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Hypothetical question on Inertia

  1. Sep 19, 2007 #1
    If we could imagine an object in space which does not obey the law of inertia ( ie it offers no resistance to motion) how would it react to a force applied to it? Since it offers no resistance to a force the concept of accleration would no longer apply (There would be no rate of change of motion). Does this mean it would instantaneously attain a certain velocity and what would that velocity be?
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  3. Sep 19, 2007 #2


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    An object that has no inertia is essentially an object that had no mass. We know what speed massless particles such as photons travel at. They always travel at the speed of light in a vacuum. They can't be slowed down in empty space. So, yes, they don't have a rate of change of velocity, since they always travel at a constant speed.
    Last edited: Sep 19, 2007
  4. Sep 19, 2007 #3
    Yes I am aware that mass implies inertia, thats why I said its hypothetical. myquestion is assuming it has mass and yet has no inertia how would it behave?
  5. Sep 19, 2007 #4
    If a duck wasn't a duck, but was a duck, how would it behave?
  6. Sep 19, 2007 #5


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    You can't invoke mass while ignoring one of its property. This makes it a meaningless question. You have no ability to pick and choose what property of an object with a mass should have.

  7. Sep 19, 2007 #6


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    Well, you MIGHT conceivably have had a universe in which that property of an object that determines the strength of the gravitational force between itself and some other object was wholly unlinked from the same object's inertial properties.

    But that is a type of universe we don't live in, probably fortunately so. :smile:
  8. Sep 19, 2007 #7


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    As everyone else said, you can't have mass without inertia, so this question is meaningless. You can still ask it, I guess, but giving an answer is going to be impossible, since you want an answer in terms of the physics you're disregarding by asking the question in the first place.
  9. Sep 19, 2007 #8
    No! Bad ZapperZ! The regime of application of our models may be limited by the scope of our experience of phenomena.

    NO! BAD G01! Consider that Euclidean Geometry is fairly applicable to reality even though it make impossible assumptions of points, lines, planes, distances, ect..., and these shapes are purely abstract and never occur exactly in nature. The fractal geometric shapes of Mandelbrot are more accurate models of nature as we experience it [1][2], but this disregards the fact that Euclidean Geometry works. Inertia is only tangentially intertwined with Physics as we currently understand it.

    We do not know "what" inertia is!

    At this current moment, our experience reinforces the fact that inertia is a result of mass, but that is not to say that view is correct. We must be open to the questions "What if..." questions if we are to see through the incorrect models which we have anchored our knowledge upon (which ever they may be).

    "There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy." - Hamlet (I, v, 166-167)

    Following along the lines of Einstein, these thought experiments can sometimes be more meaningful as experiments.

    Now, to hint at an answer, I offer the following.

    Let's take a simple example of the absence of inertia applied to motion. Richard Feynmann was once giving a lecture in Brazil (see [3]). His topic was the "lack of experimentation in the Brazil's science curriculum". He started out by taking a copy of one of the most widely used undergraduate physics books in the country and opening it to a page that gave an example of how to calculate the position of a sphere that had rolled down an tilted plane. His remark was (I don't have the book in front of me, I must paraphrase), "No! This is all wrong! These calculations neglect the fact that rolling spheres possess rotational inertia, which means that there is a slight delay in movement from the time of experiencing acceleration. The ball would be located slightly behind the ideal position. If your curriculum focused on experimentation then you would have experienced such this delay and would realize the limited scope of these equations!"

    If a macroscopic physical object lacks inertia, then it immediately responds to a change in acceleration. A rough interpretation of inertia is a "resistance to change".

    I know that this is an active topic in the String Theory, Particle Physics, Astrophysics and Condensed Matter Physics communities at the moment. Certainly, if you are interested in these views, you can visit http://www.arXiv.org/ to read some e-prints on it.


    [1] Mandelbrot, B. B. "How long is the coastline of Britian? Statistical self-similarity and fractional dimension" Science (156), pp. 636-638 (1967)

    [2] Mandelbrot, B. B. "The Fractal Geometry of Nature" W. H. Freeman and Company, San Fransisco, CA (1982) ISBN 0-7167-1186-9.

    [3] Feynmann, R. P. "Surely You're Joking, Mr. Feynman! (Adventures of a Curious Character)" W.W. Norton & Company Inc., New York, NY (1985)
    Last edited: Sep 19, 2007
  10. Sep 19, 2007 #9


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    NO! BAD OK123JUMP!
    We don't deal with 'what if?' here; that's what Science Fiction is for. Keep in mind that a lot of the people here probably wrote some of the papers that you like to paraphrase. You're dealing with world-class experts here (myself excluded), so second-guessing them is not a good idea. Challenge, by all means, but don't pretend to know more about their areas of expertise than they do.
  11. Sep 20, 2007 #10

    I have to heavily disagree, this is not relegated to the realm of "Science Fiction" any more than restricting the domain of a function to a subset and thereupon studying its behavior. There is absolutely nothing unscientific about that method, or this question. You are making reference to a heavily studied area of physics in which there are many theories, and yet, none adequately answer the question that Nova77 has posed (if one disregards the last two sentences).

    The "world-class experts" can do the following:

    Define inertia. Define it with String Theory, QFT, QED, QM, maybe even reach into the framework of Relativity and describe the nature of inertia. If any can do that, I assure you, there'll be a Nobel Prize waiting for them.

