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Space Science

New Gravity Theory Dispenses with Dark Matter 442

Posted by ScuttleMonkey from the making-quantum-theorists-happy dept.
Darkness Matters writes "According to New Scientist, a theory of modified gravity, which has no need of dark matter, has just explained why the Pioneer 10 probe is 400,000 miles off its expected course as it leaves the solar system. It sounds pretty convincing, although in dispensing with dark matter, they've had to utilize the theoretical particle, called a graviton, which appears from the vacuum of space wherever stars are densely packed, making gravity stronger."
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New Gravity Theory Dispenses with Dark Matter More

New Gravity Theory Dispenses with Dark Matter

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  • Vast improvement (Score:5, Funny)

    by Lonesome Squash ( 676652 ) on Wednesday January 25, 2006 @10:15AM (#14557650)
    So we finally get rid of this ad-hoc and hypothetical construct by replacing it with... oh.

    "See I told you guys it wasn't flying monkeys! Turns out it's flying Unicorns!"

  • so... (Score:3, Funny)

    by Albert Sandberg ( 315235 ) on Wednesday January 25, 2006 @10:15AM (#14557660) Homepage
    you're telling me dark matter doesn't matter?

  • at first I saw... (Score:4, Funny)

    by holySherm ( 916265 ) on Wednesday January 25, 2006 @10:17AM (#14557677)
    GRAVITRON! http://en.wikipedia.org/wiki/Gravitron [wikipedia.org] Damn that was a horrible ride.

  • My invisible friend (Score:5, Funny)

    by blowdart ( 31458 ) on Wednesday January 25, 2006 @10:18AM (#14557685) Homepage
    You know as a child I made up invisible things to blame and was told that was a bad thing to do. Only now I find out I was really a budding scientist...

    • Re:My invisible friend (Score:5, Funny)

      by meringuoid ( 568297 ) on Wednesday January 25, 2006 @10:40AM (#14557936)
      You know as a child I made up invisible things to blame and was told that was a bad thing to do. Only now I find out I was really a budding scientist...

      I suspect your invisible things weren't of the 'no, really, hang around and watch, you'll see, it's about to do it again!' kind. More of the 'uh... well, it went away when you came in. It's only around when you're not looking. And it knocks things over when I'm the only one in the room' variety, if you were a kid making excuses.

      That, I think, makes you a budding theologian :)

  • Gravitons are not a new concept (Score:4, Insightful)

    by csoto ( 220540 ) on Wednesday January 25, 2006 @10:18AM (#14557687)
    This has been around for years. If a hypothesis involving gravitons is explained by experimental evidence, then this hypothesis could be elevated to theory.

    Besides, didn't we use to shoot gravitons at that loud squiggly thing in Yar's Revenge?
    • Besides, didn't we use to shoot gravitons at that loud squiggly thing in Yar's Revenge?

      No! We shot gravitons at the base station. The loud squiggly thing (shot out from the base station) was a spiraling vortex of red death! Flee! Flee!
    • If a hypothesis involving gravitons is explained by experimental evidence, then this hypothesis could be elevated to theory.

      However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?

      Until that question is answered, the graviton theory is going nowhere, fast. :-/

      • Re:Gravitons are not a new concept (Score:5, Insightful)

        by MustardMan ( 52102 ) on Wednesday January 25, 2006 @10:35AM (#14557879)
        However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?

        There could be lots of reasons for this. The mechanism certainly isn't the same, but as an example of a particle which cannot be shielded, you need look no further than a neutrino, which can pass through the entire Earth.
        • It's been a while since I studied physics but, as I recall, neutrinos can not pass through Fermions. The reason that they can travel through the Earth is that the vast majority of Earth is made of nothing, with the occasional (very small) cluster of Fermions floating around in the form of atoms. A neutrino will not pass through an atomic nucleus, but the chances of it hitting one on the way through the Earth are very low. This can be increased by creating macro-nuclear structures - a lump of neutronium,

        • Re:Gravitons are not a new concept (Score:5, Funny)

          by CagedBear ( 902435 ) on Wednesday January 25, 2006 @11:36AM (#14558652)
          Sure, they can pass through Earth, but I'll be really impressed when they can pass through New Jersey without paying a toll.
        • The problem, however, is that Neutrinos don't interact with the Earth as they pass through. If they interact, then they are being shielded against. As TheRaven64 pointed out, methods do exist for shielding against neutrinos, even if such methods are only partly successful.

