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Science

Physicists Clarify Exotic Force 86

Posted by timothy from the gravity-is-pretty-exotic-stuff dept.
Azazel writes "A research group, including Purdue University physicist Ephraim Fischbach, has completed an experiment which shows that gravity behaves exactly as Isaac Newton predicted, even at small scales. Unfortunately for those in search of the so-called "Theory of Everything," the finding would seem to rule out the exceptions to his time-honored theories that physicists believe might occur when objects are tiny enough."
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Physicists Clarify Exotic Force More

Physicists Clarify Exotic Force

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  • I know this is going to get a lot of RTFM answers (even though I did), but is this just a fancy way of saying "everything as usual"?

    • Re:Did I miss something? (Score:4, Interesting)

      by Nos. ( 179609 ) <`ac.srrekeht' `ta' `werdna'> on Monday June 20, 2005 @01:49PM (#12865511) Homepage

      Well, this is the first time I've heard of the "Casimir force" force, but reading the article linked to in the article (how's that for RTFM) explains that pretty well. Now, the prevailing "ToE" is to the best of my knowledge, string theory. This theory was developed to explain the inconsistancies with Newton's and Einstein's laws when you got down to a sub-atomic level. This study is basically saying that string theory is wrong, and that Newton and Einstein (for the most part) were right.

      Now, the interesting there here is the "Casimir force" which basically, is the force of photons striking an object. We touched on this actually in high school physics. We were experimenting to find out if light was a wave of a particle. (its a wave of particles). I started to ask questions like, if that's true, wouldn't most stationary objects eventually gain mass due to a build up of photons. We never quite got into that... probably a little advanced for most people in high school physics. Sorry, back on topic. This force, becomes very powerful (comparatively) at sub-atomic levels. The force of a particle travelling at the speed of light can become very significant. In fact, it becomes more significant than gravity. So, everything as usualy, I'm not sure I'd agree, but it hopefully does get us one step closer to the ToE.

      • Accumulation of photons (Score:5, Interesting)

        by jd ( 1658 ) <`imipak' `at' `yahoo.com'> on Monday June 20, 2005 @02:17PM (#12865764) Homepage Journal
        Actually, that is a really important point and is partially answered by the photoelectric effect predicted by Einstein. (Photons have mass, they strike electrons, this generates current, the photon is not absorbed but now has much lower energy)


        Particle/wave duality is not fully explained by thinking of light as a wave of particles, as this conflicts with observations of diffraction gratings at extremely low light intensities. It is my understanding that a "refinement" is to describe light as a single photon that exists with varying probabilities across the wave. (The wave is then a probability wave.)


        QM allows objects to exist at multiple points or in multiple states simultaneously, until directly observed. If you do try to directly observe a photon, you do indeed see a single packet of energy. But if you look only at the results, you see a wave.


        By looking at light as a probability wave, a lot of apparent paradoxes don't "go away" but do fit a lot better with other known apparent paradoxes, which (to me) indicates the phenomena are related and not distinct.


        Getting back to gravity, we could be in for an interesting dilema here. With no variations so far detected, the theory of gravity being an exchange of particles seems less likely. Einstein's model of a distorted space/time would seem to be the more probable, at this point.


        This is important, as the predicted QM model for gravity could not be compatible with Enstein's model of gravity. They could not coexist, one had to be wrong. At this point, it seems likely that the particle-exchange model is the one that is wrong, which means QM in its eventual form will likely not be 100% particle based. It may need to be a heterogenius model.


        As an aside, let us assume gravity does bend space/time. Since information cannot travel infinitely fast, and as no two events can occur simultaneously, when a massive object moves, space cannot restore itself the moment the object has left. Thus, there must be something analogous to a restoring force within space/time, and therefore some parallel to Hooke's Law.


        By implication, an object moving fast enough should leave a trail, where the effect of gravity on space/time is apparent, even though there is no longer any source of that gravity present. A massive-enough object may even leave some sort of "wake", similar to that of a boat, only in gravity rather than in water.


        Hooke predicts an upper limit to expansion, though. Something stretched beyond a certain point cannot be restored to its original dimensions, but will rather be restored to some other state, with a much lower restoring force existing.


        By implication, a sufficiently massive black hole should result in a region of permanently deformed space/time, as the expansion would exceed the Universe's ability to restore.


        As far as I know, no such "massless holes" have been found, but the more the Einstinian model is verified, the more certain I am that such a thing must exist.

