that's in another brane. The second one seems (to my non-physicist mind) like it could also be explained by the same thing, just a different set of matter in a different position relative to the first.

    If our universe really is a 3D brane in a hyperdimensional space with others, isn't this exactly the sort of thing we'd expect to see? Further, wouldn't we see related effects like neutron stars unexpectedly flashing into black holes when they come into close-enough contact with dense clumps of matter in adjacent branes (IOW, when there's not enough observed mass in our own to explain the change to a black hole)?

  • "a diffuse mess, with a density that is ridiculously low, to the point where detecting its local effects is likely to remain... challenging for the foreseeable future"

    In other word something irrelevant we can safely ignore and not invest any more money or resources into.

    • Understanding the universe has been a good investment in the past. There is good evidence for something that behaves like dark matter on galactic scales. If it isn't dark matter it might be something more interesting.

  • This is a sig I've seen someone use on /. the article says to me dark matter was here, then nothing exploded.

    The 2dF Galaxy Redshift Survey http://scienceblogs.com/startswithabang/files/2013/07/kozm_LSS.jpg [scienceblogs.com]
    shows stuff coming towards us. I've heard so many space programs say everywhere you look everything is moving away from us,

  • This may or may not relate to dark matter, but the other day, an electrical storm was passing over my house, and I momentarily saw a dark spot on the wall. Is there a scientific explanation for such phenomena? I've never had visual disturbances like that otherwise.

  • So, it can't collide with itself, meaning that it has no way of getting hotter[...]

    Wouldn't gravitational interactions count as "colliding", at least for the purpose of exchanging energy? And the fact that it can't radiate away energy has more to do with it not interacting electromagnetically than its ability to collide with itself, hasn't it?

  • Am I the only one who thinks about the Pioneer anomaly after reading this article?
    If there is a constant density of dark matter in the solar system it will have a too small effect on Neptune to be detected. When moving further away from the sun the effect of the dark matter becomes stronger. So if we want to detect its gravitation we have to go as far away from the sun as possible. We should see something like the Pioneer anomaly, but we probably have to send a probe much farther out to detect any effect.

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