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Does Antimatter Fall Up Or Down? 480

KentuckyFC writes "There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field. We've never been able to make enough of the stuff to do the experiment. But at the European particle physics laboratory at CERN, where scientists have been refining the technique for making antihydrogen, researchers are designing an experiment called AEGIS that will finally settle the matter. The idea is simple — fire a beam of antihydrogen atoms and watch which way they fall — but the details are fiendish (abstract). The answer should help solve a number of important conundrums such as why there is so little antimatter in our part of the universe and what the value of the cosmological constant is."
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Does Antimatter Fall Up Or Down?

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  • Confused (Score:5, Funny)

    by wasted ( 94866 ) on Wednesday June 04, 2008 @06:06AM (#23649085)

    "There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field. We've never been able to make enough of the stuff to do the experiment. But at the European particle physics laboratory at CERN, where scientists have been refining the technique for making antihydrogen, researchers are designing an experiment called AEGIS that will finally settle the matter.

    Or will it settle (or unsettle) the anti-matter?
    • Re:Confused (Score:5, Interesting)

      by Anonymous Coward on Wednesday June 04, 2008 @06:25AM (#23649189)
      how fast things can change [particleadventure.org]:

      Gravity affects matter and antimatter the same way because gravity is not a charged property and a matter particle has the same mass as its antiparticle.

      So the above is no longer believed to be true?
      • Re:Confused (Score:5, Insightful)

        by complete loony ( 663508 ) <Jeremy...Lakeman@@@gmail...com> on Wednesday June 04, 2008 @06:43AM (#23649291)
        There's a big difference between a belief that something is most likely true, and an experiment that removes all doubt.
        • Re:Confused (Score:5, Insightful)

          by K. S. Kyosuke ( 729550 ) on Wednesday June 04, 2008 @07:25AM (#23649561)
          Until now I thought that ouside the real of mathematics (where things can be proven and no further revision is possible, save for attacking the logic of the proof), there is no such thing as "an experiment that removes all doubt"?
          • Re:Confused (Score:5, Informative)

            by dnwq ( 910646 ) on Wednesday June 04, 2008 @08:02AM (#23649841)
            An experiment that reduces doubt. Does that work for you?
          • Re: (Score:3, Insightful)

            If the 2 possible outcomes are polar opposites, and it goes one way and not the other. I'd say "removes all doubt" is a fair statement. Note I didn't use the word "prove", more like doubt in the legal prosecution sense.

            But now were just arguing semantics. Oh wait, this is /. never mind.

            • Re: (Score:3, Informative)

              by TubeSteak ( 669689 )

              If the 2 possible outcomes are polar opposites, and it goes one way and not the other. I'd say "removes all doubt" is a fair statement. Note I didn't use the word "prove", more like doubt in the legal prosecution sense.

              To get pedantic about it:
              the experiment only shows that, under a certain set of conditions, antimatter behaves a certain way.

              You've only removed all doubt if you expect every single possible type of antimatter to behave the same as antihydrogen.

              You can extrapolate from the observed behavior, but as we've seen time and time again, there are plenty of edge cases & fringe behaviors that are completely unanticipated.

      • Re:Confused (Score:5, Interesting)

        by smilindog2000 ( 907665 ) <bill@billrocks.org> on Wednesday June 04, 2008 @09:18AM (#23650717) Homepage
        While this is a reasonable guess, it's about the same as guessing heavier objects fall faster. Consider electron-hole pairs in a silicon lattice. They act very much like electron-positron pairs. However, electrons fall down, and holes fall up. To me, it would seem odd if anti-matter fell down.

        Why is there so much matter around, and no anti-matter? Perhaps because they repel each other? There is some evidence that nearby galaxies are made of matter and not anti-matter, but the universe is very big, and time could be effected differently by anti-matter gravity (speeding up). Why are galaxy clusters accelerating in their separation from each other? Could anti-matter still be present somewhere, causing the acceleration? Why is matter in the universe so clumped together, and not more uniformly spread out? Could there be clumps of antimatter between the clumps of matter?