          If it was not possible to shield against gravitons, then how do they interact with bodies such as the Earth? If they do interact with objects, and it is possible to shield against them, then why doesn't placing one object in front of anoth
          • then why doesn't placing one object in front of another reduce the influence of the body that the second object is shielding against?

            Because gravitons don't "push", they only "pull". The only reason we can shield against, say, an electron, is because the electric field can "push" as well as "pull".

            Since a gravitational interaction is always purely attractive, any material you put in the middle, to first order, doesn't have any effect. If you've got particle A, B, and C, if A exchanges with C (A==C), and the
      • If gravity is manifest as a particle, why can't we shield against it?

        We can hardly shield against neutrinos, right?

      • Pick your own snark:
        A) How do you think UFOs work?
        B) Ironically, any substance that blocks gravitons would be so dense that it, itself, would be a source of gravitons.
        C) Could God create a source of gravity so strong that even God could not escape it?
      • I'd guess it due to equalibrium sort of like heating a block of steel to 1200 C, the exposing it to photons from a blackbody source at 1200 C it produces no net effect every photon absorbed stimulates a photn emmision.
        My question is if gavitons are particles as well as waves, these should unify which strong, weak, and electromagnetic; and should have equivilent properties. I wonder what the gravitional equivilents to heat and temperature are? Heat and mass should have equivelency; but it seems that matter a
      • Oh I think it is coming together together quite well. Attractive theory isn't it?

      • Re:Gravitons are not a new concept (Score:4, Informative)

        by barawn ( 25691 ) on Wednesday January 25, 2006 @12:15PM (#14559180) Homepage
        However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?

        You typically think about stopping "particles" with other particles - like a wall. That's a very classical idea. But in order for a particle to stop at a wall, it needs to interact with that wall - in fact, it needs to either be "absorbed" by the wall, or it needs to be totally deflected by the wall. But in either case, it needs to interact with the wall.

        Neutrinos, for instance, don't interact with much, since they only interact via the weak interaction. So we really can't shield from neutrinos that well, although you could build weird gadolinium-doped materials which would probably cut down on the flux of neutrinos more than others. Thankfully, neutrinos interact just like normal matter when they *do* interact, and so you could conceivably shield against them - just not easily.

        As for gravitons, though, the situation changes - now you have to ask "can we build a material that interacts with gravitons?" Well, yes - all matter does. But annoyingly, that material itself would produce gravitons as well, and in terms of the SVTG theory, it sounds like it "conducts" them through, too. It's a lot like magnetic shielding - putting a material that interacts with magnetic fields isn't enough to shield a field from you. You need a high-permeability material - that is, one that makes it easier for a magnetic field to flow around you rather than through you.

        You can even realize this based on the spin that the graviton has: a graviton would be a spin-2 particle, and any interaction with a spin-2 particle as its mediator must be an attractive potential. Without the possibility of repelling a graviton, you can see that you can't build a shield.

        Note that we don't have any fundamental spin-2 particles other than a graviton, so it's understandable that naive ideas don't work.

      • Re:Gravitons are not a new concept (Score:5, Informative)

        by radtea ( 464814 ) on Wednesday January 25, 2006 @01:45PM (#14560411)
        However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?

        Because gravitons are spin 2. There's no nice "intuitive" way of explaining it, but within the mathematical framework of modern quantum field theory spin 2 particles always produce an attractive force between things that those particles interact with. This means that there is no possibility of "anti-gravity", which is what is required to shield against gravity. Electromagentic shielding is possible because photons, being spin 1, can produce either attractive or repulsive forces. This is how it comes to be that there are two electrical charges of opposite sign, whose differential displacements in matter allow us to create electromagentic sheilding. The spin-2 nature of gravitons means there is only one "gravitational" charge, called "mass".

        The most one would be able to do with gravity is cancel out gravity waves in a small region by generating out-of-phase waves. The static field cannot be cancelled in this manner, so there is no way of shielding it.

