Follow Slashdot blog updates by subscribing to our blog RSS feed


Forgot your password?

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."
This discussion has been archived. No new comments can be posted.

Inertial Mass Separate From Gravitational Mass?

Comments Filter:
  • General Relativity? (Score:3, Interesting)

    by gyrogeerloose ( 849181 ) on Monday June 14, 2010 @05:10PM (#32570918) Journal

    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 jfengel ( 409917 ) on Monday June 14, 2010 @05:17PM (#32571022) Homepage Journal

    The "show" here is a proof, or rather, a calculation. They describe what kind of experiment can be used to test the calculation (on a Bose-Einstein condensate in free-fall).

    The experiment isn't trivial, and these theoreticians won't be the ones doing it. They publish the theory, and everybody else looks at it to see if it's worth the time and money to set up an experiment. That's pretty much canonical science going on there, and doesn't merit being dismissed as "just a pretty theory".

  • Sure, here you go (Score:1, Interesting)

    by scorp1us ( 235526 ) on Monday June 14, 2010 @05:24PM (#32571118) Journal

    Professor Fran De Aquino's Webpage [] explains in detail what is going on, and how to do it. He even has the paper "Engineering the Simplest Gravity Cell []"

    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.

    One of the more awesome things is that when you are at +/-0.159g, you disappear from regular space-time because you are too weakly interacting with it, like a neutrino.

  • Ringworld (Score:2, Interesting)

    by cats-paw ( 34890 ) on Monday June 14, 2010 @05:24PM (#32571120) Homepage

    "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.

  • 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 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:Quantum (Score:5, Interesting)

    by vlm ( 69642 ) on Monday June 14, 2010 @05:45PM (#32571422)

    Imagine if you could lift an aircraft carrier sized ship in to space with nearly no energy, then accelerate to .999 light speed with no more thrust than a model rocket.

    Note that one situation means low/zero gravitic mass, the other means low/zero inertial mass. You might be able to arbitrarily control both. You might be able to trade one off for another. Or maybe only modify one. Also, the problems with SR and QM are at a small scale, so your aircraft carrier might only be one atom in diameter or something.

    Finally, I haven't read the paper, but it'll be interesting to see how it gets around various perpetual motion type problems. Right off the top of my head, extracting energy from a pendulum where gravitic and inertial mass are different and varying is going to be a serious issue.

    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.

    Similar problems at a quantum scale. Otherwise it would be too easy to accelerate two beams of "reduced inertial mass" deuterium to an arbitrarily high velocity and then increase their gravitic mass at the collision point until they fuse.

    Finally, the most interesting apps might be arbitrarily increasing inertial and gravitic mass. Increasing gravitic mass would make gravity wave detectors much simpler to make. The odds of increasing the gravitic mass of something small on a spacecraft to something large like a planet seem unlikely aka artifical gravity. Increasing inertial mass might be useful for weapons, armor, pretty much anywhere you use lead, tungsten, or DU.

  • 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-)

  • 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...

  • by justin12345 ( 846440 ) on Monday June 14, 2010 @07:14PM (#32572560)

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

    Reverse engineering things like Star Trek to come up with plausible explanations is fucking stupid.

    Just because something is stupid doesn't mean it isn't fun. Don't be a party pooper. Star Trek was good enough science fiction that many, many people working in both physics and engineering will point to Star Trek as inspiring them to pay attention in science class.

    These days I prefer my reading a little more challenging, but if it wasn't for my Dad waking me up early to watch Captain Kirk shoot phasers at Klingons and punch out guys in rubber suits, I probably would have just learned how to play baseball or something.

  • 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.
  • by justin12345 ( 846440 ) on Monday June 14, 2010 @07:47PM (#32572900)
    I always figured that all the noises and being thrown around the ship was a features of the ships "GUI". It gives the crew visceral feedback in situations where the forces involved would be incomprehensible to the human brain. A few bumps and bruises are a small price to pay for the realization that the weird blue beam being fired at you is several orders of magnitude more energetic then it looks
  • Re:No GR in Article (Score:2, Interesting)

    by Anonymous Coward on Tuesday June 15, 2010 @12:38AM (#32574758)

    What you have to realize though is that if the experiment yields m_i=m_g then QM fails. Either result is very meaningful--and based on their paper I'd say a m_i!=m_g is much more likely--we just have too much data confirming the assumptions they used.

  • by CrazeeCracker ( 641868 ) on Tuesday June 15, 2010 @12:40AM (#32574766) Homepage

    Ok, this question has been bugging me for a short while, and this seems like the ideal place to bring it up, since it's somewhat on topic:

    (1) We're always told how inertial mass and gravitational mass, while two distinct things, are always the same (up until today, anyway).
    (2) We also know that mass increases with speed, which we use to explain why objects can't accelerate to the speed of light (infinite force required to overcome inertia, etc.)
    (3) This would logically imply that gravitational mass increases with speed as well, and would further mean that gravitational attraction between two objects depends not only on their separation, but also on their relative velocities.

    Are my conclusions correct? 'Cause that's kinda counterintuitive (although that's what tends to happen at the frontier of physics).

Never buy from a rich salesman. -- Goldenstern