        Evidence suggests that there simply is no anti-matter left in the universe, but it's fun to speculate upon implications of anti-matter falling up.
        • Re: (Score:3, Interesting)

          by somersault ( 912633 )
          Surely more massive objects do accelerate towards the eart slightly faster than less massive ones, since the objects themselves will be exerting a gravitational pull on the earth themselves? So the natural human instinct to believe that heavier objects fall faster is semi-right (I say semi right because it's right, but for the wrong reasons!) in that case, just not observably so over short distances.
          • Re:Confused (Score:4, Informative)

            by mshannon78660 ( 1030880 ) on Wednesday June 04, 2008 @11:28AM (#23653233)
            Uh, no, actually, they don't. Because F=Gmm'/r (m being the mass of one object, and m' the other, G being the gravitational constant and r being the distance between them), and a=F/m (acceleration equals force divided by mass). When you substitute one for the other, you get a=Gm'/r - which is completely independent of the mass of the object being observed. Now, you could argue that (as you seem to) that the earth is also accelerating towards the other mass, leading to what seems to be a higher rate of falling - but (contrary beliefs about relativity aside), you can't really treat all of the acceleration as relative in that manner - that simplification of relativity only works for uniform motion - it is possible to detect acceleration (although you can also replace it with a gravitational field, but that would then invalidate our original formulas - can't change horses in mid-stream like that).
            • Re: (Score:3, Insightful)

              by somersault ( 912633 )
              Yes I would be arguing that the earth would also be accelerating however infinitescimally quicker towards the other object. I know that the differences are purely academical and have pretty much no bearing in real terms, but consider you had 3 different orbs of the same volume but different masses, say A=1 large mass unit, B=2 large mass units and C=1 large mass unit.

              If you left any of these orbs near each other in space then they would exert a significant gravitational pull on each other and would both be
        • Re:Confused (Score:5, Informative)

          by courtarro ( 786894 ) on Wednesday June 04, 2008 @11:05AM (#23652759) Homepage

          Consider electron-hole pairs in a silicon lattice. They act very much like electron-positron pairs. However, electrons fall down, and holes fall up. To me, it would seem odd if anti-matter fell down.

          Holes are a virtual particle with no mass, whereas anti-matter has mass. Electrons actually exist, but holes are simply a place where an electron can fit. Take the example of a helium balloon in a car: if you slam on the breaks, it flies to the back of the car because all the air is rushing forward, and the balloon's relative vacuum gets pushed backward. A hole behaves similarly. Antimatter, on the other hand, has mass just like its matter counterpart and therefore isn't directly comparable to a hole.

        • Re: (Score:3, Interesting)

          If you think about it: Antimatter can be considered as matter which is travelling backwards in time (Please check up on the famous CPT-theorem on this subject for more info).

          In this view it would be quite natural that antimatter is 'falling' upwards.

          In addition it would explain why we don't see antimatter in our universe: The antimatter universe simply evolved into the other (negative) direction of the timeline.

          On the other hand - matter and antimatter as just another manifestation of energy - it should

    • Re: (Score:3, Funny)

      by camperslo ( 704715 )
      Excuse me, but isn't asking is something will fall up or down asking a loaded question?
      Shouldn't we instead be asking how it will fall?

      What if it falls on an imaginary axis and ends up travelling in time or to a parallel universe?

  • It's a cool experiment, but it's news once you get the result, not "a few years" before.

    Unless they're trying to drum up interest for funding...
  • I guess it depends on the existence of gravitons. Does the universe really bend or is that just a description of the gravitational force, or is gravitons exerting the force, and in that case does anti-gravitons exists, and what happens when gravitons collide with anti-matter?
    • Futurama quote

      Professor: And the microwave radiation, combined with the gravitons and graviolis from the supernova, blasted us through time itself.

