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Inertial Mass Separate From Gravitational Mass? 405

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 maxwell demon ( 590494 ) on Monday June 14, 2010 @05:43PM (#32571406) Journal

    The law of gravity says that fat people are more attractive than thin ones.

  • Re:Quantum (Score:3, Insightful)

    by Dragoniz3r ( 992309 ) on Monday June 14, 2010 @05:45PM (#32571426)
    Yeah, I mean, quantum tunneling? Quantum confinement? Those effects totally just cancel out and never do us any good!

    C'mon guys. I've never seen a response so short-sighted as to discard a physics breakthrough so quickly.
  • 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 MozeeToby ( 1163751 ) on Monday June 14, 2010 @06:01PM (#32571690)

    Seriously, how did that get rated up? Do the mods just say "oh that sounds interesting" and mod it up without even looking at the links or think about what the person is saying? Yes, I'm sure some random guy on the internet has come up with a convenient, easy, reproducible way to produce an anti-gravity device and it somehow slipped our attention. Thanks for filling us in GP!

  • 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.

  • I'm no Einstein (Score:1, Insightful)

    by rudy_wayne ( 414635 ) on Monday June 14, 2010 @06:35PM (#32572102)

    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.

    1g is a measurement we made up. Since we define it as being equal to the gravity we normally feel, then yes, the gravity we feel on earth is the same as a spaceship accelerating at 1g.

    I'm no Einstein, but, DUH!!

  • Re:Quantum (Score:1, Insightful)

    by Anonymous Coward on Monday June 14, 2010 @06:50PM (#32572310)

    Good post, but one problem I would like to address:

    Changing inertial mass would do pretty weird things to rotating flywheels. I suppose you could make a spinning flywheel break apart with immense violence at a very low rotational speed. Or rotate a spinning flywheel at insane speeds without it flying apart. All at the same stored energy level. Theres probably a perpetual motion machine that would involve extracting constant energy at a constant torque at high vs low RPMs.

    Best I can tell, you're describing flywheels that are held together by gravity. This is generally not the case for modern flywheels, which rely on chemical and/or metallurgical bonds, i.e. electromagnetically-based, and not gravitational attraction, to stay in one piece.

    As for the perpetual motion machine idea, the simplest one I can think of does not involve rotating units (except maybe for a crankshaft which is not part of the mass-modification system). It would work like so:

    • 1. Reduce the gravitational mass of a vertically-oriented piston.
    • 2. Lift the piston to its highest point.
    • 3. Restore the gravitational mass of the piston.
    • 4. The piston falls, transforming its potential energy to kinetic energy.
    • 5. Repeat.

    Note that the energy consumed in steps 1 and 2 needs to be less than the energy delivered in step 4 for this to be a perpetual motion machine. My bet is that it isn't. And even if it seems like it is, you're probably just measuring it wrong. If anything, there may be a transfer of energy from the Earth (or wherever you're getting your gravity from) to the device in step 3, much like how a spacecraft executing a gravitational slingshot maneuver actually transfers momentum from the planet/star/whatever it is using to itself. TANSTAAFL.

    That said, just because there is no perpetual motion doesn't mean it isn't useful. Anything that makes it easier to get out of this gravity well will be appreciated.

  • by jfengel ( 409917 ) on Monday June 14, 2010 @06:52PM (#32572344) Homepage Journal

    The process of science goes back and forth between theory and experiment. The theory step is important, since it helps guide experiment.

    So it's not "just" a pretty theory, in the sense of one that sits on the shelf and doesn't do anything. It makes prescriptions; it's participating in the back-and-forth between theoreticians and experimentalists.

  • by electricprof ( 1410233 ) on Monday June 14, 2010 @10:29PM (#32574092)
    What I really want to know is ... which makes my ass look bigger? Inertial? Gravitational?
  • by MichaelSmith ( 789609 ) on Monday June 14, 2010 @11:25PM (#32574420) Homepage Journal

    ... things like Star Trek ... weren't engineered to begin with

    Actually, they were. FAR more so than any previous scifi to come out of Hollywood. (And note the SciFi / SF distinction. Star Trek is much closer to SF than just about anything "studio" before Babylon 5.)


    Silent Running

    Logans run


  • by HeckRuler ( 1369601 ) on Tuesday June 15, 2010 @11:21AM (#32578838)
    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.

Every little picofarad has a nanohenry all its own. -- Don Vonada