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Origin of inertia

  1. Dec 9, 2012 #1
    What is the origin of inertia?

    This is a question asked by many people. I believe even Einstein pondered on this and did put some of his thoughts in a paper, but nothing really conclusive.

    In Newtonian gravity, inertia appears to be just something that is inherent to mass. However, given relativity theory, this is no longer a satisfactory explanation. Inertia of an object increases with velocity according to relativity. (I assume that means inertia of a body also increases in a stronger gravitational field, though I have not seen that spelt out anywhere in my limited reading).

    So, if inertia increases with velocity or gravity, what is the source of the base inertia of a body? It appears that a body at rest and at ideal zero gravitation still has some base inertia. Where does that come from?

    I have come across some explanations like (a) Mach's principle and (b) expansion of Universe from Big Bang, as tentative theories of the origin of basic inertia. None seem to be convincing, esp. since there is no mathematical model to back up the explanations.

    Is there any comprehensive theory on the origin of inertia, or is it still a mystery of physics?
  2. jcsd
  3. Dec 9, 2012 #2
    well, like the mass of a electron, charge, space, time,etc...it's pretty much a mystery....

    Some insights here:


    My own view is that everything popped out of some kind of 'bang'...so they are probably all related in ways we do not yet fully understand. We do have explanations of many observed phenomena, and have managed to put together a lot of what we know in the Standard Model of particle physics. But there are also many remaining gaps.....unknowns.

    A related way to check further on an answer to your question, might be via the Higgs field....I don't know a lot about that, but I do know the Higgs has two responsibilities - to give mass to the force carriers of the weak force, and to give mass to the fermions. But one doesn't usually see Higgs results along the lines of inertia.....usually just mass. So when we understand exactly why inertia and mass are so intricately interwoven, likely it will offer some new insights and understanding.
    Last edited: Dec 9, 2012
  4. Dec 9, 2012 #3
    That is my understanding too. There isn't anything out there I can find which comes even close to explaining.

    However, Relativity Theory at least does deal with 'change' of inertia, so I have this feeling that if we look deeper, there may be an explanation of the base inertia also somewhere. May not be explicitly in the existing Relativity framework, but some extension may be possible. (I am trying to look at it from the classic perspective, rather than quantum, as I feel there should be an explanation from either perspective).
  5. Dec 9, 2012 #4
    yeah, that would be nice...but nature may not be so accommodating. Or perhaps I should say our mathematical models are not so refined currently.

    For example, there is not a definite line differentiating virtual particles from real particles [mass if you wish] — "real particles" are viewed as being detectable excitations of underlying quantum fields, virtual particles, not so much. Virtual particles are also excitations of the underlying fields, and appear in quantum math, but are detectable only as forces, not particles.

    Generally a measurement would correspond to an operator of some sort [ a mathematical component] and the effect of a measurement on the state [wave function] would be to make it jump into some eigenstate….which eigenstate is a matter of chance!!

    ok, this is much better:

    The following quote is from Roger Penrose celebrating Stephen Hawking’s 60th birthday in 1993 at Cambridge England.....this description offered me a new insight into quantum/classical relationships:

  6. Dec 9, 2012 #5
    I can understand that. Still, my thought was that inertia is a macro phenomenon, so should be possible to explain through classical theories.

    I was hopeful that Relativity theory may have some explanation in it or there may be an extension, known to more knowledgeable forum members, that I may not have come across.

    My logic was - 'Since Relativity theory can predict change in inertia, perhaps there is a handle in it that can even explain base inertia'. This is not very firm logic, but more a hopeful quest.

    BTW, very interesting quote.
  7. Dec 10, 2012 #6
    Looking for a possible theory that might help answer my question, I came across this article today: http://adsabs.harvard.edu/full/1953MNRAS.113...34S, by D. W. Sciama.

    I was surprised to see that it was in fact published as long back as 1952. While the author admits this is not a fully complete theory, it certainly provides some thoughts on the origin of inertia based on Mach's principle, and develops it mathematically to some extent.

    Does anyone know if this particular theory has any credibility, or whether it has been proven wrong, or any further work has happened along these lines?
  8. Dec 10, 2012 #7


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    arindamsinha, Dennis Sciama was probably the last person to take Mach's principle seriously. It's an old-fashioned idea that went nowhere.
  9. Dec 10, 2012 #8
    Oh, I see. Thanks for the info.
  10. Mar 3, 2013 #9
    Before I ended up with with the understanding that inertia is a Major Outstanding Mystery, I read things like this. I have to say that the author's reasoning here strikes me as a bit circular.


    "This 'new view' of the origin of inertia says that all objects with size will resist any change to these relations (spatial, temporal, or both). In order for the object to be accelerated, it is necessary to overcome the object's intrinsic resistance to changes in these relations. This might be the cause of the resistance that inertial bodies have against being accelerated.

    So, maybe Einstein was wrong on the origin of inertia. Maybe it has nothing to do with Mach's principle or the matter in the Universe at large, but just with what's going on within every object's spatial and temporal relations. "

    It's inertia which is determining the spatial/temporal relations of an accelerating rod, so you can't use those relations as a cause of inertia. Right?
  11. Mar 3, 2013 #10
    Thought I'd check wikipedia:

    Source of inertia

    There is no single accepted theory that explains the source of inertia. Various efforts by notable physicists such as Ernst Mach (see Mach's principle), Albert Einstein, D Sciama, and Bernard Haisch have all run into significant criticisms from more recent theorists.

  12. Mar 3, 2013 #11


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    I always understood that it is not old fashioned, but merely qualitative and not quantitative, which makes it hard to implement in physical theories.
  13. Mar 4, 2013 #12


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    Hm, well in that case I guess Dennis Sciama was not the last person after all. :smile:

    Mach's principle is in total discord with modern physics, and relativity in particular. Remarkable that we even still discuss it. It imagined that local properties of matter are determined (somehow) by the distant universe. In particular it said that the local inertial (nonrotating, nonaccelerating) reference frames are the ones in which the distant stars on average appear fixed.

    At least three things wrong with this. a) there is nothing known by which this influence could be conveyed. A new field would have to be postulated for the purpose, for which there is no other evidence. b) there is no need, because both SR and GR already contain a perfectly valid and reasonable definition for local inertial reference frames. c) there are solutions of Einstein's equations which violate Mach's principle, e.g. ones in which the local frames do rotate with respect to the distant one.
    Last edited: Mar 4, 2013
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