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Science

Does Antimatter Fall Up? 255

Posted by Soulskill
from the where's-scotty-when-you-need-him dept.
New submitter Doug Otto sends word that researchers working on the ALPHA experiment at CERN are trying to figure out whether antimatter interacts with gravity in the same way that normal matter does. The ALPHA experiment wasn't designed to test for this, but they realized part of it — an antihydrogen trap — is suitable to collect some data. Their preliminary results: uncertain, but they can't rule it out. From the article: "Antihydrogen provides a particularly useful means of testing gravitational effects on antimatter, as it's electrically neutral. Gravity is by far the weakest force in nature, so it's very easy for its effects to be swamped by other interactions. Even with neutral particles or atoms, the antimatter must be moving slowly enough to perform measurements. And slow rates of motion increase the likelihood of encountering matter particles, leading to mutual annihilation and an end to the experiment. However, it's a challenge to maintain any antihydrogen long enough to perform meaningful experiments on it, regardless of its speed. ... The authors of the current study realized that [antiatoms trapped in ALPHA] eventually escaped or were released from this magnetic trap. At that point, they were momentarily in free-fall, experiencing no force other than gravity. The detectors on the outside of ALPHA could then determine if the antihydrogen was rising or falling under gravity's influence, and whether the magnitude of the force was equivalent to the effect on matter."
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Does Antimatter Fall Up?

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  • by Anonymous Coward on Tuesday April 30, 2013 @05:39PM (#43595239)

    Maybe our universe is a 'matter bubble' in a 'sea of anti-matter'. WE are the anti-matter.

    To us, our normal matter is so common but that's only because we're sitting right smack in the middle of it. That would explain the repelling forces and show why dark matter could exist outside the bounds of the observable universe.

    • by Brucelet (1857158)
      Maybe any number of crazy things happen beyond our cosmological horizon. We'll never see them so it's not relevant. The observable universe is matter-dominated.
    • by scheme (19778)
      Then we should see a very bright border as matter and anti-matter annihilate on the edges. As far as I know, that doesn't exist so being a bubble of matter in anti-matter doesn't seem likely.
      • by Calydor (739835)

        Force field. *nod* We are the same experiment to the outer universe as the one described in the summary, caught in the equivalent of a magnetic trap.

      • by tmosley (996283)
        How fast is the universe expanding, and how big is it? We might have just not seen it yet.

        Pretty scary thought, that the universe is doomed to destruction by a wall of arbitrarily energetic photons traveling inward from its edges.
      • by alexgieg (948359)

        Then we should see a very bright border as matter and anti-matter annihilate on the edges.

        Unless we can't see it because it's all wrapped up by the counter-annihilation of space colliding with anti-space!

  • If they have created hydrogen atoms already, why wouldn't they just check to see if those atoms fall to the bottom of the container, or float to the top? I would guess these atoms are stored in a vacuum, so buoyancy isn't a concern.
    • by Electricity Likes Me (1098643) on Tuesday April 30, 2013 @05:47PM (#43595343)

      The problem is there's something like a 1000. Total.

      Actually measuring them accurately is a challenge, although no one in the physics community really expects the answer to be "they fall up" at this point. It would be a huge upset if they did.

      • Ah so just getting the detectors in place. Thanks.
        • Re: (Score:2, Informative)

          by Anonymous Coward

          No, hydrogen atoms in a vacuum are going to be more effected by nearby weak magnetic fields then by gravity. And even more so by what direction they where traveling. So you'd first need to slow them down which is hard to do when all you can use is magnetic force. Of course you could try to accurately measure there trajectory at two or more points and then figure out how gravity effected the atom. But it's hard to get real good measurements of atomic trajectories with strong magnetic fields. If we could mak

      • ... no one in the physics community really expects the answer to be "they fall up" at this point. It would be a huge upset if they did.

        To the physicists, maybe.

        For the rest of us... SUPERPOWERS!!!

