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Dark matter becomes less 'ghostly'
- mherrmann 12y agoAs someone with little background in physics I've always found dark matter to be a very inelegant concept. To introduce a new kind of "matter" which can hardly be observed sounds very arcane to me. It reminds me of the Luminiferous aether which was postulated as the medium in which light travels in the late 19th century. I'm looking forward to new theories that can explain what we now observe as "dark matter" without having to invent a new kind of matter specifically for it!
- rotorblade 12y agoThe main subject for such an approach is 'Modified Gravity', where one in different ways would modify gravity to reproduce 'dark-matter' effects and still reproduce the same results as GR has.
- burke 12y agoYou're right, of course. The problem is that inflationary cosmology did a phenomenally great job of explaining so many things about the universe, with testable and subsequently verified predictions (detailed predictions of patterns in the CMBR IIRC). The only catch is that it predicts the total mass/energy content of the universe to be about 5ish times what we actually observe in the form of matter interacting with electromagnetic force.
- inclemnet 12y agoAn important fact about the luminiferous aether is that it was abandoned as an idea because it was inconsistent with experiments. In contrast, dark matter has continued as an idea and become a popular explanation because new experiments have always fit very well with what it would predict. We don't know what dark matter is, or if it even is a kind of matter rather than e.g. modified gravity (though modified gravity has many problems of its own), but I think it is a common mistake that people are so eager to associate it with aether on the grounds only of what they find 'elegant'. Edit: Also, the notion of a kind of matter that can hardly be observed is not particularly weird or 'arcane'. For instance, neutrinos have many of the right properties. I think they are ruled out as dark matter candidates mainly because their mass is too low to fit with observations.
- cozzyd 12y agoStandard model neutrinos are ruled out because they are relativistic and cannot play a role in structure formation (see http://en.wikipedia.org/wiki/Hot_dark_matter http://en.wikipedia.org/wiki/Hot_dark_matter )
- Dylan16807 12y ago>An important fact about the luminiferous aether is that it was abandoned as an idea because it was inconsistent with experiments. Is it? As I understand it the concept of an aether is not so much provably wrong as utterly meaningless; once you factor in various dilations there is no effect that would be caused by an aether. Nothing could possibly measure it, and it has no impact on predictions.
- marcosdumay 12y agoThe Aether was proved wrong, and Special Relativity (that makes different previsions) was only developed after that.
- JoeAltmaier 12y agoI understand it was simply proved irrelevant - it cancelled out of the equations. Until recently, when Maxwells Equations were re-derived using some quantum field as the Aether, which yielded many surprising (and true) results. So maybe now, not so irrelevant.
- qwerty33467 12y agoCan you give a citation for the Maxwell Eq. aether paper?
- JoeAltmaier 12y agoFudge. It was an article listed here. I'm paging down, not finding it.
- eloff 12y agoIt's quite inelegant, but physicists have eliminated most of the other sensible explanations. Remember, there's no law in the universe that us humans have to like the underlying physical laws of the universe. Frankly it's amazing that they have as much elegance and mathematical beauty as they do.
- TeMPOraL 12y ago> Frankly it's amazing that they have as much elegance and mathematical beauty as they do. I'm not surprised; humans always keep searching for more elegant models. There seems to be something in the brain that associates positive emotion with models that compress information well.
- astrobe_ 12y agoHowever, using an heliocentric reference to describe the movement of planets is both much more "elegant" and "right" than an earth-centric reference. It suggests me that nature is relatively "simple", and that excessively complicated models are "wrong" or at least suspicious.
- jarin 12y agoIt's also not completely accurate. The Sun is not the center of the planets' orbits, but it is one focus of their orbital ellipses. Also, using the Sun's reference frame is only an approximation due to forces from other stars, the galaxy, etc.