    • Re:Gravitons (Score:5, Interesting)

      by jandersen ( 462034 ) on Wednesday June 04, 2008 @07:31AM (#23649611)
      Gravitons are like photons: simply distortions in the underlying field. When two masses move relative to each other, the change in position corresponds to a change in the force between the two, but this change isn't communicated instantaneously. Instead the change travels as a distortion in the force-field - ie. a graviton (or several, as the case might be). This is what it means, intuitively at least, when they say that "the graviton mediates the force of gravitation"; and the same goes for the other mediators of force: photon, gluon and W- and Z boson. The perceived conflict is an artifact of limitations in the viewpoint of quantum mechanics.

      The gravitational field as a scalar field surperposed on a flat space-time is just another way of describing gravitation - the curved geometry of general relativity is a better model, although it is more difficult to get a handle on. Perhaps it would be worth trying to tackle the other forces in the same way, as geometry in some sort of space-time. Perhaps we can even derive quantum mechanics as a special case of such a model; mathematics has certainly come a long way since the time of Einstein and Bohr, and it isn't unreasonable to hope that we are now approaching a situation where we can solve those old problems, that neither had the tools for.
      • What gravitons? (Score:4, Interesting)

        by Roger W Moore ( 538166 ) on Wednesday June 04, 2008 @04:16PM (#23658189) Journal

        Gravitons are like photons: simply distortions in the underlying field...

        Are they? Ever seen evidence of one? Gravitons are a purely theoretical construction and, worse of all, one that does not work. While you can construct a quantum field theory of gravity it does not work to arbitrary energies. You have to impose a cut-off threshold and since there is no valid reason for doing so the theory is broken...hence all the theoretical activity trying to reconcile GR and Quantum mechanics.

        The gravitational field as a scalar field surperposed on a flat space-time is just another way of describing gravitation

        You mean a vector field since gravity has direction, rather than the Tensor field of GR.
  • It will fall down (Score:5, Informative)

    by little1973 ( 467075 ) on Wednesday June 04, 2008 @06:11AM (#23649111)
    According to GR, gravity is the curvature of Space-Time. As the anti-matter moves through space it has to follow this curvature. If it does not, that means GR is wrong (which may be the case, but I doubt this experiment will disprove GR).
    • Re: (Score:2, Interesting)

      Exactly right. And the value of the Cosmological Fudge Factor errrr Constant is zero.
    • by thue ( 121682 ) on Wednesday June 04, 2008 @06:49AM (#23649317) Homepage
      Yes, Silly physicist PhDs doing unneccesary experiments. They could have been told the result of the experiment just by asking a random commenter on Slashdot. :)

      Because our understanding of physics is so consistent that it is a waste of time to test the cornercases of our theories *cough*quantum gravity*cough*dark matter*cough*dark energy*cough*

      :P
      • Re:It will fall down (Score:5, Informative)

        by Anonymous Coward on Wednesday June 04, 2008 @08:02AM (#23649827)
        I'm a physics PhD and this is definitely one of the experiments where there is no reason to expect things to behave differently from the theory. Any reasonable theory already allows us to put low limits on the difference in gravitational behaviour between matter and anti matter and there certainly is no theory of gravity that I know of where antimatter "falls up". There are some where it might fall differently.

        There also is some direct evidence that if you have differences they are not due to gravity:

        Reference e.g.:

        http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatter_fall.html

        "The only direct experimental result on antimatter and gravity comes from Supernova 1987A. This supernova in the Large Magellanic Cloud emitted both neutrinos and antineutrinos, some of which were eventually detected on Earth. Those neutrinos and antineutrinos took 160,000 years to reach Earth, and while travelling were bent from a "straight line" path by the gravity from our own galaxy. The bending with gravity changed the time needed to reach Earth by about 5 months, yet both the neutrinos and the antineutrinos reached Earth at roughly the same time (within the same 12 second interval). This shows that the neutrinos and antineutrinos "fell" similarly, to a very high level of precision (about 1 part in a million). [4] and [5] provide some background information on this."
      • by Hankapobe ( 1290722 ) on Wednesday June 04, 2008 @08:13AM (#23649961)

        Yes, Silly physicist PhDs doing unneccesary experiments. They could have been told the result of the experiment just by asking a random commenter on Slashdot. :)

        Why not? This is where I get my IP legal advice!