        • Wow, loved that comment. I had never thought of the "wake" effect of a massive object moving (or disappearing) and space/time not being able to completely restore itself afterwards. Even though my understanding of physics is quite limited (high school level plus some reading and disucssions since then), I do tend to grasp most concepts, and the idea that gravity was a force travelling as a particle never quite felt right. However, the idea that a massive object could bend space/time did.

          I'm part way thr

          • Hawking currently favors the idea of "brane"s. His analogy is that our universe may be like a bubble in boiling water. Other bubbles/universes are out there in the 'water', whatever that may be. I got to hear him give a presentation a couple years ago and it sounded as if he thought universes may interact in some way (collisions, etc.).

        • What? (Score:5, Informative)

          by HerbieTMac ( 17830 ) <5excelroa001@sneakemail.com> on Monday June 20, 2005 @03:04PM (#12866208)
          Sorry, but you are misinformed. Gravity does not warp space-time, gravity is the warping of space-time.

          So, no, you will not see a "wake" of gravity because you are an observer, you will be affected by the gravity of the object at a point. Since the object itself cannot move faster than the speed of light, the gravity well will always be able to restore faster than the object moves.

          You may be thinking of frame-dragging, which is a different phenomenon.

          BTW, what moderator decided that this comment was "Interesting"? What I wouldn't give for a "-1, Uninformed" mod.

          • Re:What? (Score:3, Informative)

            by jd ( 1658 )
            In order for that to be true, you must assume certain other things to be true:
            • Space/time can bend without any means of exchanging information with the object (without some exchange of information, space/time would not be affected by any mass within it). Both the exchange of information and the consequence are referred to as gravity, but it would be more correct to describe the latter as a gravitational well or gravitational field, as distinct from gravity itself.
            • Two events can occur simultaneously (t

            • Re:What? (Score:3, Informative)

              by HerbieTMac ( 17830 )
              Sorry but you are confusing things.
              • As an observer, you cannot perceive more than one point of gravity (or any other light-speed wave) at a time. In other words, you perceive the effects of gravity as a point source.
              • If you want to argue that space and time are quantized, I am afraid that I cannot engage you in a discussion as you will have to assume a radical reworking of mathematical analysis to fit observational data. If you have some reason to assume this quantization, I will listen, otherwise you ar

              • Re:What? (Score:5, Interesting)

                by jd ( 1658 ) <`imipak' `at' `yahoo.com'> on Monday June 20, 2005 @05:18PM (#12867352) Homepage Journal
                Let's start with the easy one: Is space/time quantized? To test this, you must point a telescope at a great enough distance that the angle produced by the increment is clearly visible and clearly uniform. Have telescopes seen clearly visible and "unnaturally" uniform regions of space? Uh, yes. That's exactly what is seen in the early Universe, despite the obvious problems this would create (such as no way for structures to form). The solution to this problem is to say that we are seeing a small enough region of quantized space that variation is impossible, that we are not seeing the variation that must exist because we aren't looking at a continuun. Ergo, yes, we have observations that can best be explained by quantized space/time.

                Now onto the rest of my post: The physicists at Warwick University, for the Einstein Celebration, considered my theories on relativity to actually be pretty good. :) I had produced a summary of the derivation of relativity and from that derived what overall physical phenomena must underpin the entire theory.

                The quantization of space/time is guaranteed. Why? Because matter is quantized, and matter and energy are simply different facets of the same thing, energy must be quantized. (Matter is merely condensed energy, it is therefore the same stuff, just in a different state.)

                If energy is quantized, then fields must be quantized, as fields define energy. If fields are quantized, then space and time are quantized, as fields are defined over these.

                The scale of quantization is extremely small. A Higg's Particle is the smallest unit of matter definable, but in order to have energy to condense, the scale on which a photon itself exists must be smaller still. There may well be smaller particles in the "quantum foam", which is fine as they don't have to be stable. The Higg's Particle is stable and is likely the smallest object that can be stable.

                What does that give us for scale, though? Without knowing even the theoretical mass of the Higg's Particle, that is hard to even guess at, but a guesstimate based on existing data would imply quantization of space at around about 10^-50 m, and something comparable for time.

                No, you do not perceive gravity as a point source, because you are not a point. That is why objects in a gravity well will stretch. EACH point of you will experience gravity differently and not from "one source" but rather from the composite value.

                (If you are between Earth and the moon, you will experience gravity from each. At the right point, you will be held stationary because the interference will produce no net force. If that were not the case, the Universe would be in serious trouble. As would most of physics, as a lot depends on overlapping fields.)

                We are talking about energy differentials. It is a grave mistake of the first order to distinguish between phenomena that are, in fact, the same thing. All objects travel at the speed of light, at different angles to space/time. It is the angle that produces relativistic mass, reliativistic length and relativistic time.