        All of this, of course, depends on the imperfect mathematics of quantum field theory, which may or may not be an accurate description of the universe.

    • Re:Gravitons are not a new concept (Score:4, Insightful)

      by G3ckoG33k ( 647276 ) on Wednesday January 25, 2006 @10:24AM (#14557759)
      Well, as the article states "it has yet to pass the most crucial test - how to account for the afterglow of the big bang".

      So, it won't be the theory you are after until some more time.

    • If a hypothesis involving gravitons is explained by experimental evidence, then this hypothesis could be elevated to theory.

      There's no such thing as being "elevated to theory".

      A hypothesis is just a proposed explanation for some phenomenon. A theory is just a bunch of them taken together because it seems to makes sense to do so (e.g., we group a bunch of hypotheses into something we call the "theory of evolution", and another bunch into a "theory of gravity", because it doesn't make sense to make a "the

  • Please be real! :D (Score:3, Insightful)

    by SalsaDoom ( 14830 ) on Wednesday January 25, 2006 @10:18AM (#14557691) Journal
    Man, I hope this is real so much. I've always hated dark matter. You know what dark matter reminds me of? Aether. The whole idea of dark matter reminds me of a stupid hack -- which I suppose you have to deal with when it comes to topics like physics now and then.. its not like we can just go and look so readily ;P

    Anyway, these "Gavitons".. I think I've had them in computer games for a while now, its about time we 'discovered' them. Aethe-- I mean Dark Matter was such a cranks idea anyway... anything has to be better then "OOoooh! There must be... some.. uh, invisible undetectable matter.. that uh, has mass. But you can't see it, because.. its dark! yeah thats the ticket." Given an unlimited choice of possibilities I could have came up with something better, and it probably would have been about as scientifically valid too. :)

    Hurray for gavitons! Prepare the graviton pulse cannons! :D
    --SD
    • "I've always hated dark matter."
      And emotion has anything to do with science? I understand your distaste for it, but you know, people hated that the earth wasn't flat, that the clouds didn't really obscure the angels, that the earth really is more than 6,000 some years old (even today you get arguments against that), etc., etc. That doesn't mean it's not real. I'm going to wait until the evidence comes in before I make a decision.

  • Sure, it's off course (Score:5, Funny)

    by darthservo ( 942083 ) on Wednesday January 25, 2006 @10:20AM (#14557702)
    If you get Gravitrons [wikipedia.org] spinning fast enough, you can unpredictably launch things anywhere, especially smaller objects such as probes or children.

    Oh...graviton.

  • New Theory (Score:2, Funny)

    by 3CRanch ( 804861 )
    Its funny how easy it is to explain something when you get to come up with your own theoretical items.

    Lets see...how 'bout its off course cause the Universe has shifted due to a USABLE poll on Slashdot...
    • Its funny how easy it is to explain something when you get to come up with your own theoretical items.

      Yes, well, that's true, but at the same time that's how the atomic and quantum theories got started too.

    • Its funny how easy it is to explain something when you get to come up with your own theoretical items.

      No it's not. The MOND researchers had to find the simplest model they could that fits observation without hypothesising dark matter but instead making small tweaks to known existing laws. In general this is a far from trivial task and checking to see if your ideas fit the data is pretty laborious.And the MOND researchers haven't been working by some special rules saying that they are allowed to make up "the

  • Well, at least we don't have to worry about Dark Matter critters lurking in deep space.
    http://www.schlockmercenary.com/d/20040311.html [schlockmercenary.com]

  • scalar-tensor-vector gravity ? (Score:3, Funny)

    by Black Parrot ( 19622 ) * on Wednesday January 25, 2006 @10:26AM (#14557777)
    Couldn't they make up their mind?

