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Inertial Mass Separate From Gravitational Mass? 405

Posted by Soulskill
from the what-an-interesting-mass-effect dept.
CPerdue writes with this excerpt from the MIT arXiv blog: "The equivalence principle is one of the more fascinating ideas in modern science. It asserts that gravitational mass and inertial mass are identical. Einstein put it like this: the gravitational force we experience on Earth is identical to the force we would experience were we sitting in a spaceship accelerating at 1g. Newton might have said that the m in F=ma is the same as the ms in F=Gm1m2/r^2. ... All that changes today with the extraordinary work of Endre Kajari at the University of Ulm in Germany and a few buddies. They show how it is possible to create situations in the quantum world in which the effects of inertial and gravitational mass must be different. In fact, they show that these differences can be arbitrarily large."
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Inertial Mass Separate From Gravitational Mass?

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  • by The_mad_linguist (1019680) on Monday June 14, 2010 @05:05PM (#32570820)

    Because once we have inertial drives, it's only a little while before we can colonize other planets.

    The technology lens itself very well to that.

    • by Anonymous Coward on Monday June 14, 2010 @05:07PM (#32570856)

      The technology lens itself very well to that.

      I sea what you mean.

    • I realize that this all works only at that quantum level but what implications, if any, does this have for Einstein's general theory of relativity?

      • by wurp (51446) on Monday June 14, 2010 @05:21PM (#32571062) Homepage

        General relativity is known to be incompatible with quantum mechanics. People are still trying to come up with a theory that reconciles the two.

        This is similar to the way we knew:
        * the constant speed of light (regardless of reference frame) was incompatible with the classical laws of momentum and energy [resolved by Special Relativity]
        * the equations for low energy blackbody radiation and high energy blackbody radiation were incompatible with one another [resolved by quantum mechanics]

        I haven't RTFA, but if they have something testable, I would think this means we have a basis for making quantitative measurements of what happens where GR and QM collide. (And hence a basis for coming up with a unifying theory.)

        • No GR in Article (Score:5, Insightful)

          by Roger W Moore (538166) on Monday June 14, 2010 @06:02PM (#32571698) Journal

          I would think this means we have a basis for making quantitative measurements of what happens where GR and QM collide.

          Not quite. They make no assumptions about GR in the article, what they have done is come up with a way to test one of the assumptions of GR - assuming the article passes peer review, arXiv is just a preprint server. There are too possible outcomes to the test they propose: m_i=m_g or m_i!=m_g. In the first case nothing has changed and in the second case one of GR's core assumptions has been dismantled so GR cannot be a fundamental theory since there is a phenomenon which it cannot explain. Hence QM and GR will never 'collide' because GR will have disappeared to be replaced by something else - possibly something which QM has no problem with.

          My personal guess is that any such experiment will show that m_i=m_g but it will be an interesting test to do and potentially result in a far more accurate test of the equivalence principle.

      • Re: (Score:3, Informative)

        by maxwell demon (590494)

        It allows new measurements to find potential deviation in the relation of inertial and gravitational mass. If no deviations are found, then this means nothing for general relativity (the equations would just contain the same quantity under two different names). If deviations are found, then it probably means that GR must be modified.

    • by EdZ (755139)
      The first comment is a veiled Bergenholm reference? My faith is Slashdot is restored!
  • by bfmorgan (839462) on Monday June 14, 2010 @05:07PM (#32570844)
    Would this lead to science fictions "Inertial Dampeners"?
    • by sznupi (719324) on Monday June 14, 2010 @05:16PM (#32571008) Homepage

      I will take the option of seatbelts while sitting at the bridge of your spaceship, thank you very much.

      • by Ungrounded Lightning (62228) on Monday June 14, 2010 @05:48PM (#32571470) Journal

        I will take the option of seatbelts while sitting at the bridge of your spaceship, thank you very much.

        Reverse engineering things like Star Trek to come up with plausible explanations is lots of fun.

        My take on near misses with photon torpedoes making "bang" sounds and throwing people around the bridge (besides the needs of dramatic presentation).