                This can be proved by simple trigonometry, and is likely where Einstein got the equations in the first place. Relativity is just a restating of Pythagoras, as all equations are based on the same formula: R' = sqrt(1 - v^2/c^2), where R' is the relativistic version of the variable of interest.

                When re-written, this becomes R'^2 + V^2 = C^2, which is basically Pythagoras.

                However, the consequence of this is both simple and profound. If all objects indeed travel at the speed of light, at different angles to space/time, and indeed relativity is nothing more than the projection onto the plane of interest, then gravity is a direct consequence of this motion through space/time.

                (Relativistic mass, by this logic, is simply a force exerted at 90' to space. The distortion of space is the result of this. This means tha

                • Re:What? (Score:1, Flamebait)

                  by HerbieTMac ( 17830 )
                  Sorry there, little dude. You are obviously more creative than most cranks but you still score pretty high on Baez's 17pt scale. Let's see if we can't point you in the right direction.

                  Starting with some algebra. If R'=sqrt(1-v^2/c^2), then (R'^2)*c^2+v^2=c^2. You forgot a term. Remember this sequence: simple math FIRST, then general relativity. The other way around just needlessly complicates things.

                  Second, your formula doesn't even support your statement that all objects travel at the speed of lig

                  • Re:What? (Score:2, Interesting)

                    by mattpalmer1086 ( 707360 )
                    Hmmm... just finished reading an excellent book about cosmology (The Fabric of the Cosmos, by Brian Greene). In it, the author clearly states that all objects are constantly moving at the speed of light through space-time.

                    If you are stationary in space, then you are "moving" at the speed of light through time. Any motion through space reduces your velocity through time - but it always adds up to the speed of light.

                    I must admit, that idea made me stop and re-read that section of the book a couple of tim

                    • Re:What? (Score:2, Insightful)

                      by AKAImBatman ( 238306 ) *
                      If you are stationary in space, then you are "moving" at the speed of light through time. Any motion through space reduces your velocity through time - but it always adds up to the speed of light.

                      That's more or less what relativity says. It's not so much that your velocity always adds up to the speed of light, however, as it is that we are travelling on a four dimensional vector. i.e. Just as a car traveling in a diagonal path has a slower southward velocity than a car travelling at the same speed but hea
                  • Second, your formula doesn't even support your statement that all objects travel at the speed of light. This is a nonsensical statement and I suppose it only figures that it needs nonsensical math to back it up, but wow you take it to extremes.

                    Actually, this has become a fairly standard and accepted way of formulating SR from a simple geometric viewpoint.

                    It's okay to criticize people's theories, but if you're going to insult them and call them a "crank", at least make sure you're completely correct first
                  • Yeah, he blew the description by getting all Alex Chiuy on us.

                    30 second version.

                    Einstein postulated that the speed of light is the same for all oberservers. With this and the Pythagorean formula you can find the time dialation.

                    Dude sits in a box that has a device that emits a single photon from the floor towards the top. From his perspective the box is C*Tdude in height. Speed-o-light times the time it took for a single photon to travel from the bottom to the top of the box as mesured by the dudes cloc

          • Re:What? (Score:4, Interesting)

            by Curtman ( 556920 ) on Monday June 20, 2005 @04:57PM (#12867186)
            "BTW, what moderator decided that this comment was "Interesting"?"

            What deity bestowed the ultimate truth and power to judge the value of opinions upon you? I found the comment interesting. This is a great forum for discussion of news items, and that is what I come here for.

            What I wouldn't give for a "-5, Callous Pedantism" mod.
            • Yeah, you're right man. That last comment was uncalled for. Consider it withdrawn.
              • I know how hard it is to keep from getting caught up in these discussions sometimes. I do it far too often myself. Thanks for clarifying, Let me know if I step out of line too please. :)

        • Photons have mass? (Score:5, Informative)

          by Nasarius ( 593729 ) on Monday June 20, 2005 @03:26PM (#12866408)
          Photons have mass

          No! Photons have momentum. This does not imply that they have mass.

          • Re:Photons have mass? (Score:1, Informative)

            by Moekandu ( 300763 )
            Photons have mass

            No! Photons have momentum. This does not imply that they have mass.

            Actually, it more than simply implies that they have mass. You cannot have momentum without mass. You can have velocity without mass (in the case of neutrinos, I think), but not momentum. In fact, this is how you calculate momentum:

            p = mv

            p = momentum
            m = mass
            v = velocity

            • Firstly, neutrinos have mass. Secondly, you're using Newtonian calculations for relativistic speeds, which doesn't always work.
            • You're about 100 years behind. Newton's equations were amended by Einstein, Planck, and others.