    • Re: (Score:3, Interesting)

      by Enrique1218 ( 603187 )
      That assertion may not be true. A prominent physicist once suggest that antimatter in not antimatter at all but rather matter traveling in anti-time or backwards in time from are perspective. That explains why we measure opposite charges from normal matter. If placed in curvature of space, it would appear to move up against the gradient. That interpretation would preserve GR. I would also remind you that every law like GR is just an approximation.
  • Obvious? (Score:5, Funny)

    by neokushan ( 932374 ) on Wednesday June 04, 2008 @06:14AM (#23649133)
    It doesn't-matter.
  • by ShooterNeo ( 555040 ) on Wednesday June 04, 2008 @06:40AM (#23649267)
    I wish the results were that antimatter falls upwards. If that were true, while it would have no practical use in the near future, it would be a hole in physics that our far descendants could exploit.
    • by pla ( 258480 ) on Wednesday June 04, 2008 @07:46AM (#23649703) Journal
      If that were true, while it would have no practical use in the near future

      Not necessarily - Merely opening that particular conceptual door would lead to a massive influx of funding and revisited anomalous past results.

      Interesting thing about experimentation, even the most honest of researchers tends to throw away "bad" results (in the sense of not publishing them, not in the academically-dishonest sense of omitting them from the data). If the scientific community suddenly accepted the possibility of spooky-effect-X, you can bet that dozens or even hundreds of research groups would dredge up their past efforts to see if effect-X explains their results.

      Case in point, l'Acedemie des Sciences and meteorites. Up to the turn of the 19th century, only idiots would dare claim that rocks could fall from space... Until the scientific community decided they could, at which point a huge body of past evidence appeared practically overnight supporting the existance of such falling objects.
    • by krnpimpsta ( 906084 ) on Wednesday June 04, 2008 @07:50AM (#23649731)

      I wish the results were that antimatter falls upwards. If that were true, while it would have no practical use in the near future, it would be a hole in physics that our far descendants could exploit.
      Dude, please don't exploit holes in physics. I don't want my access to the universe revoked just because God banned us all from our reality for hax. All it takes is one noob hax0r particle physicist to ruin it for us all.
  • I'm probably wrong & welcome being corrected, but I can't shake this thought.
    I'm under the impression Electromagnetic fields & Gravitational fields function similarly or the same.

    I'm thinking of a field situated like the simultanious implosion of a uranium or plutonium bomb, with these antimatter things trapped in the center.
    What if on one point of the field the magnetic strength was weakened, perhaps by touching it with another magnetic field, while on the oppisite end the strength was increas
  • ... that there was any question about how antimatter behaves in a gravitational field. I thought that
    it had mass just like anything else, and is therefore affected by gravity just like any other particle with mass.



    Wouldn't "falling up" mean that antimatter has negative mass ? And if so, how does this comply with energy/mass conservation laws ?

    • Yes, that is how I read it. Mass is only known to us through its gravitational effect; if anti-matter falls away from matter, it must mean that it has negative mass. There is no conflict with energy conservation - it only means that the total energy (or mass) of a closed system doesn't change.
  • ... why should antimatter be any different? In fact every particle with mass yet measured behaves the same way in gravity. I can't see any reason why antimatter should be any different.
    • by Tom ( 822 ) on Wednesday June 04, 2008 @06:59AM (#23649375) Homepage Journal
      And science is all about the difference between "I think..." and "I've tested..."

      If it behaves exactly as predicted, you can make another mark and continue. If not, you've found something potentially very important.

    • by Urkki ( 668283 ) on Wednesday June 04, 2008 @07:08AM (#23649441)
      Antimatter *could* be different because the mathematics of GR allow it, and we haven't actually done the experiment before. I wouldn't put much faith in human intuition in these matters, considering how counter-intuitive entire GR is...