          - Photon torpedoes are established as matter-antimatter nuclear bombs.
          - These can be expected to produce some extreme EMP as a side-effect of their detonation and the "gamma light" from it striking any nearby matter.
          - The artificial gravity / inertial compensation for multi-G impulse engine thrust (and any oddball forces from warp drive and changes to it) has to be variable to handle such variable conditions.
          - The EMP interferes with its control mechanism. Not enough to smear the crew like paint over a nearby bulkhead. But enough for a near-miss to throw them around in their seats and rattle the ship enough to create the "bang" sound in the air. (Perhaps also the "whoosh" of a passing spacecraft, due to an electromagnetic "wake" from its systems - though that was clearly established as use of artistic license after the soundless flybys in the first trial footage were unsatisfying.)
          - The engineers made the artificial gravity system VERY reliable. (Note that it keeps working when most of the ship's mechanisms, including other life support, is on the fritz.) And they made it good enough to keep the crew largely intact through "impacts" that seriously degrade the other systems and structural integrity of the ship. But they weren't able to get it down to no noticeable effect.
          - And the designers didn't add seatbelt-equivalents until the first movie (after Admiral Kirk, done with his five-year missino, had given them hell about it.) B-)

      • I will take the option of seatbelts while sitting at the bridge of your spaceship, thank you very much.

        You'd rather be carried out in a bucket?

    • It wasn't until I started reading SF rather than just watching Star Trek that I realised how inertial dampeners would be useful. Of course being able to absorb an impact or two without turning the occupants to jelly would be nice but if you can lower the inertial mass of your spaceship can't you accelerate at ridiculous rates?

      Holidays on Nereid, here I come!

      • by Ungrounded Lightning (62228) on Monday June 14, 2010 @05:34PM (#32571258) Journal

        ... if you can lower the inertial mass of your spaceship can't you accelerate at ridiculous rates?

        See E. E. Smith's _Lensman_ series for an exploration of that.

        My own take: All bets are off since the principles are currently unknown. But assuming that things like energy conservation and action/reaction remain valid, an "inertial damper" seems likely to function as a way to transfer thrust evenly from the engines to the matter of the ship, crew, cargo, etc. (Or deliberately unevenly to achieve a convenient artificial gravity without spinning the ship.)

      • by Tetsujin (103070) on Monday June 14, 2010 @05:37PM (#32571298) Homepage Journal

        It wasn't until I started reading SF rather than just watching Star Trek that I realised how inertial dampeners would be useful. Of course being able to absorb an impact or two without turning the occupants to jelly would be nice but if you can lower the inertial mass of your spaceship can't you accelerate at ridiculous rates?

        That was a key idea in "Lensman"... (And it's a pretty silly idea, though I enjoy how the books explore the exploitation of this idea)

        Inertial dampeners don't imply that you're negating the mass of the passengers, however - just that you're translating external forces to make them also apply to the ship's contents. Whether this means some kind of accelerometer/tractor beam combo, or if you imagine some kind of pervasive force field acting to translate external forces smoothly and continuously onto everything inside the ship - the idea of an inertial dampener is beyond our technology, but it doesn't necessarily break conservation of mass.

        • Re: (Score:3, Insightful)

          by HeckRuler (1369601)
          I cannea take it no more cap'tin

          To damp - to reduce
          To dampen - to make moist

          So unless you got some quantum sponge or something, yer getting it wrong! Please use "inertia dampers" instead.
      • by Dragoniz3r (992309) on Monday June 14, 2010 @05:40PM (#32571340)
        Indeed, and in fact this concept plays a large role in the middle book of the Revelation Space series by Alastair Reynolds (which I recommend that you read, if you have not).
      • Re: (Score:3, Informative)

        by sznupi (719324)

        This was a fun read about inertia...
        http://physics.fullerton.edu/~jimw/general/inertia/index.htm [fullerton.edu]

        One of the probable explanations seems to be - inertia is equivalent to the gravitational force that acts on the body...from the rest of the Universe. With a disclaimer that this would need propagation of gravitational disturbances into and from distant future!