- cozzyd 12y agoThe entire development of particle physics has been rather inelegant. Only later has it become a (sort of) coherent framework. For example, take a look at http://en.wikipedia.org/wiki/Muon#History http://en.wikipedia.org/wiki/Muon#History
- PhantomGremlin 12y agoI also find dark matter to be "inelegant". A slashdot quip from a while ago summarized it for me: Dark Matter is the Phlogiston of Contemporary Cosmology Of course, just because I find it inelegant doesn't mean it can't exist. The predictions of quantum mechanics, verified by the behavior of the billions of transistors in the very computer I'm typing this on, are way "crazier" than what is postulated for dark matter. I'm looking forward to new theories that can explain what we now observe as "dark matter" without having to invent a new kind of matter specifically for it! Carver Mead is a very famous guy who also wants to shake things up[1]. Maybe he's turned into a crank in his old age, but, to steal from Ghostbusters: "the usual stuff isn't working". [1] http://www.theregister.co.uk/2013/02/20/carver_mead_on_the_future_of_science/ http://www.theregister.co.uk/2013/02/20/carver_mead_on_the_f...
- motoboi 12y agoAbout elegance (or the lack of it) as requisite to a particles model, you may find interesting the book Imperfect Creation, by Marcelo Gleiser.
- phkahler 12y agoAm I right in thinking you're being deliberately kind with your use of the word "inelegant"? I usually get a downvote when the subject comes up because I can't hold back. ;-)
- coliveira 12y agoIt is normal in physics to use temporary labels for things that are not known. Dark matter is labeled as that because it has properties similar to visible matter. Once we learn more about it, a better name will probably be found, similarly to how the ether concept was replaced by relativity theory.
- speakeron 12y agoThere are several strands of evidence which lead to the belief in the existence of dark matter. It's not just observations of the dynamics of galaxies (which is probably what most people have heard of), but also calculation of the amount of baryonic matter made in the big bang. So alternative theories like MOND may explain galaxies but have nothing to say about the big bang. The potential existence of dark matter candidates, which some suspect to be sparticles predicted by Supersymmetry (maybe the LHC throws something up there or, as always, it's just the next collider away), is another reason to pursue dark matter ideas.
- andrewflnr 12y agoThat's what they thought about neutrinos at first, too. At first they were just a kludge to make (IIRC) lepton or spin conservation hold during alpha decay. Then they actually found the things.
- phkahler 12y agoWhat is this: According to widely accepted ideas, the visible matter in galaxies exists inside clumps of dark matter. Without dark matter's gravity to stabilise them, galaxies like the Milky Way would tear themselves apart as they spin. Is this the tired old "galactic rotation curves don't fit kepler's law" still kicking around?
- cousin_it 12y agoI thought that was actually true. Why are you calling it tired and old?
- phkahler 12y ago>> I thought that was actually true. Why are you calling it tired and old? Because there is no reason to expect stars going around a galaxy to follow keplers laws for two body systems. I have yet to see the measured rotation curves compared to an "expected" curve where the author doesn't use Kepler as part of the expectation. Just a year or two ago, I saw a paper in Nature where they were looking at a gas cloud forming a star, and they compared its rotation to something based on Kepler. I just had to shake my head - the correlation was there but not a great fit of course. The curve is interesting, the comparison not so much.
- blackhaz 12y agoWhy wouldn't they follow a Keplerian orbit?
- tim333 12y agoKepler's laws can be a pretty good approximation to reality and probably good enough for figuring if the galaxy contains unseen matter.
- rotorblade 12y ago> Because there is no reason to expect stars going around a galaxy to follow keplers laws for two body systems. The 'Kepler' law for galaxies are modified to accommodate for a mass distribution. See e.g.: https://en.wikipedia.org/wiki/Galaxy_rotation_curve#Halo_density_profiles https://en.wikipedia.org/wiki/Galaxy_rotation_curve#Halo_den...
- guelo 12y ago> Dr Thomas Kitching, a co-author of the study from University College London, said ... "It's too early to say if this is a dark matter effect, or caused by normal astrophysical processes. What we need to do now, is make simulations of these collisions to distinguish these possibilities."
- thom 12y agoThere was a recent episode of BBC radio's In Our Time covering dark matter, which was useful to someone uninitiated like me: http://www.bbc.co.uk/programmes/b054t3s2 http://www.bbc.co.uk/programmes/b054t3s2