  • "Hypothetical particle" (Score:5, Informative)

    by ChowRiit ( 939581 ) on Wednesday January 25, 2006 @10:26AM (#14557779)
    While the graviton has never been observered, it's not as wildly unlikely as that article seems to suggest. Of the four fundemental forces (strong/weak nuclear forces, E/M and gravity) only gravity hasn't had a "force mediator" particle (one which "carries" the force, for example photons for electricity and magnetism) observed in lab experiments. However, as gravity is the weakest force (by an order of I believe around 10^-28 times, or similar), this is not unlikely. However, it is extremely unlikely there ISN'T a particle which mediates gravity, ergo the (pretty reasonable) assumption of the existance of the graviton. Assuming it exists, further things can be predicted about it from other laws of Physics, hence we have a particle we've never seen but are pretty sure exists, with certain properties.

    • Re:"Hypothetical particle" (Score:5, Informative)

      by Billosaur ( 927319 ) * <wgrother AT optonline DOT net> on Wednesday January 25, 2006 @10:54AM (#14558115) Journal
      Gravitons [bookrags.com] are supposed to be the exchange particle [gsu.edu] for gravitation, as the photon is for electromagnetic force. The graviton and photon are chargeless, massless particles, differentiated by their spin. The strong and weak nuclear forces are also mediated by exchange particles (W & Z for the weak, Gluons for strong quark interactions, Pions for strong nucleon interactions). The graviton's importance is in serving as a moderator of the gravitational force; if this theory is correct, then it won't be long before someone will come up with an idea for exposing the graviton to the light of day.
    • My most frustrating questions in High School physics was "How does gravity work"?

      It seems we don't really know much more now than we did way back when.
    • Photons emitted (as from the Sun) which hit another piece of matter (such as a light sail) PUSH on the struck matter due to the transfer of momentum. For a particle to hit (or go through) a mass and cause the mass to go TOWARD the direction the particle came from, it would have to have NEGATIVE momentum.

      What's even more problematic is that unlike any other particle I can think of, the graviton has its effect on matter, but isn't itself affected as it passes through. When Pluto is eclipsed by Jupiter (surely

  • Some clarification (Score:5, Informative)

    by kebes ( 861706 ) on Wednesday January 25, 2006 @10:29AM (#14557811) Journal
    The way the post is worded, I think there is some misunderstanding what a "graviton" is. The graviton is the force-carrying particle of gravity, in a similar way to the photon being the force-carrying particle for electromagnetic phenomena. Although the graviton has not yet been directly observed, there is little doubt among physicists that it does exist. The current best theories we have (standard model of particle physics, etc.) strongly suggest that it exists.

    The post makes it sound like suggesting that gravitons exist is outlandish... but this is rather accepted. Instead, it seems that their theory is a particular attempt to quantize gravity (there have been many attempts over time, with all ultimately being unsatisfactory). Whether or not their new theory is useful remains to be seen.

    Also, in TFA, they say: "In this case, a hypothetical particle called a graviton - which mediates gravity - appears in large numbers out of the vacuum of space in regions crowded with massive objects such as stars." Again, it is generally accepted that in any reasonable theory of quantum gravity, gravitons will be the force-carrying particle for gravity. Where there is a large gravitational field, virtual gravitons will be exchanged to mediate the force (more info on virtual particles [wikipedia.org]). This is nothing new. And in particle physics, virtual particles can always appear and disappear from the vacuum.

    So again, I think we can't coment much on this theory without reading the actual paper (anyone have a link?). I would like to understand what is actually novel about their formulation. Also, they are not the first to try and reformulate the basic laws of gravity to get rid of the "dark matter anomaly" and none have been found to be consistent with all the experimental data.

    • The post makes it sound like suggesting that gravitons exist is outlandish... but this is rather accepted.

      IANAP [or a Cosmologist], but Pioneer 10 is pretty damned far out there at this point. So far, in fact, that it must take, what - several hours? several days? - for something travelling very, very fast [as in "The Speed of Light"] to get from here [that big fat gravity source called "our sun"] to there [the Kuiper belt, or wherever the hell Pioneer 10 finds itself these days].

      Is not one of the big

      • Re:Uhh - Action at a Distance? (Score:5, Informative)

        by kebes ( 861706 ) on Wednesday January 25, 2006 @11:07AM (#14558273) Journal
        I'm not an expert in particle physics either, but here's what I know:

        Is not one of the big problems with "gravitons" that gravity appears to act more or less instantaneously at great distances? And isn't that a little troubling from the "Action at a Distance is Big No-No" point of view?

