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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 wurp (51446) on Monday June 14, 2010 @04:21PM (#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.)

  • by abigor (540274) on Monday June 14, 2010 @04:22PM (#32571082)

    They "show how it is possible to create situations", according to the summary. I think the experiment they outline in Appendix D of the paper satisfies that sentence.

  • by gurps_npc (621217) on Monday June 14, 2010 @04:26PM (#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.

  • by maxwell demon (590494) on Monday June 14, 2010 @04:34PM (#32571256) Journal

    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 Ungrounded Lightning (62228) on Monday June 14, 2010 @04:34PM (#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 @04:37PM (#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.

  • by Dragoniz3r (992309) on Monday June 14, 2010 @04:40PM (#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).
  • by maxwell demon (590494) on Monday June 14, 2010 @04:40PM (#32571342) Journal

    Neutron: Electrically neutral particle. One of the particles out of which atomic nuclei are built.
    Interferometry: Measurement of the interference of waves. Remember that according to quantum mechanics, particles also show wave-like properties, especially interference.

  • by Anonymous Coward on Monday June 14, 2010 @04:41PM (#32571368)

    From Princeton's WordNet on the definition of "dampen":
    # S: (v) dampen, damp, soften, weaken, break (lessen in force or effect) "soften a shock"; "break a fall"

    A damper is either a movable iron plate to control the draft in a furnace, a device that decreases the amplitude of oscillations, or a depressing (as in emotional) restraint. Inertia is not a furnace, an oscillation, nor an emotion.

    This post was brought to you by the Arrogant Pedants' Society.

  • Re:Sure, here you go (Score:3, Informative)

    by somersault (912633) on Monday June 14, 2010 @04:55PM (#32571578) Homepage Journal

    What he has discovered is that it is the PLASMA above the properly charged surface that creates a gravity shielding effect, and shielding includes inversion. Yes, -1g is possible.

    Except it don't work on water..

  • by sznupi (719324) on Monday June 14, 2010 @05:04PM (#32571738) Homepage

    This was a fun read about inertia... []

    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 part of fundamental mechanisms of our world. OTOH with the potential to bring even more wild scenarios if it's possible to break away from said limits?

  • Re:Quantum (Score:3, Informative)

    by TomXP411 (860000) on Monday June 14, 2010 @05:10PM (#32571800)

    You may be correct about what scientists think they are are saying, but it rarely comes out that way. A physicist may say something like "FTL drive is impossible," and he may be thinking, at least until someone discovers a way to transform the underlying space-time matrix, but what people hear is "That's the final word, and it will never change."

    If the "until someone discovers differently" qualifier went without saying, people wouldn't be starting these ridiculous movements like "Mundane Science Fiction."

  • Re:Sure, here you go (Score:5, Informative)

    by Anonymous Coward on Monday June 14, 2010 @05:12PM (#32571836)

    "Professor" Aquino is widely known as a total nut. For Newton's sake, his theory "includes not only force particles and matter particles, not only general relativity and Quantum Gravity, but also a theory of consciousness"!! He can't publish his papers at the "Journal of New Energy"! Heck, one of his abstracts starts with "The existence of imaginary mass associated to the neutrino is already well-known" (and as a particle physicist, I've never seen any theory or experiment that even suggests an imaginary mass). He was worked at INPE (which is a very respected research institution) in a data-taking-monkey position; then got a job at the Maranhão state university (where there is NO research at all). He is listed at UEMA as having only a masters' degree (no PhD, so he can't have a research position). Please, don't mention him on an article about science. It's just like mentioning a 1940 VW Beetle when discussing today's F1 cars.

  • by maxwell demon (590494) on Monday June 14, 2010 @05:36PM (#32572124) Journal

    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.

    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 gravitiational mass as A.

    Or to put it in formulas:

    Be miA (miB) the inertial mass of A (B), mgA (mgB) the gravitiational mass of A (B), G the gravitational constant, r the distance between A and B, and aA (aB) the acceleration of A (B). Then we have miA aA = mgA mgB/r^2 and miB aB = mgA mgB/r^2. Note that the right side is the same in both cases, so if the inertial masses are the same, then also the accelerations will be (especially you'll find that, irrespective of the inertial masses, energy and momentum are conserved). If it were not so, Coulomb interaction would violate those conservation laws, too (because in the above equations, you can easily replace gravitational mass and gravitational constant by charge and 1/(4 pi epsilon0)).

    Now what can be shown by your argument is that splitting the gravitational mass further into a "field generating mass" (i.e. one that determines the strength of gravitational field of the object) and a "field reacting mass" (the one which says which force the object experiences in a given gravitational field) and allowing those to be independent would violate the conservation laws. In that case (with mgg being the generating mass, and mgr being the reacting mass), we would get

    miA aA = G mgrA mggB/r^2, miB aB = G mgrB mggA/r^2

    As you can see, now the terms on the right hand side are not equal any more, and therefore the very scenario you described can happen (just replace "inertial mass" with "field reacting mass" and "gravitational mass" with "field generating mass" in your text).

  • by maxwell demon (590494) on Monday June 14, 2010 @06:17PM (#32572596) Journal

    Well, the throwing people around the bridge might be explained much simpler:

    • The inertial dampers need a short time to react to acceleration information.
    • for ship-generated accelerations (impulse engine), the information is already available at the time the acceleration starts, because it's transmitted to the dampers at the same time as to the engine.
    • The force of a photon torpedo cannot be completely predicted. So the inertial dampers can only incompletely compensate the acceleration.
  • by splogic (1797526) on Monday June 14, 2010 @06:44PM (#32572878)
    This is just another example of quantum trickery. No fundamentally new principles have been derived. The potential applications are probably in material manufacturing, electronics, and perhaps astronomy, as most quantum applications are. As for "implications" for the macroscopic world, there are basically none. From what I understand, they basically used electromagnetic fields to play quantum tricks. In my opinion, nearly everything you can do with QM is basically like playing with shadows. You can make all the bunny rabbit and alligator shadows you want, but the basic physics of light is still the same.
  • Re:I'm no Einstein (Score:4, Informative)

    by ceoyoyo (59147) on Monday June 14, 2010 @07:26PM (#32573278)

    The point is that, according to the euivalence principle, X g of acceleration due to gravity is indistinguishable from X g of acceleration due to anything else. The article used the specific example of the 1 g you feel at the surface of the Earth.

  • by Ungrounded Lightning (62228) on Monday June 14, 2010 @08:42PM (#32573790) 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.)

    Gene R. and his cohorts put together an engineering manual for the authors (which eventually was published and made available to the general public) in order to maintain technical consistency across episodes and keep things plausible enough that techies - much of their target audience demographic - wouldn't be constantly having their "willing suspension of disbelief" broken by glaring errors.

    and, let's face it, Star Trek is terrible science fiction.

    Compared to what went before it moved very far in the right direction. It was just about the best SF to hit moving images up to then (with a very few exceptions, such as a few episodes of Twilight Zone.)

    You darned whippersnappers just don't UNDERSTAND how bad it was, back when 110 BPS was a fast connection.

In 1750 Issac Newton became discouraged when he fell up a flight of stairs.