      • by Shimbo (100005) on Tuesday April 30, 2013 @05:52PM (#43595397)

        Actually measuring them accurately is a challenge, although no one in the physics community really expects the answer to be "they fall up" at this point. It would be a huge upset if they did.

        There's a (possibly apocryphal) story about a physics professor. Whenever he dropped his chalk, writing equations on the board, he would look upwards. When one of the students finally asked him why he did this, he replied, "If one day it fell upwards, I wouldn't want to miss it."

      • by ndogg (158021) <the.rhorn@gmail . c om> on Tuesday April 30, 2013 @06:08PM (#43595531) Homepage Journal

        It would be a huge upset if they did.

        Actually, I'm pretty sure a lot of them would have the opposite reactions. When the Higgs Boson was finally found, a lot of physicists were actually disappointed because it meant there wasn't really much in the way of new physics to be discovered.

        • by ceoyoyo (59147) on Tuesday April 30, 2013 @06:13PM (#43595579)

          Scientists like upsets. They wouldn't BE upset, it would be an upset.

        • by harrkev (623093)

          Well, in all fairness, nobody was really sure about the Higgs. A lot of people were hoping, but nobody was willing to bet the farm on the mass (or even the existence of the Higgs).

          Now, gravity on, the other hand, is a completely different aniamal.

          IANAP (I am not a physicist), but the way that I understand it is that gravity is simply following a straight line in curved space-time. So, a straight line is a straight line for both matter and anti-matter. If anti-matter flies up, then that totally blows the

          • by X0563511 (793323)

            This seems to be just another "Yup, relativity still works" type of experiment.

            The way you say that suggests you don't see much value in that. Confirming that what you believe to be true, is true, is very important.

        • by Goaway (82658)

          That is "upset" as a noun, not an adjective.

    • by Hentes (2461350)

      If I understand it right, they also wanted to check whether the gravity of antimatter is a fraction of the gravity of normal matter. So they needed some other force to measure gravity against.

    • by Charliemopps (1157495) on Tuesday April 30, 2013 @06:44PM (#43595893)

      Because these are individual atoms. Very hard to detect unless they are clumped together as a mass... as in the millions. The only way to know their position is to force them to be where you want them via a magnetic field... etc... which ruins your chances of measuring any gravitational effect which is unfathomably tiny at atomic scales. You could make a whole pile of them (very difficult indeed) so it would act more like classical matter... the problem there is that by the time you had that much, when it hit the bottom of your container you'd find out just exactly what e=mc2 is all about and likely need to start looking for a new research facility.

      • by chihowa (366380)

        But you get a nice annihilation event whenever they touch matter. That makes it much easier.

        So you constrain it in a a known location with an electric field (or two, a DC and an AC field, like an electrodynamic balance) in a vacuum, then let go and see where the two photons from the annihilation come from. With a large enough vessel and a sensitive PET-like setup, you should be able to tell whether it hit the top or the bottom of the vessel.

        • by GryMor (88799)

          And you'll likely end up measuring the bias of your trap rather than the effect of gravity on the particle after the asymmetrically decaying field of your trap has given it a kick.

          • by chihowa (366380)

            Pft. That's just an engineering problem. You have plenty of charged matter with known properties to test it on (though the detector will of course be different). Also simple stuff like turning the device upside down will help suss out instrument bias. At 0.6% equatorial vs polar gravity, you could likely tell a difference in travel time even with a bias in the trap.

            So overall, probably not a show stopper.

    • Its difficult because for a single atom gravity is very weak. Small magnetic or electric fields (or field gradients) can interact with the magnetic field, or electric dipole moment of the atom. Also the atoms are moving inside of the trap. The speed of their motion depends on temperature: (at room temperature it is > 1 kilometer/second). I assume they cool the anti-hydrogen, but the atoms may still be moving so quickly that gravitational effects are not very large.

    • by Mt._Honkey (514673) on Tuesday April 30, 2013 @08:58PM (#43596801)

      I'm an ion trapper, and though I don't work on this experiment, I've heard their group leader speak on exactly this topic a year or so ago, so hopefully I can do it justice from memory.