      I mean, we see water falling off edges of waterfalls etc. Why should the edge of the world be any different? ;-)
      • by Sycraft-fu ( 314770 ) on Wednesday June 04, 2008 @08:34AM (#23650171)
        It's not science. Even doing a nice bunch of calculations and saying "Well this shows that anti-matter should do this," is not science, or at least not the important part. Science is testing beliefs by experiment. So regardless of what we think anti-matter will do, and regardless of how sure we think we are, we still need to test it. That's how science works. You come up with an idea, you test it. If the test falsifies it, you come up with a different idea and test it. If the test supports it, you come up with more tests to try and falsify it.

        Through this process, we come to understand the natural world, and come to be fairly certain that our understanding is correct. Math and theoretical work is great, but actually testing those theories is what makes science what it is.

        So even if we are 99.999999% certain that our calculations are solid and anti-matter does something, we still need to test it. There are plenty of things that we've been certain about that, when we tested it, turned out not to be the case.
    • Re: (Score:2, Informative)

      by Vulch ( 221502 )
      One model for anti-matter is that it is a normal particle travelling backwards in time. The energy from a particle/anti-particle annihilation is the energy released by it changing direction under this model. As far as the particle is concerned, it is behaving normally in a gravitational field by falling downwards, but when we look at it from our usual time axis it appears to be falling up.
    • Re: (Score:3, Insightful)

      Because all the other properties of antimatter that have been tested so far are different unlike every type of normal matter ...

      This is a Black Swan problem

      Theory : All Swans are White
      Proof : every swan I see is white, every swan ...
      Problem : Australia was then discovered along with the black swan.....
  • by Bromskloss ( 750445 ) <auxiliary DOT ad ... privacy AT gmail> on Wednesday June 04, 2008 @06:57AM (#23649369)
    my relativity teacher told his class, is a function of time: At first, it was non-zero, then people said it was zero, then it might be non-zero after all.
    • Re: (Score:3, Informative)

      The original theory did not need it, but it predicted the universe was expanding, and conventional wisdom at the time said this was not the case... so Einstein add the constant to get a steady state universe ...

      Then Hubble found the universe was expanding so Einstein took it out again ....

      Then the inflational big bang model and observations that the current expansion of the universe was accelerating seemed to require it again ... so it was put back ...

      This is why experiments need to be done ...
    • by Lijemo ( 740145 ) on Wednesday June 04, 2008 @09:46AM (#23651165)

      my relativity teacher told his class, is a function of time: At first, it was non-zero, then people said it was zero, then it might be non-zero after all.

      I saw a paper in the Journal of Irreproducible Results advancing the theory that the age of the universe is a nonlinear function of time.

      They plotted on a graph the age people (or rather, western civilization) thought that the age of the universe was at various points in history--when the 19th century geologists said it had to be at least hundreds of thousands of years old, when the 20th century astronomers said that it had to be even older than that-- and plotted the points on a graph. They formed a smooth curve demonstrating (I think) a geometric increase.

      So their theory was that, assuming all the age-of-the-universe estimates were correct, that means the beginning of the universe is moving backwards in time, away from us. In 1000ad, the universe really was 6000 years old, and now it really is 14.5 billion years old, and in another century, it will probably be in the trillions of years old

      (I love the Journal of Irreproducible Results!)

  • So anti-matter falls up in gravity field and Guiness bubbles sink in a gravity field what happens when you mix the two?
    But how does this help explain why our area of space has so little anti-matter? If this was true then it would mean we are at the bottom of the universe, as opposed to what; or is the current thinking that anti-matter is not effected by gravity and this experiment would provide that it is?
    • by cnettel ( 836611 )
      If anti-matter "falls" up, then masses of ordinary matter repel antimatter. This would mean that there will be no gravitationally stabilized chunks of matter and anti-matter (which would sooner or later annihilate), but rather that over scales where the gravitational force dominates, we would get separated regions.

      Like many other posters, my "physical intuition" (indoctrination in GR) tells me that this is most unlikely, but it would have tremendous consequences for cosmology.