        No, according to the theories gravitons would travel at the speed of light. In fact, bear in mind that the exchange of virtual particles is what prevents "action at a distance", if you like. Instead of gravity (or magnetism) having an effect "just because", the theory explains that it is because virtual particles are flying back and forth between the two objects in question. In the case of gravity, it is virtual gravitons, and in the case of magnetism, it is virtual photons. Both travel at the speed of light, which explains why force effects (like gravity and magnetic fields) are not instantaneous: they propagate at the speed of light (this has been measured and is not in dispute).

        Pioneer 10 is pretty damned far out there at this point.

        Apparently Pioneer 10 is 89 AU from the sun [nasa.gov]. 90 Astronomical Units is 12 light-hours [google.ca]. Still, your point is well-taken... gravity operates over distances of millions and even billions of light-years... so how can these "virtual gravitons" cover such distances? After all, supposedly virtual particles exist only for a short time!

        My apologies to the hard-core particle physicists for this simplistic explanation, but here goes: When you look at the Heisenberg Indeterminacy Principle [wikipedia.org], you find that there is a relation between space and momentum. We all know the famous "the more accurately you localize a particle, the more spread out its velocity is"... it turns out that this implies a similar relation for energy and time. What it means is that high-energy particles can "pop into existence" for very short periods of time... but low-energy particles could exist for longer times. This is what allows virtual particles to do their thing. Very strong forces (nuclear forces and electromagnetic) involve high-energy virtual particles, which can only travel short distances before "disappearing"... that's why those forces operate over short distances.

        But gravity is very very weak (by comparison). So that means that a virtual graviton can pop into existence, and travel for a long distance and time (millions of years) before disappearing. That's what, in fact, causes gravity to operate over such vast distances. So in fact the distance-scales and force intensities are intrinsicaly related in quantum treatments. So "a short time" means something different for EM-forces and gravity-forces.

        I hope this (simplistic) explanation is somewhat useful to someone.
        • That's a neat explanation, thanks. I didn't realise the relationship between the 'strength' of a force, and the distance over which it can act.

          Does this mean that there is an absolute maximum distance that a virtual particle can travel? So if I send out a beam of perfectly coherent light, my friend at the other edge of the universe* would never recieve it?

          I always assumed that, say, the strong nuclear force did exist past its short range, it just wasn't strong enough to hold the protons in a nucleus togethe
          • Note: some of the replies to my posts have fixed mistakes I made in the explanations. For instance, it is more correct to specify that the maximum range over which a force acts has to do with the mass of the particle (which is related to particle energy, of course). Massless particles can operate over (in principle) infinite distances, so there is no upper bound to EM and gravity forces... however the "perceived distance" over which a force acts also has to do with how quickly it decays, and I believe this
          • Does this mean that there is an absolute maximum distance that a virtual particle can travel? So if I send out a beam of perfectly coherent light, my friend at the other edge of the universe* would never recieve it?

            The photons from your light beam are real, not virtual. There is no limit to how long they can exist.

      • Is not one of the big problems with "gravitons" that gravity appears to act more or less instantaneously at great distances? And isn't that a little troubling from the "Action at a Distance is Big No-No" point of view?

        There may be ways to determine whether the effect of gravity moves instantaneously, or only at the speed of light, but it seems gravity is too weak for humankind to (currently) make an experiment to detect it.. I can't imagine measuring the speed of gravity without carefully orchestrated black
      • That leads to the question I've been asking my science teachers for decades: if the sun were to suddenly disappear (hypothetically, of course), would the earth continue to follow its orbit for 8 minutes, or would it immediately go flinging off into space?

    • links to paper... (Score:5, Informative)

      by kebes ( 861706 ) on Wednesday January 25, 2006 @10:52AM (#14558092) Journal
      Sorry to reply to my own post, but here are, possibly, the scientific papers in question. Doing a search on arXiv for the names of the authors (Joel Brownstein John Moffat) [arxiv.org] provides a paper entitled "Gravitational solution to the Pioneer 10/11 anomaly" [arxiv.org] (warning: PDF).