              See Ask A Scientist [anl.gov].

            • Re:Photons have mass? (Score:5, Informative)

              by WaterBreath ( 812358 ) on Monday June 20, 2005 @04:46PM (#12867106)
              In fact, this is how you calculate momentum...
              Not for photons.

              This is how you calculate momentum for photons:

              p = h / lambda, where lambda is wavelength.

              Alternatively:

              p = hf / c, where E is energy, and f is frequency.

              More info here:
              http://scienceworld.wolfram.com/physics/Photon.htm l [wolfram.com]

              And here:
              http://scienceworld.wolfram.com/physics/Energy.htm l [wolfram.com]

              You can "back-calculate" a supposed mass for a photon, once you know its momentum, by using the p = mv equation. But this often called a "fictional" mass, because it is purely relativistic. If you took away a photon's speed, it would have neither mass nor momentum, and would essentially cease to exist. Mass as an fundamental physical quantity exists even in the absence of velocity. This cannot happen with a photon...

              Unless you subscribe to the view that photons do not always travel at c in vacuum. But I will not argue that here. Not enough space, and I don't want to be in a flamewar.
              • Er... ignore the "where E is energy" part.
          • Maybe it doesn't imply, but experimentation and observation suggests they do. For example, blackholes create such a powerful gravity well, that light cannot escape. If photons do not have mass, why are they affected by gravity. And its not just blackholes, light can be seen to "bend" around large objects like planets. Unless I'm mistaken, the general belief is that a photon does have mass.
            • They are affected because gravity is the warping of space. A black hole is a sign of very strongly curved space, therefore photons are affected by gravity.

            • Re:Photons have mass? (Score:3, Informative)

              by doppe1 ( 856394 )
              It is not generally accepted that photons have mass, they are generally believed to have no mass.

              The bending of light around large objects is not due to the planet excerting a force due to gravity on the photon, but instead the presence of the planet bending the space-time around the planet, then the photon travels in a straight line through this curved space-time.

              This means that the photon does not need to have mass to be bent by light.

              • I thought that mass and energy were interchangeable. E=MC^2? Wouldn't the energy of a photon curve space like a mass would? Granted it would be small...
                • Yes. Photons do, in fact, "gravitate" in the sense you're speaking of.
                • No, the equation you have quoted you have misinterpreted.

                  The mass defined here is the effective mass, the full equation is E^2=(pc)^2 + mc^2. Where m = m_0 {sqrt[1-(v/c)^2]}^(-1), and m_0 is the rest mass, the mass a particle has when it is not moving. So for a particle with mass, at rest, then the equation is E=m_0 c^2. But for a photon, which doesn't have mass, m_0=0, and the second term is zero, all its energy comes from the first term, so E=pc, momentum times the speed of light.

                  If a photon had mass

            • Unless I'm mistaken, the general belief is that a photon does have mass.

              No offense intended, but you are mistaken.

              If photons do not have mass, why are they affected by gravity?

              According to relativity, gravity bends space. It doesn't act directly on other mass. Rather space acts on mass, by telling "how to move", which is along paths called "geodesics". A geodesic is a path demarking the "shape" of spacetime in a region. Light moves along geodesics, which is basically a way of saying that it perc

          • Photons have no rest mass. Rest mass is the magnitude or length of the energy-momentum pseudotensor. Whether it has mass depends on how you define mass. If you use a semi-classical definition of mass as that which resists motion and creates gravity, then mass is energy, which photons have. But a lot of physicists use mass to mean rest mass, which is 0 for a photon. If you try to decelerate a photon, it keeps moving at c, but it decreases in frequency. If you try to bring it to a rest, you will red-shift it
      • No, no, no, no. String theory is attempting to explain the observations of angular momentum of hadrons. Specifically that they appeared to be directly proportional to the square of their energies.