        Which would be...most interesting. Possibly actually strenghtening speed limits present in our Universe, with those limits being probably even more crucial

    • by AdmiralXyz (1378985) on Monday June 14, 2010 @05:26PM (#32571166)
      That's "dampers", unless you're talking about devices that make the bridge slightly moist when the ship is subject to acceleration.
    • by DarrylM (170047)

      Would this lead to science fictions "Inertial Dampeners"?

      Not only that, but if we can establish a low-level warp field around the station, then we can move it to the mouth of the wormhole really quickly.

  • by Count Fenring (669457) on Monday June 14, 2010 @05:08PM (#32570880) Homepage Journal
    I would submit, courteously, that your mother's inertial and gravitic masses are arbitrarily large.
  • Ringworld (Score:2, Interesting)

    by cats-paw (34890)

    "But we had purchased a reactionless, inertialess drive from the Outsiders. You may have guessed their price. We are still paying in installments. "

    I seem to remember that in one of his other stories, the figure is a trillion stars, which was the worth of an entire, technologically advanced, planet.

  • by gurps_npc (621217) on Monday June 14, 2010 @05:26PM (#32571156) Homepage
    The rocket equation tremdously limits maximum speed. Even with an anti-matter powered rocket, the maximum theorectical speed would be 0.1 C (1/10th the speed of light).

    In a gravity well, this explains why we need so much fuel to get out. But that assumes that inertial mass acts like gravitional mass. If we change that, then suddenly we use HIGH inertial mass but low gravitational mass as rocket exhaust, tremendously reducing the mass of the rocket's fuel, which has exponential gains in increasing the potential payload.

  • Dark matter? (Score:4, Interesting)

    by jschen (1249578) on Monday June 14, 2010 @05:31PM (#32571226)
    Assuming that these guys are right, would the presence of two different effects that we currently group together allow us to generate a model of the universe that doesn't require the vast majority of matter to exist as (currently) undetectable dark matter?
  • by hugi (878361)
    de von Ausfern -schplenden -schlitter -crasscrenbon -fried -digger -dangle -dungle -burstein -von -knacker -thrasher -apple -banger -horowitz -ticolensic -grander -knotty -spelltinkle -grandlich -grumblemeyer -spelterwasser -kürstlich -himbleeisen -bahnwagen -gutenabend -bitte -eine -nürnburger -bratwustle -gerspurten -mit -zweimache -luber -hundsfut -gumberaber -shönendanker -kalbsfleisch -mittler -raucher von Hautkopft auf Ulm would be proud of his fellow citizen.
    • Let me interrupt you, Herr de von Ausfern -schplenden -schlitter -crasscrenbon -fried -digger -dangle -dungle -burstein -von -knacker -thrasher -apple -banger -horowitz -ticolensic -grander -knotty -spelltinkle -grandlich -grumblemeyer -spelterwasser -kürstlich -himbleeisen -bahnwagen -gutenabend -bitte -eine -nürnburger -bratwustle -gerspurten -mit -zweimache -luber -hundsfut -gumberaber -shönendanker -kalbsfleisch -mittler -raucher von Hautkopft auf Ulm, and ask you, just quickly...

  • by selven (1556643) on Monday June 14, 2010 @05:35PM (#32571274)

    Consider two giant bouncyballs in space, with the same inertial mass but where ball A has 4 times the gravitational mass of ball B. They start off some distance apart from each other, with velocity 0. As they attract each other, B will be accelerating 4 times faster than A since A has 4 times the gravity, and at one point they will meet. When they meet, A will have velocity -1 and B velocity +4. When they bounce off of each other, A will, naturally, have velocity +4 and B velocity -1. Now, B is still accelerating (or rather, decelerating) toward A 4 times faster than A is toward B, and when their relative velocity reaches 0, A will have velocity +3 and B will have velocity +3. Thus, each bounce accelerates the entire system by +3 with ZERO energy input, thus violating conservation of momentum and conservation of energy.

    This is why any universe with a concept of conservation of energy and/or momentum must have the property inertial mass = gravitational mass. Now, if we can somehow break this rule with energy input, those of us interested in interstellar travel might have a completely new type of engine on our hands.

    • Re: (Score:3, Informative)

      by maxwell demon (590494)

      Consider two giant bouncyballs in space, with the same inertial mass but where ball A has 4 times the gravitational mass of ball B. They start off some distance apart from each other, with velocity 0.