      There are a couple challenges. One is "letting go". The atoms are trapped by very strong magnetic fields, and those have to be turned off rapidly to "let go" of the atoms. They turn off the superconducting magnet coils by heating them above their critical temperatures to make them normal-conducting and dumping all that energy into heat ("quenching" the magnet). Then the atoms are free to move around, but they weren't just sitting perfectly still in the traps, they had some thermal motion, which could fling them in any direction, including up. They've had trouble getting the atoms as cold as they had planned. They hoped they would be around 3 K, but I think they were stuck at 10 or 20 K for some unknown reason. So they aren't really just "dropping" the atoms. More atoms will go down than up if they are affected by gravity as expected, but it isn't remotely universal. Additionally their current trap is horizontal because the beam comes in from that direction, so there are only a few vertical cm in which to build up that bias.

      Perhaps the bigger issue is actually knowing which way the atoms went. Their current trap was designed to do laser spectroscopy of atoms sitting in the trap, not tracking atoms as they fly around the beamline. What they do is wait until an anti-atom hits a surface and annihilates with a normal atom, and detect the radiation that is released from the annihilation. The radiation flies off in every direction though, so it takes some doing to build a radiation detection array that can reconstruct where in the apparatus the annihilations actually take place. As I mentioned, the current trap was not optimized for this particular study, so the reconstruction ability is pretty weak.

      They are working on building the next generation of the experiment that will include a vertical trap, better detection arrays, and colder atoms, so that should be able to get to a better detection.

  • Obviously there must be some credence to this idea for such an experiment to take place, but since my understanding is that gravity is an inherent effect of mass warping space, wouldn't anti-matter possess mass in the same way that matter does, so why would gravity act differently?

    Just asking. Not trying to claim anything.

    • by Electricity Likes Me (1098643) on Tuesday April 30, 2013 @05:51PM (#43595373)

      Obviously there must be some credence to this idea for such an experiment to take place, but since my understanding is that gravity is an inherent effect of mass warping space, wouldn't anti-matter possess mass in the same way that matter does, so why would gravity act differently?

      Just asking. Not trying to claim anything.

      Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions - it's perfectly logical to have something which "weighs" a 1000kg when experiencing electromagnetic acceleration, and only 10kg when experiencing gravitational acceleration.

      For normal matter, this is the case. For antimatter it's presumed but not actually tested, and therein lies the rub. Even a slight deviation would be huge - and have big implications for the question of why the universe has so much matter in the first place.

      • by emt377 (610337)

        Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions - it's perfectly logical to have something which "weighs" a 1000kg when experiencing electromagnetic acceleration, and only 10kg when experiencing gravitational acceleration.

        The discussion is about mass, not weight. Weighing something is a very indirect way to determine mass; but regardless, it's about mass, not weight. If it were about weighing schemes a term other than mass would have been used.

        • Inertial Mass comes from Newton's second law:

          F = m_i * a

          That is, inertial mass determines how much an object will be accelerated by a particular force.

          Gravitational Mass comed from Newton's law of graviation:

          F = G * m_g1 * mg2 / r ^2

          That is, the magnitude of the gravitational forces between two objects.

          The question is whether the two definitions of mass are interchangable (e.g. does m_i = m_g1?). That appears to be the case for normal matter, which we can tell because all objects accelrate at the same rate

      • Re:I must be stupid (Score:4, Informative)

        by BitterOak (537666) on Tuesday April 30, 2013 @07:21PM (#43596191)

        Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions

        Well, if you believe General Relativity, they darn well better be equivalent. In fact, Einstein took the Equivalence Principle as one if his starting points when developing GR. If the Equivalence Principle fails (which it must if anti-matter falls up), then they will have disproven Einstein's theory, which would be very big news, indeed.