        Casimir force has nothing, let me repeat, nothing to do with the force of photons striking an object. You are undoubtedly confused and I can't even begin to guess from where you gleaned this information. The Casimir force arises from the relative density of quantum vacuum fluctuations of the space between two

        • Re:Did I miss something? (Score:3, Informative)

          by Nos. ( 179609 )

          You are undoubtedly confused and I can't even begin to guess from where you gleaned this information

          Well, as I said, I read the article about Casimir force linked to in the original article ( [purdue.edu] http://news.uns.purdue.edu/UNS/html4ever/030811.F ischbach.casimir.html [purdue.edu]) which contains this paragraph:
          The Casimir force has to do with the minute pressure that real and virtual photons of light exert when they bump against an object. High quantities of photons are constantly striking you from all directions, emitt

          • Sorry for jumping on you like that. Fishbach must have been misquoted by the journalist because that is a blatantly false statement. Although the Casimir effect calculations above absolute 0 do take into account radiation, that is a correcting factor, not the effect itself. For a lay explanation of the Casimir effect, see this article on Physics Web [physicsweb.org].
      • We were experimenting to find out if light was a wave [or] a particle. (its a wave of particles).
        Light is definitely not a wave of particles. In fact, light is the propogation of two orthogonal (perpendicular) oscillating fields, one electric, and the other magnetic, through space, with the oscillations being centered about a point traveling at speed c.

    • Re:Did I miss something? (Score:4, Insightful)

      by TripMaster Monkey ( 862126 ) * on Monday June 20, 2005 @01:50PM (#12865525)


      Yes and no...what's noteworthy about this experiment is what they didn't find. Much like the Michelson-Morley experiment [wikipedia.org] in 1887, which set out measure the 'aether', and instead failed utterly to detect any such thing, this experiment was devised to detect exceptions to the behavior of gravity on a quantum scale, and found no such exceptions.

      Ephraim's not giving up yet, though...he plans on developing another experimental apparatus that is a million times more sensitive than the one that was used in this experiment. Also, even though this experiment was nominally a 'failure', the fringe benefit of clarifying the Casmir force is a big success.

  • This sound slike common sence. But hey, I'm just a computer geek.

    • The problem is there are two different common senses in effect. One the one hand we know gravity exists, so common sense says it should exists everywhere. On the other hand there is no explination to account for gravity (We can measure it, but cannot explain it), and the some theories that when looked at alone seem to imply that gravity doesn't apply.

      So they have proved one side right, but only at the expense of making some otherwise well tested theories fail.

      Gravity is easy for the uneducated masse

  • Gravity at small length scales (Score:5, Informative)

    by dr. loser ( 238229 ) on Monday June 20, 2005 @01:54PM (#12865579)
    IAAP (I am a physicist), and here's the deal:

    There are suggestions out there that one way to test for the existence of extra "compactified" spatial dimensions (the kind of stuff needed in string theories) is to look for deviations from Newton's 1/r^2 gravity at small distance scales. See, for example, here [lanl.gov].

    The problem is, it's very hard to measure just the gravitational interaction between two objects separated at micron scales. Gravity is incredibly weak compared to common forces like electrostatics and magnetic interactions, and even more exotic things like Casimir forces (related to the van der Waals interaction).

    The Purdue team has shown that the measured Casimir force in their experiment acts just as expected, setting a new limit on how screwy gravity can be at these distance scales.

    For what it's worth, there are two other big efforts in this area. The one at Stanford [stanford.edu] is led by Aharon Kapitulnik, and is so sensitive that their apparatus can detect the different forces on Au and Si in the earth's magnetic field due to diamagnetism (!). The one at Washington [washington.edu] is reportedly even more sensitive, and there are rumors [blogspot.com]circulating that they may have seen something exciting.

    The really cool thing here is how table-top solid state experiments may have something profound to say about high energy physics, without any big accelerators.
    • Well **** me! I was going to call your bluff on the van der Waals force being related to the Casimir force but wisely I did a bit of web searching first and found that they are related and that this has been known since 1955. On the one hand I've studied quantum field theory and read papers on the Casimir force, and on the other hand I've worked with computational chemists who put the vdW force into their models all the time. But I had no clue these things were related. I had merely assumed that the vdW for
      • Try this [wikipedia.org], particularly the external link to the 1999 hep-th paper.

        In short, when you assume "action at a distance" and calculate the instantaneous forces between fluctuating dipoles, you get the van der Waals interaction. When you do full local treatment of the quantum EM fields, including retardation effects, you get the Casimir force.

    • Correction to the above.... (Score:5, Informative)

      by dr. loser ( 238229 ) on Monday June 20, 2005 @02:32PM (#12865871)
      I succeeded in tracking down the actual paper [aip.org] from the Purdue folks. What they've really done is come up with a clever experimental scheme that measures the gravitational interaction independent of the Casimir force - basically it's a background-free measurement. Very slick.