      OK.

      As they attract each other, B will be accelerating 4 times faster than A since A has 4 times the gravity, and at one point they will meet.

      Wrong. B will have a four times as strong gravitational field than A, therefore A will also have four times the acceleration it would have if B had just the same gravitiat

  • When a body accelerates all its components are accelerated at the same rate. However, when body is subjected to a gravitational attraction, the part of the body nearest the attractor experiences a slightly larger attraction than is experienced by the other end of the attracted body (since the force experienced depends on the distance ** 2 from the attracting body). Unless you start talking about single point, infinitely small bodies, the difference in attraction across the gravity gradient will be real.

    S

    • by BradleyUffner (103496) on Monday June 14, 2010 @05:51PM (#32571514) Homepage

      When a body accelerates all its components are accelerated at the same rate.

      Not quite. Acceleration starts at a specific point and "pushes" its way through the object at the speed of sound in the material of the object. If you had a 10 mile long metal bar and were strong enough to shove one end, the other end wouldn't move instantly. Your force would start a compression wave along the metal bar, traveling at the speed of sound though the metal, until it reached the other end. Same with a rocket, the engines apply acceleration at their connection point and the acceleration pushes its way through the materiel. This is why they have to be built out of such strong stuff, it has to be able to withstand the compression forces of the acceleration without fracturing due to stress.

      • by petes_PoV (912422) on Monday June 14, 2010 @07:19PM (#32572620)
        Assuming the acceleration is provided by mechanical means. But if the body in question is a conductor and the accelerating field is a uniform magnetic field, the acceleration is applied to all the particles in the body at the same time and in the same amount. Provided the accelerating force is uniform, it can still, theoretically be distinguished from gravity by its lack of a gradient.
  • The claim is extra-ordinary but unfortunately the proof is not. It is well known for a long time that the equations of quantum mechanics violates equivalence principle. Precisely for this reason, we don't have satisfactory theory of quantum gravity. So there is nothing new in terms of it. If I interpreted the contents of the paper right, the authors are suggesting a way to create an experiment which can show that m_i and m_g are indeed different, but these experiments have not been performed yet.
  • by peter303 (12292) on Monday June 14, 2010 @06:03PM (#32571716)
    The key part is the null-grav Bose-condensate at the base. When the temperature falls below 91 micro-kelvins, the resulting phase-change decouples inertial mass from equivalent mass and the gravitational force disappears.

    There a few bugs to be worked out however. First, the grav-shield must be aligned within ten arc-seconds perpendicular to main gravitational body (Earth) or gravity leaks through. Second, stray cosmic rays have the disturbing habit of energizing the condensate about the phase-change temp and destroying the null-grav effect. I hope to have fixes by next week.
  • by DynaSoar (714234) on Monday June 14, 2010 @06:14PM (#32571872) Journal

    If he's right, they'll call it that "Kajari Drive". That just doesn't ring for me. We need someone else to refine this and make it go. An Archer maybe, or a Cochrane. Now those are names a real space drive can wear. Hell even inter-compartment conduits get names like Jefferies Tubes. Kajari? No way. He can have an episode of his own when they serialize history (as we know they have, so we can see it but consider it fiction thus avoiding paradox), but not the name of the drive.

  • by joeyblades (785896) on Monday June 14, 2010 @06:25PM (#32572000)

    "The equivalence principle is one of the corner stones of general relativity. Now physicists have used quantum mechanics to show how it fails."

    Alternatively, they could choose to look at this equivalent assertion: The wave-particle duality of matter is one of the cornerstones of quantum mechanics. Now physicists have used general relativity to show how quantum mechanics fails.

    Of course, in actuality, they haven't shown anything yet...

  • Cartman (Score:3, Funny)

    by Hognoxious (631665) on Tuesday June 15, 2010 @05:26AM (#32575762) Homepage Journal

    I don't have a lot of gravitational mass ... it's my bones that have a lot of inertia.

Fools ignore complexity. Pragmatists suffer it. Some can avoid it. Geniuses remove it. -- Perlis's Programming Proverb #58, SIGPLAN Notices, Sept. 1982

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