        • Re:I must be stupid (Score:4, Interesting)

          by Electricity Likes Me (1098643) on Tuesday April 30, 2013 @08:10PM (#43596539)

          Inertial mass and gravitational mass are observed - for normal matter - to be exactly equivalent. There's no actual reason they should be though, since they're the product of very different interactions

          Well, if you believe General Relativity, they darn well better be equivalent. In fact, Einstein took the Equivalence Principle as one if his starting points when developing GR. If the Equivalence Principle fails (which it must if anti-matter falls up), then they will have disproven Einstein's theory, which would be very big news, indeed.

          Which would hence be the value of a test that it is in fact enforced. Again: we can only assume it's true because the laws we know work in other cases assume it's true. But there's no implicit reason to think that we aren't simply observing a whole lot of local cases where some higher principle is simplifying to General Relativity, or where at the fringes there's a small correcting constant which isn't significant in most normal situations.

          This type of measurement is where new physics comes from - it's why there's people who have been measuring alpha to ever greater precision, even though we've no reason to think it'll deviate if current theory is a complete explanation.

    • Well... There are two conjectures which need to be tested.

      1. Are anti-particles just like normal particles except with their direction of time reversed?
      2. Do anti-particles have negative energy?

      If they are the "travelling backwards in time", then gravity would be repulsive.

    • If they changed the definition of antimatter to "has an anti-higgs-boson-field" such that the matter actually has an inverse affect on the universe, that might cause it to have "anti-mass" which would warp the anti-space referenced by the anti-field. However, this is not how we have traditionally defined anti-matter; the original definition was actually due to the fact that the universe has significantly less mass than it should, and "anti-matter" was hypothesized as an explanation. So by definition, anti

      • Re:I must be stupid (Score:5, Informative)

        by Guy Harris (3803) <guy@alum.mit.edu> on Tuesday April 30, 2013 @06:29PM (#43595773)

        However, this is not how we have traditionally defined anti-matter; the original definition was actually due to the fact that the universe has significantly less mass than it should, and "anti-matter" was hypothesized as an explanation.

        Actually, the original modern definition of anti-matter was "Dirac's relativistic equation for the wave function of the electron had negative energy states as well as positive energy states, which was a bit weird, so it was proposed that all the negative energy states were filled, and if you knocked an electron out of one of the low-energy states, a "hole" would be left behind, and that hole behaved like an electron, except that it has a positive charge". It was later seen in the real world (particles moved in a magnetic field as if they had the mass of an electron and a +1 electrical charge). See, for example, the Wikipedia article about the positron [wikipedia.org].

    • All theory says that anti-matter should behave like matter becasue both kinds of matter have positive energy (hence the release of energy when they annihilate). Negative energy is harder to produce than anti-matter, and it is possible that there are fundamental limits to its production. Negative energy would produce anti-gravity (at least I think, given my rudimentary knowledge of GR). As "Electricity Likes Me" said in his/her reply, it is *possible* that contrary to what is expected, anti-matter floats
      • As far as I'm aware, negative energy is still purely hypothetical. There is currently no reason to believe such a thing actually exists or is even allowed by physics.

    • by SEE (7681)

      since my understanding is that gravity is an inherent effect of mass warping space, wouldn't anti-matter possess mass in the same way that matter does, so why would gravity act differently?

      It's expected that gravity would work normally. However, we don't know that until we see antimatter fall.

      Moreover, if it didn't fall just like matter, that would be important information for constructing a new theory (which, since we still have GR and QM contradicting each other, we know we need).

    • In general relativity, gravity is a warping of space and EVERYTHING falls at exactly the same speed. This has been tested to very high accuracy in a variety of experiments. (see eotvos experiment)

      There are other theories of gravity where this doesn't necessarily need to be true and antimatter and matter might fall at different rates. The eotvos type experiments have indirectly tested this since there is some amount of virtual antimatter in normal objects (from quantum fluctuations), but a direct measurement

  • Is it black?

    Does it live under water?

  • If antimatter interacts with gravity in such a way that it "falls" up or pushes against the force like magnetic fields pushing against each other, does this mean that antimatter would make anti-gravity platforms possible?

    I'm a science plebe who watches/reads too much sci-fi, this was the first thing that came to my mind.

    • by habig (12787)

      Sure, in principle.