    • Free Link (Score:1, Informative)

      by Anonymous Coward
      Physical Review Letters is pricey. Here's the free copy off arXiv [arxiv.org]. Anonymous for no karma whoring.
    • I was under the impression that under string theory, gravity didn't deviate significantly from classical until you got down near the Planck scale. So if I interpret the article right, they have ruled out some TOEs. But if I've interpreted what I've read about strings correctly, this experiment probably can't rule them out.

      Or am I all wet?

    • Re:Gravity at small length scales (Score:5, Informative)

      by thermopile ( 571680 ) on Monday June 20, 2005 @03:41PM (#12866557) Homepage
      IAAAP (I Am Also A Physicist), and let me (humbly -- your explanation was really good) add some more meat to your description.

      Physicists have a really, really hard time explaining *why* gravity is 10^42 times weaker than all other forces. (If you really want to split hairs, it's about 10^38 times weaker than the Weak Force, but what's an order of magnitude among friends?) Gravity appears to be a completely different manifestation than the electromagnetic, weak, and strong forces of nature. This irks many, and they try to rectify that by a Grand Unifying Theory (GUT).

      One recent shot at explaining all this was well laid out in this article in Physics Today [physicstoday.org] (subscription required, sorry) from 2002. In short, it theorized that gravity exists in 11 dimensions, not just 3, over short distances. Over some distance, the force known as gravity would "collapse" back down to our traditional 3. The fact that it acted over 11 dimensions, not 3, made gravity drop off as something like 1/r^10. This could help explain the apparent weakness of gravity.

      IIRC, the authors predicted that gravity would get measurably stronger at small distances, as it was acting in many dimensions at once. Towards the upper end of their estimates, they predicted that gravity could be measurably stronger at distances around 3-5 millimeters.

      As I read this latest discovery, it appears to throw water on that attempt to unify gravity with everything else. Back to the drawing board.

      • "they predicted that gravity could be measurably stronger at distances around 3-5 millimeters"

        Wouldn't gravity changing at 3-5 millimeters mean that people would notice its effects in everyday life?

        • Re:Gravity at small length scales (Score:1, Interesting)

          by Anonymous Coward
          Nope. The only gravity you notice in everyday life is from the Earth, with the Moon and Sun causing tides but not otherwise making any noticable difference. All these things are huge, and their centres (where the net total of all their gravity comes from) are far away. Gravity from nearby stuff is pretty insignificant - you can go next to 10000 kilograms of lead and you don't end up standing at a weird angle (although a sensitive scientific instrument can measure the gravity from that much mass fairly easil
      • As far as I can tell, this result rules out extra dimensions larger than about the width of a atom. While string theory allows extra dimensions on this scale, the "natural" scale is still fifteen orders of magnitude smaller. This result doesn't seem to rule out string theory, just an enticing (but unnecessary) possibility- "large" extra dimensions.

        Can anyone familiar with this experiment elaborate?

  • Yes. . .everythings normal BUT. . . (Score:3, Insightful)

    by robotkid ( 681905 ) <alanc2052@nospAm.yahoo.com> on Monday June 20, 2005 @02:00PM (#12865627)
    The reason why this is interesting is not because "oh look, gravity still works". Most physicists have no doubt that whenever you test it, gravity will still work. In that sense, it is kindof what another poster said, "everything's normal, big deal!"

    But what's eating all the theorists is that they have absolutely no idea why. The venerable laws of gravitation are empirical, in the sense that noone knows where it comes from other than the fact that it is associated with mass. All the other forces of nature have a quantum explanation, and have a particle that transmits them (most notably electromagnetism and photons). Noone has been able to satisfactorily reconcile gravity with any fundamental (quantum mechanical) nature of a particle.

    It's almost scary that we know more about what binds subatomic particles together than what keeps the moon orbiting the earth. It's also ironic that most people's only introduction to physics is newtonian physics which is presented in textbooks as complete and understood. It's true we have the math to predict the effect of gravity to arbitrary precision, but I'm sure engineers can back me up that just because something has a robust empirical law doesn't mean anyone really understands how it works.

    • It's always seemed to me that we won't understand gravity until we understand mass and inertia. And we won't understand mass an inertia until we understand time.

      That's my non-insightful insight for the day...
      • For those of you scoffing at the notion of Time being an unknown, I'd like to point out the following strangeness about Time:
        1) Between 8am and 5pm, Time is slower than between 5pm and 8am.
        2) Friday, 5m - Monday 8am goes by a lightning speed compared to "Normal Time"
        3) One year passes faster than 365.4 days
        4) Right now, it's 9:30pm, and I'm wasting precious Time on this useless post.
        4a) So I *seriously* hope you are too!
    • Bravo, the more I learn, the more I am annoyed with the way things were taught to me in school. There are so many things that are taught as a complete and proven body of knowledge as opposed to pointing out the very interesting holes where we don't understand what is going on. Maybe students would be more interested if they understood not everything has been done.
      • I wholeheartedly agree. I think in gradeschool the emphasis is always on learning facts instead of concepts especially where science is concerned. . . I'm certain we'd have alot more interest in science if we could convey just how much we don't know instead of giving the impression that everything's been worked out and it's all unimportant minutiae you could always look up in the encyclopedia if you were really desperate. . .