    I remind you that at one point in time, the "world-class experts" widely believed that the Earth floated through Aether. Professors, researchers, technicians and students developed highly detailed annals of Chemistry, Physics, Astrophysics, Biology and a host of other technical works, predicated upon the idea that this "substance" existed. Through and through, the Scientific Method has whittled away at those theories which were too divergent from reality - it will continue to do so.

    Lastly, if I have been untruthful about my facts, or I am incorrect in my statements, I implore everyone to point that out - that is the essence of the Scientific Method.
    Last edited: Sep 20, 2007
  12. Sep 20, 2007 #11
    Danger don't be a danger to progress. A hundred years ago the thought of a man on the moon would have been called science fiction. To be a "world class expert" (what ever that means) within the field of the known is no big deal. If you stop questioning you aren't going anywhere. As for inertia (as I'm sure you know) we don't have an explanation.
    Now to get back to my question does any one have an answer, "science fiction" is acceptable!!!!!!!!!!!
  13. Sep 20, 2007 #12
    Let's say that an object had a nonzero gravitational mass but no inertial mass. What would happen to it? I believe that was the original question.
  14. Sep 20, 2007 #13

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    Feynman was complaining about bad physics in the textbooks--a simple error that would have been detected if they did the experiment. Or if they had a clue about basic Newtonian mechanics. What does this have to do with the question at hand?
    No, I'm afraid "science fiction" is not acceptable here.
  15. Sep 20, 2007 #14
    Then the equivalence principle wouldn't be true, and the whole of GR would come crashing down. Then you're left with the problem of finding a field theory of gravity that doesn't violate special relativity (unless you're throwing that out too?)
  16. Sep 20, 2007 #15
    This question is not plausable. It is like asking the old Superman Comics book question. If a immovable mass is hit by a mass that can not be stopped what happens? Standard theory would say they would form a black hole due to the huge increase of mass due to the conservation of energy.
    Last edited: Sep 20, 2007
  17. Sep 20, 2007 #16
    Capnahab, that may not be true.

    Until the Manhattan Project, the question of whether there was an exact conservation of energy and momentum in the quantum realm was widely debated. A Quantum Theory of Optical Phenomena was formulated by J.C. Slater, in collaboration with Bohr and Kramers in 1925[1] that formulated a framework where these two concepts need not always apply. Somewhat recently, R. Pakula has used this framework and published an e-print[2] showing that the reasons leading to the assumption that a photon has mass 0, also attributable to the standardization of Quantum Theory from the Manhattan Project, are flawed. If he is correct in his framework, then, this is a situation where other processes take effect and there is not an exact conservation of energy and momentum in QM.

    I relate this to the current topic by saying that there are credible views that exist contrary to the of the main viewpoints. Due to the fact that we do not know what causes inertia, we cannot assume that it is inherently generated by mass - we only have tangential evidence of their connection. If you assume that this is not the case, you fall into the post hoc ergo proctor hoc fallacy. We need direct evidence and a concise theory that links the two - I claim that both are lacking.

    Thus, let us deeply consider what inertia is, rather than skipping this whole topic by making some half-baked reference to Science Fiction.


    [1] Slater, J.C. "A Quantum Theory of Optical Phenomena" Phys. Rev, Vol. 25, No. 4, pp. 395-428

    [2] Pakula, R. "A realistic model for radiation-matter interaction" arXiv e-Print archive, June, 2004 http://arxiv.org/abs/quant-ph/0405055
    Last edited: Sep 20, 2007
  18. Sep 20, 2007 #17
    Okay, so the problem of gravitational charge vs inertial mass is exactly the one addressed by the equivalence principle. Specifically, the weak equivalence principle, or uniqueness of free fall. This has been experimentally tested, through a series of very accurate experiments by Eoetvoes, Dicke, Braginsky, and that's only up to the late 1970's. Misner, Thorne and Wheeler, in Gravitation, gives the following figures:

    5 parts in 10^9 that the Earth imparts the same acceleration to wood, platinum, copper, asbestos, water, magnalium (90% Al, 10% Mg), copper sulphate and tallow.

    7 parts in 10^10 for Earth on platinum, copper, bizmuth, brass, glass, ammonium fluoride and an alloy of 30% Mg, 70% Cu.

    1 in 10^11 of Sun on aluminium and gold.

    1 in 10^12 of Sun on aluminium and platinum.

    From this, it's possible to infer the response of neutrons, protons, electrons, virtual electrons-positron pairs, nuclear binding energy, electrostatic energy, etc. So we get 1 in 10^10 for neutron and proton, 2 in 10^7 for electrons vs nucleons, 1 in 10^4 for virtual pairs, 1 in 10^7 for nuclear binding energy and 3 in 10^9 for electrostatic energy.

    The full details are, like I said, in Gravitation by MTW, 38.3.

    Does this answer the question?
  19. Sep 20, 2007 #18
    I believe that your post is a great place to start. Good one.

    Have these results been experimentally verified for particles?
    Last edited: Sep 20, 2007
  20. Sep 20, 2007 #19
    I would say that these experiments are about particles. What do you have in mind?
  21. Sep 20, 2007 #20
    As an example, if one could take a free particles, such as a free-electron beam, and experimentally verify the inertial inferrance from MTW, that would be a verification for particles.

    Since I began my focus on Laser Physics, I have not come across such a report - that is not to say that one does not exist. Are you aware of any?
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