      However, a thorny engineering problem would be stopping the tons of antimatter holding up the platform from interacting with the normal matter around it. If it did, *boom*. That's the way you can get 100% of the "E" out of the "m" in E=mc^2.

    • Assuming it "falls up", it'd be a heck of an engineering challenge to a) produce enough antimatter, b) trap it safely and c) keep it trapped safely.

      It'd be easier to use hydrogen balloons. Not to mention safer.

    • In principal, yes... as long as you don't mind the platform spontaneously detonating and vaporizing the earth when the containment field fails.

    • by Immerman (2627577)

      Absolutely. Of course a tiny floating 100kg platform would require 100kg of antimatter, and I wouldn't want to be anywhere nearby if your EM-containment field failed. If it were a solid chunk of anitmatter it probably wouldn't be *too* bad - a small surface explosion when it contacted the platform followed by a hail of antimatter meteors falling upwards while blasting out gamma rays from air-molecule collisions. A tank of anti-H though, that would mix very quickly and release all the energy at once - and

      • The far more significant application would be that we would finally have a substance exhibiting (hopefully) negative spacetime curvature, which we could use to stabilize wormholes.

  • by idontgno (624372) on Tuesday April 30, 2013 @06:10PM (#43595551) Journal
    Negative matter DOES react to tractor beams in reverse, being repelled by the nominally attractive force.
  • I'd love it if we found out that antimatter falls upwards, but I'd be even more interested if anyone could conjecture on what that would mean.

    Could that mean that antimatter warps space in the opposite direction as matter so that it has a repelling affect?

    • by b4dc0d3r (1268512)

      I'm no particle physicist, but i can generalize.

      It would mean that we have a preliminary report on an unfinished experiment. Or more specifically, an experiment not intended to explore this subject has not ruled out the possibility.

      What this actually means to us is that experiments intended to find this result have not been proven useless already, and they could be conducted using the existing ALPHA setup. ALPHA appears to be the most successful anti-particle creation mechanism, making it the obvious plac

    • by Burz (138833)

      It might mean that as black holes evaporate by giving off Hawking radiation, they would also build up electrical charge.

  • Assuming floating up meant it to have negative mass, antimatter would have negative energy and therefore matter and antimatter with congruent weight would annihilate without any visible energy-output.
    • Re:E=mc^2 (Score:4, Interesting)

      by iggymanz (596061) on Tuesday April 30, 2013 @06:27PM (#43595761)

      we already know antimatter doesn't have "negative mass" in that sense, it responds with expected inertia to acceleration by electromagnetic forces. we already know the yield of annhilation too (relativistic mass is positive). question is just of response to gravitational field of normal matter, which way the force vector points.

      • by AJWM (19027)

        Right, we know it has positive inertial mass. We haven't yet properly observed their gravitational mass. We assume the two are equivalent; they may not be.

        Actually, physicists have antimatter all wrong. A positron actually does have a negative charge but also has negative inertial mass, so it will react to an electromagnetic field the opposite way an electron does. We just observe that as reversed charge.

        (Yes, I did just make that up, tongue firmly in cheek.)

  • As I wrote in a comment to the submission [slashdot.org]: Photons fall down. Is there such thing as an "anti-photon"?
  • Background info (Score:5, Interesting)

    by AdamHaun (43173) on Tuesday April 30, 2013 @06:29PM (#43595783) Journal

    The Usenet Physics FAQ [ucr.edu] has some background information [ucr.edu] on the theory behind this question. It's 14 years old but still worth reading. One interesting bit:

    Based on what we currently know, we would expect that the only significant force acting on a piece of falling antimatter is gravity; by the equivalence principle, this should make antimatter fall with the same acceleration as ordinary matter. However, some theories predict new, as yet unseen forces: these forces would make antimatter fall differently than matter. But in these theories, antimatter always falls slightly faster than matter; antimatter never falls up. This is because the only force that would treat matter and antimatter differently would be a vector force (mediated by the hypothetical gravivector boson). Vector forces (like electromagnetism) repel likes and attract opposites, so a gravivector force would pull antimatter down toward the matter-dominated Earth, while giving matter a slight upward push.