        I was floored on my first day of intro physics when the professor told us it's

        • was floored on my first day of intro physics when the professor told us it's just an assumption that intertial mass was the same as gravitational mass. It's been shown to 10 decimal places or so but not formally proven (I think it stems from the same inability to describe gravity that the original post was about). My college profs were always very good about pointing out what we do and don't know. my only beef there is that only the physics majors get the good profs who tell us this stuff. . someone who w

          • I certainly agree everyone shouldn't be taught as if they were physicists. And engineers do care about completely different things than physicists, granted. But they way it is now, they don't even speak each other's language.

            I guess my problem is that physicists, especially in academia, are encouraged to pursue things with no obvious practical use. Excessive attention to practicality is viewed as pandering for funding. Engineers, on the other hand, are taught the complete opposite. . .practicality is k

            • As for "teaching only as much as a major needs", that's the exact problem right there. How does a junior physics professor who studies general relativity have any idea at all what type of physics a neurobiologist is going to find useful? He doesn't, so they learn how balls bounce instead of how an MRI works.

              Obviously any school worth your money is going to try and place the right professors for the right job. Look at the factulty that the my school has for biomedical physics. Doctors in addition to

        • I was lucky to start school in the 1970s, and continue in schools (mostly public) which kept trying to develop students' ability to find our own facts, compare them, and compare them to our own experience. Teaching us to learn. So I find that I have learned to remember many facts, but that I'm even better at finding new ones, or rediscovering forgotten ones. With the Internet, and lots of other informed people with whom to discuss, the problem is not so much one of memory anymore, but a good BS detector. An
    • It's almost scary that we know more about what binds subatomic particles together than what keeps the moon orbiting the earth.

      If you find that almost scary then do you almost cream your pants considering that we know more about what binds subatomic particles together than the nature of our own mind? I mean we do "know" that there is a certain relationship between physical phenomena and psychical events, but yet we don't know what thoughts are and how it is even possible that the "I" can create them.

      I
  • In laymans terms: It's been a big topic in physics as to exactly how small a scale you have to go to before the quantum forces take over from gravity. All this experiment showed is that it's smaller than where these folks were looking.

    So no big breakthrough, but it is nice that they narrowed down the field of search a bit.

    • Nope - this is nothing to do with when quantum effects "take over" from gravity. The reverse, in fact - those forces already swamp the measurement of such a tiny force as gravity - they had to find ways to rule them out to reveal the gravitational influence.

      The experiment is looking for evidence that gravity does not follow Newton's law at very small scales. This is predicted by some theories (notably string theory). Confirmation that gravity behaves "normally" up to these atomic scales rules out some t

  • Explaining Gravity (Score:3, Interesting)

    by Squiffy ( 242681 ) on Monday June 20, 2005 @02:33PM (#12865879) Homepage
    Slightly off-topic:

    So they still haven't observed the graviton and are still having trouble explaining why.

    What I'd like to know is, why aren't physicists trying harder to explain gravity as a "pseudo-force" like the centrifugal "force" and the Coriolis effect? That's not just a rhetorical question. What makes physicists so sure that the graviton even exists? I trust that there must be some deep reasoning involved -- what is it?

    • Re:Explaining Gravity (Score:1, Informative)

      by Anonymous Coward
      Maybe because they move too fast.

      The minimum observed limit on the speed of gravity is >= 2*10^10c.

      http://www.ldolphin.org/vanFlandern/gravityspeed.h tml [ldolphin.org]

      Yes, 20 billion times the speed of light.

    • Re:Explaining Gravity (Score:5, Informative)

      by jpflip ( 670957 ) on Monday June 20, 2005 @03:48PM (#12866627)
      You're right that we've never observed a graviton. However, most physicists would say that this is hardly a surprise. There's no trouble explaining why - any effects of quantum gravity (any behavior where you'd have to know about gravitons and not just about general relativity) probably shouldn't kick in until the Planck energy scale (the energy scale associated with the observed strength of the gravitational force), which is something like 10^16 times greater than any energy ever achieved in an accelerator. Some theorists have come up with ways in which quantum gravity effects become manifest at lower energies (such as the extra-dimension theories the experiments in this post are designed to test), but your naive guess would be that we shouldn't have seen quantum gravity yet.