      I'm not an expert in gravitational theory, so I would appreciate others correcting any mistakes I make. The abstract to the paper says: "The theory allows for a variation with distance scales of the gravitational constant G, the fifth force skew symmetric field coupling strength (omega) and the mass of the skew symmetric field = 1/(lambda)."

      I think this is quite a departure from what is conventional accepted about gravity. The gravitational constant, G, [wikipedia.org] sets the scale for the force of interaction of gravity. It is normally assumed that this value is constant throughout the entire universe. They seem to be allowing that this value changes with distance, so that the interaction of gravity is different at small and large length-scales. That they are able to come up with a fit to actual experimental data is quite amazing... although so many bits of astronomical data have been computed assuming a particular (and constant) value of G, so to compare with "established facts" they will have to reconsider all of these previous calculations.

      • Re:links to paper... (Score:4, Informative)

        by barawn ( 25691 ) on Wednesday January 25, 2006 @03:07PM (#14561280) Homepage
        I think this is quite a departure from what is conventional accepted about gravity. The gravitational constant, G, sets the scale for the force of interaction of gravity. It is normally assumed that this value is constant throughout the entire universe.

        This isn't a new idea. This idea has been around a long time - it comes from Mach's principle [wikipedia.org] (yes, that Mach) which essentially states that the inertial mass of an object only means anything in context of other objects. Taken with the equivalence principle, this means that gravity depends on the spatial distribution around it.

        General relativity does not satisfy Mach's principle - you can create an "empty Universe" which solves Einstein's equations. There have been several modifications to GR to try to satisfy Mach's principle. The simplest one is Brans-Dicke [wikipedia.org] theory, which does exactly what SVTG does - allows G to vary. Brans-Dicke theory is essentially identical to general relativity if the coupling between the scalar field and the tensor field (gravity) becomes infinitely weak. Sounds like SVTG is an extension of Brans-Dicke, allowing a scalar, vector, and tensor component for gravity.

  • Testable with planetary motion? (Score:4, Insightful)

    by G4from128k ( 686170 ) on Wednesday January 25, 2006 @10:30AM (#14557822)
    If gravity isn't quite what it seems to be in terms of strength versus distance, then studies of planetary systems should show the effect. The relationship between orbital radii and orbital periods (and orbital path) would not be quite consistent with the 1/r^2 rule for Newtonian gravity. Admittedly the distance and mass scales of a our planetary system are far smaller than the galactic scales discussed in the theory, but our ability to make extremely precise measurements of planetary distances and orbits should compensate for that.

  • Dark Matter Is Chuck Norris (Score:5, Funny)

    by Himring ( 646324 ) on Wednesday January 25, 2006 @10:33AM (#14557851) Homepage Journal
    Scientists have actually calculated that Dark Matter is, in actuality, Chuck Norris. He recently flew to the west coast, and this threw off the Pioneer 10 probe by 400,000 miles.

    Scientists have also not yet revealed the real reason behind the ban on human cloning. The real reason human cloning is outlawed is because scientists fear Chuck Norris being cloned. They theorize that two simultaneous Chuck Norris roundhouse kicks could possibly destroy the universe....

  • Nibbler (Score:5, Funny)

    by rharder ( 218037 ) on Wednesday January 25, 2006 @10:36AM (#14557886) Homepage
    If there's no dark matter, what's that stuff coming out of Nibbler?

  • Here we go again (Score:5, Informative)

    by Anonymous Coward on Wednesday January 25, 2006 @10:37AM (#14557902)
    Now we're going to see the inevitable parade of, "See, I knew dark matter was a stupid idea all along".

    First off, dark matter, even if it ultimately turns out to be wrong, is not a stupid idea; it explains a wide variety of independent phenomena [slashdot.org] (and contrary to the eternal "it's just epicycles" cry among Slashdotters, it is testable, falsifiable, and predictive).

    Second, this new work is, well, new. Only one [arxiv.org] of the three papers (other two: here [arxiv.org], here [arxiv.org]) has passed peer review so far. When a theory like dark matter has amassed evidence in its favor over a period of decades, it takes a lot to overturn it.