  • by njcoder ( 657816 ) on Wednesday June 04, 2008 @07:07AM (#23649433)

    Does Antimatter Fall Up Or Down?
    Yes!
  • by itsdapead ( 734413 ) on Wednesday June 04, 2008 @07:54AM (#23649753)

    Just a few technical details to sort out, first :-)

    ...and, of course, if your antimatter-powered airship crashes, the phrase "Oh, the Humanity!" is going to be even more applicable. Maybe without the "the".

  • by franknagy ( 56133 ) on Wednesday June 04, 2008 @08:02AM (#23649835) Homepage
    As I rememeber, this has already been tested by drifting positrons down the length of the SLAC accelerator tube and measuring the beam deflection due to gravity (at least 20+ years ago).

    Yes, anti-matter does fall down just like matter.
  • I Bet (Score:3, Funny)

    by Symbolis ( 1157151 ) <symbolis&gmail,com> on Wednesday June 04, 2008 @08:10AM (#23649905)
    It falls sideways, just to screw with those smarmy scientists.
  • by mbone ( 558574 ) on Wednesday June 04, 2008 @08:12AM (#23649943)
    There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field.

    Uh, no there are not. Gravity (or geometry, same thing in the theory) depends on mass energy in General Relativity. Stuff (with mass energy) follows the metric (the local geodesic). Even photons (which are their own anti-particles) follow the geodesic - and that has certainly been tested. Equivalence principle tests also show that different sorts of nuclear matter (including neutrons) individually follow the geodesic. Anti-matter certainly has mass energy, and (with matter) can be converted to photons and is no different in the theory. In other words [springerlink.com] "there is only one type of geodesics and there are no antigeodesics for antimatter."

    The original article talks about "flavors" of General Relativity. Ain't so such beasties. Period. If you go to the real original article [arxiv.org], you find a proposal for a 1% test of the equivalence principle for antimatter, and no such claims of flavors. Now, the equivalence principle has been tested to better than parts per trillion, and part of the mass energy in ordinary matter is made up of antimatter (in virtual particle pairs), so (based on the experimental evidence) I would claim that this test will be negative and is not actually that interesting as new physics. (The articles say that these older tests are "model dependent," but they are not model dependent enough to matter for this.)

    That doesn't mean that this shouldn't be done (everything should be tested in physics, and different tests are always useful), but the prediction of General Relativity is clear : if anti-matter has anti-gravity, then General Relativity is wrong. The experimental evidence is also clear : this isn't going to be accurate enough to matter. Will make for some good public relations, though.
    • Re: (Score:3, Informative)

      by mbone ( 558574 )
      A brief search on the subject reveals this [sciencedirect.com]

      we conclude that the Principle of Equivalence between particles and antiparticles holds to a level of 6.5, 4.3 and 1.8 x 10-9,

      Just a little bit better than 1%.
  • by mark-t ( 151149 ) <[moc.talfdren] [ta] [tkram]> on Wednesday June 04, 2008 @12:39PM (#23654625) Journal
    That it doesn't fall at all? Up, or down? That antimatter doesn't interact gravitationally with normal matter in any way. but only with other antimatter?
  • No. (Score:4, Informative)

    by blair1q ( 305137 ) on Wednesday June 04, 2008 @03:23PM (#23657289) Journal
    Sorry, this is all silly.

    "Anti-matter" is not the opposite of matter. It is matter with opposite charge or other opposite properties. Mass has no opposite. It is there, or it isn't. (Or it is and then it isn't and then it is and then it isn't, on the level of quantum fluctuations of the gravitational field.)

    And no, a hole isn't "anti-matter". It's merely a void in the surrounding matter. It rises because of a principle known as "buoyancy" which is really the gross action of the matter surrounding it causing pressure differentials as a function of distance from the CM. As long as the surrounding matter can flow, it can fill any displacement of the void, and the mass will tend to move down rather than up, and the void will therefore appear to tend to move up rather than down.

    And no, hot-air balloons don't rise because of the heat, they rise because they make the air in the balloon less dense than the air outside the balloon. How they do it is not relevant. A rigid balloon and a vacuum pump would work, too.

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