      What you describe (gravity as pseudoforce) is actually something like the way gravity works in general relativity. In that theory, mass warps the fabric of spacetime. Objects travel in the straightest lines they can in this curved space, and we perceive the bends in those paths as being because of a "force" between masses. This theory has been extremely successful in explaining all sorts of large-scale phenomena (not to mention the fact that it is very theoretically beautiful).

      The problem is that general relativity and quantum field theory (the theoretical framework of "particles" being exchanged that works so well for the other forces) seem to be fundamentally incompatible. General relativity is fundamentally a theory of the way the geometry of spacetime changes. Field theory is formulated on a pre-existing, static background spacetime. You get into mathematical trouble however you try to get these together.

      You can continue in (at least) two ways. Particle physicists are usually more inclined to think that the field theory point of view is fundamental, and that whole geometry thing is just the way things look on large scales. This leads to string theory and the usual discussion of gravitons. If you treat the geometric point of view as more fundamental, you try quantizing spacetime and get loop quantum gravity. String theory is more popular, but no one knows what the right answer is (both may even be different points of view on the same thing!).
      • What about this [calphysics.org]? These people seem to think gravity might be described as a side effect of electromagnetism.

      • String theory (Score:3, Interesting)

        by Pseudonym ( 62607 )

        Nobody has ever adequately explained to me why string theory is popular. It isn't actually a "theory", since it hasn't made any testable predictions. There is no problem which it has (yet) solved. Its desirable features (e.g. supersymmetry) are not known to be useful in the first place. Even its motivating examples don't seem to fit the theory. (Hadrons look like strings, but no known string models look anything like hadrons.)

        As far as I can tell, the argument seems to be that high-energy physicists n

        • Re:String theory (Score:3, Interesting)

          by jpflip ( 670957 )
          There is definitely a certain amount of what you describe. Particle theorists need something to do, and string theory is the best game in town. It has essentially no testable predictions currently, and it was motivated originally by hadron physics (which the current implementations have nothing to do with).

          I think the thing that really got people excited about string theory was the fact that it's a quantum theory of gravity that works at all. That's pretty powerful, since people had been trying crazy t
    • I think the terminology is throwing you off.

      Centrifugal "force" and the Coriolis "force" are not real forces because in both cases you would see it's just inertia and nothing more if you were observing from the proper inertial frame . It just looks like an extra force when we make the assumption that our rotating Earth is an inertial frame or that the merry-go-round we are on is an inertial frame (both cases are rotating which means they are being accellerated and are therefore improper frames). So the

  • exotic force? (Score:1, Offtopic)

    by BortQ ( 468164 )
    And here I thought the article would be talking about the exotic force of latin women...
    • These are physicists. They have only theoretical experince with women.
      • A friend of mine (whom I'm pretty sure only had theoretical knowledge of women) often liked to quip (in a yoda voice)

        "social life. . heh. . .women. ..heh. . .a physicist craves not these things.. ."

        I think he went on to say econ majors were the dark side of the force but he was usually drowned out by hysteric laughter at this point.

        I have to tell you, though, it doesn't help that most of the women in physics I knew were completely asexual. Except for the one diamond in the rough that was an avid

  • Brian Greene loses a "brane" cell...

    http://en.wikipedia.org/wiki/Brian_Greene [wikipedia.org]

  • IAAP, and just skimmed the PRL on this. I'm a bit surprised to see they have only found (for $\lambda\approx 200nm$) that $\alpha\leq 10^{12}$. Here it's defined through $V(r)=V_N(r)[1+\alpha e^{-r/\lambda}]$ where V_N(r) is the expected Newtonian gravity.

    So, as I see it, they've shown that this "other" interaction is less than a million million times stronger than Newtonian gravity, right? Until $\alpha \approx 1$, I wouldn't say they've "shown that gravity behaves exactly as Isaac Newton predicted". This is interesting, of course, but there's a long way to go. Fortunately they conclude their Letter by saying they expect to be able to get limits on $\alpha$ down to 10^6 with $\lambda\approx 100nm$. We'll be looking forward to it.

    As a side note, it'd be really nice if /. learned to render TeX for any non-physicists who might be reading this.

    • The clever bit is that they've come up with an experimental technique that let's them avoid having Casimir forces contribute a background to their measurements of G.

      You're right about TeX. I suppose we can wait for /. to start supporting MathML....

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