  • Background (Score:5, Informative)

    by Okian Warrior (537106) on Tuesday April 30, 2013 @06:45PM (#43595897) Homepage Journal

    The question of whether anti-matter experiences anti-gravity goes back as far as I can personally remember (1970's) and probably some decades before that.

    For most of the past 300 years in physics, experiment has led theory. We measure something, it leads to a theory, and then experiments are done to check the theory. Examples abound of theories that explain previous observations, and also predict something new - probably the most famous is relativity predicting the precession of Mars, but there are lots of others. (Newton predicting elliptical orbits based on the inverse square law of gravity comes to mind.)

    Since about 1970 the situation is reversed - theory has led experiment. We have a satchel of theories which attempt to explain questions in physics which have no discriminatory evidence. Theories such as "Super Symmetry", "Loop Quantum Gravity", and "String Theory". I'm reading a book right now [amazon.com] which claims 10^500 different string theories (yes, that's 10 with 500 zeroes after it), and lamenting the fact that few of these actually make testable predictions.

    Relativity predicts that anti-matter should have positive gravity, but this has never been tested.

    Until recently, the only antimatter we had access to has been charged particles: anti-protons and anti-electrons. Measuring the gravitational force on a charged particle is nigh impossible because the EM force is so large (relative to the gravitational force) that any EM effects swamp the readings. You can't just see if the particle falls in the container, because it's essentially impossible to shield a container well enough. It's like trying to measure the mass of a cork floating in a tornado.

    Anti-hydrogen would work, but until recently we had none to test. Antiparticles tend to have high velocities when produced - they have to escape their anti-nemesis which is also produced - so they have to be slowed down enough to "pair" to make the neutral antimatter particle.

    The vacuum used for the experiments has a big effect also. Depending on the level of vacuum used, any particle has a "mean free path [wikipedia.org]" before it will impinge on another particle. You have to get your anti-particles to slow down, form antimatter, and conduct the experiment before another particle comes in and annihilates it. This requires insanely good vacuum which is both hard to achieve and highly expensive.

    The ALPHA [web.cern.ch] experiment at CERN now produces antimatter, so the referenced paper asks the question: what is the ratio "F" between the inertial mass and the gravitational mass of antihydrogen? For normal matter it's 1 and for "antigravity matter" it would be -1.

    The paper reports that they have measurements within specific confidence levels that F < 110 almost certainly, and F < 75 at the 95% confidence level.

    If the experiments outlined in the paper are continued (and perhaps refined), over time they can statistically narrow the results and ultimately settle the question by experiment.

    I think that this would be a good thing, it would confirm (or contradict) by experiment something that is predicted by theory.

  • If antimatter is gravitationally repulsed by matter, then it could help explain dark matter. Instead of requiring a huge expansion of the Standard Model, it may simply be that the vacuum is gravitationally polarized.

    http://arxiv.org/pdf/1106.0847.pdf [arxiv.org]

    (I'm a big fan of Hajdukovic. Whether he's right or wrong, he asks fascination questions).

  • Ultracold so it doesnt hit the container quickly. Made at CERN too.
  • I have been thinking this for twenty years.

    Think of E=MC2.

    Faster than light travel is only impossible when you have a net positive mass. If your mass is net zero, (meaning in your magnetic grip you hold matter and antimatter in the same functional unit but not touching each other (two magnetic bottles), then you could travel faster than the speed of light.

    • by jmv (93421) on Wednesday May 01, 2013 @01:42AM (#43598097) Homepage

      Anti-matter still has a positive mass. Otherwise when a positron meets an electron it wouldn't release any energy. Personally, I highly doubt it "falls up", as that would be inconsistent with general relativity because anti-particles would not follow a curved space. What would be really cool is if it was found that anti-matter curved space in the opposite direction as matter, making gravity repulsive. I highly doubt that's the case, but it would certainly be a cool discovery.

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