    Even if their STV theory does ultimately pan out (and there have been many alternate proposals in the past that have ended up failing), it will take years to be hashed out in the literature and subjected to far more tests; so far they have only passed a few of the observational tests that dark matter does, even assuming that their papers are correct, which no one has checked — there are no followup studies by other authors at this point.

    Basic lesson: for every revolutionary new theory that works, there are a hundred that don't, and it can take a long time to decide which is which. New Scientist is not doing anyone a service by jumping on the latest unpublished preprint of the month and hyping it as the revolution of the century, as they tend to do.

    • Re:Here we go again (Score:4, Interesting)

      by Dr. Zowie ( 109983 ) <slashdot.deforest@org> on Wednesday January 25, 2006 @11:40AM (#14558704)
      and contrary to the eternal "it's just epicycles" cry among Slashdotters, it is testable, falsifiable, and predictive).
      Hey, don't run down epicycles! They're testable, falsifiable, and predictive (of future planetary position) -- they're just more weakly predictive, and more complicated, than Newton's laws of gravity. But when Keplerian orbits were invented, those were no more predictive than epicycles. Both are descriptive theories.

      Epicycles are actually a valid description of planetary orbits, and are still used today to analyze perturbations in planetary and protoplanetary systems. (A particle in a circular orbit will, if perturbed a small amount, acquire both drift and epicyclic motion relative to its original path).

    • Re:Here we go again (Score:3, Insightful)

      by ave19 ( 149657 )

      When a theory like dark matter has amassed evidence in its favor over a period of decades, it takes a lot to overturn it.

      Don't forget, though, that as long as ANY nook or cranny remains uncovered by the theory of Dark Matter, there is room for a replacement. Dark Matter has been predictive, but there is still a problem with it, none has been detected, and as we search for it and fail, we make the remaining possible types of it more and more exotic. The whole situation reminds me of the search for ethe

    • Re:Here we go again (Score:3, Insightful)

      by radtea ( 464814 )

      First off, dark matter, even if it ultimately turns out to be wrong, is not a stupid idea;

      I agree--dark matter is not even an idea. It is a family of ideas. They are all reasonable ideas. But having watched the growth of dark matter theories in the past twenty years--hot dark matter, cold dark matter, warm dark matter, MACHOs, WIMPs, etc ad nauseum--I think there is some justification for a degree of skepticism regarding any dark matter theory.

      All I ask of proponents of any dark matter theory is that the

  • Heim Unified Field Theory (Score:3, Insightful)

    by scorp1us ( 235526 ) on Wednesday January 25, 2006 @10:53AM (#14558104) Journal
    The Heim Unified Field Theory [heim-theory.com] or here is astonishingly accurate theory. Derived from Eenstien's thoery of relativity he postulates 12 (6 major) dimensions. He also postulates gravitophotons. It has been very suceesful in:
    • determining mass of most fundamental particles to experimental accuracy (the others are damn close)
    • Explaining the problem of entropy and the big bang. (In his model matter forms after expansion begins, so the laws of thermodynamics are consistent)
    • explains the effects we currently attribute to "dark matter" without needing actual matter.
    • brings relativity and quantum mechanics into accord.


    I highly suggest you read up on it if you like physics.
  • If there is actually any substance to this, the headline should be that someone has produced a GUT by working out a quantum theory of gravity.

    Instead we get "we've explained away dark matter and explained the Pioneer anomaly".

  • Yoda knows about Dark Matter (Score:4, Funny)

    by digitaldc ( 879047 ) * on Wednesday January 25, 2006 @10:58AM (#14558167)

    From Wikipedia [wikipedia.org]:
    Detecting a graviton, if it exists, would prove rather problematic. Because the gravitational force is so incredibly weak, as of today, physicists are not even able to directly verify the existence of gravitational waves, as predicted by general relativity.

    Yoda: Hard to see, the dark side is.

  • It's only a theory (Score:5, Insightful)

    by smooth wombat ( 796938 ) on Wednesday January 25, 2006 @11:05AM (#14558265) Journal
    It's only a theory folks. We shouldn't be teaching it because it hasn't been proven. It's not a fact.

    (For the humor impaired I'll give you a few moments to let the words sink in)
  • Could someone point me to a good reference (book or website) that can explain to a layman our current understanding of gravity?

    My most frustrated question in high school physics was "How does gravity work?" It seems we don't know anymore today than we did way back when.
    • What do you mean by "how does gravity work?"? If you want some equations to tell you how to make accurate predictions then there are countless books and "It seems we don't know anymore today than we did way back when." is only true if by "we" you mean "people who haven't studied gravity". For an almost layman's level introduction to general relativity try John Baez's intro [ucr.edu].

      Or do you mean "how does it work? how does an object here get pulled towards an object there? what is the connection between them?". In

    • Re:Dark Matter? Gravitons? (Score:4, Insightful)

      by Obvius ( 779709 ) on Wednesday January 25, 2006 @11:28AM (#14558551)
      I read physics at university. An optional Third year course was 'General Relativity'. In the little booklet they gave us to help in choosing what courses to 'major' in (it was an english university), there was an asterisk next to 'General Relativity', as well as 'Cosmology' as a matter of fact. The asterisk denoted 'mathematically rigorous - to be considered only by students with particularly strong mathematical backgrounds'. My friends and I didn't take it - we did things like Computational Physics and Astrophysics instead. In fact, one afternoon, for a laugh (crazy guys that we were) we sat in on a General Relativity lecture to see if we could even keep up. It was a thirty minute lecture on 'Aphelion Procession Using the Scwarzchild Geodesic'. We didn't stand a chance - ball-breakingly tensor analysis. My point is, at that time I knew a hell of a lot more physics than your average guy in the street and I didn't have a clue what was going on in that General Relativity lecture. I read around, spoke to people smarter than I was, spent a fair bit of time trying to get my head around General Relativity I didn't even scratch the surface. And I was a straight-A student back then. I just don't think there exists such a thing as a layman explanation of our understanding of gravity. That other splendid bugger Dr Richard Feynman once said something like 'If a theory can't be reasonably well explained in a single undergraduate lecture then we don't really understand it at all.' It may be that we don't really understand the theory of General Relativity - maybe there is a far more elegant theory explaining gravity that could be explain gravity in simpler terms. For certain, though, that theory does not currently exist. It's a shame, because like you I was always frustrated by the absence of a simple answer to 'How does gravity work?', Why is it always attractive and never repulsive? Some things are just really, really difficult to model and the only models we have are 'mathematically rigorous'. In the words of JBS Haldane 'The universe is not only stranger than we imagine, it is stranger than we can imagine'. http://en.wikipedia.org/wiki/Richard_Feynman [wikipedia.org] http://en.wikipedia.org/wiki/General_relativity [wikipedia.org] http://en.wikipedia.org/wiki/Tensor_analysis [wikipedia.org]
  • Comment removed based on user account deletion
  • I thought Dark matter effectively functioned like negative gravity so as to account for the accelerated expanding universe? How does the graviton explain this phenomenon?

  • This doesn't even seem to explain the data. (Score:5, Informative)

    by Razor Sex ( 561796 ) on Wednesday January 25, 2006 @12:05PM (#14559062)
    One of the most significant pieces of evidence for dark matter is the rotation curve of galaxies. If the Universe functioned like we thought it did, the rotation curve of a galaxy should be a downward sloping curve - the further out a star is from the galactic center (where the mass of the galaxy is concentrated), the slower its orbit should be. This is what Kepler's Law tells us - that the orbital speed of an object decreases inversely with the square root of the orbital radius.

    What we find, however, is that the rotation curves of galaxies are nearly flat, meaning that the mass distribution of galaxies must be nearly equal all the way through. This means there must be a large amount of matter that we don't see. There aren't enough dwarf stars, planets and other things like that to make up this mass. Of course we haven't counted or seen all of these, but if you do the math, there would have to be a ridiculous amount of these - more than is likely. Hence, we have dark matter.

    This new theory says that the force of gravity should be stronger near the galactic core, where the stars are packed most densely. So the core is even more massive than we thought, meaning that the rotation curve of the galaxy should be even more skewed - far from flat. So either New Scientist seriously misrepresnted his theory, or it doesn't even deserve a cursory thought. MOND at least seems plausible.

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