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Mach's Principle

  1. Apr 4, 2006 #1
    I'm sorry if I sound too dopey, I simply don't know much about the title.

    I've seen various texts (such as this) and none were satisfactory to me. What I've understood from what Mach says, it's way too ridiculous.
    An accelerating charge radiates, for instance. We can tell this without looking at any other charge, but observing the radiated photons.

    How could it be that Einstein paid attention to it?
    Last edited: Apr 4, 2006
  2. jcsd
  3. Apr 4, 2006 #2
    The various attempts to explain inertia based upon the collective influence of other bodies in the universe always involve some new physics (postuates) that as yet are not observed. Sciama developed a theory based upon matter radiation - if interested you can find it at p 131 in "Unity of the Universe" Einstein attempted to incorporate Mach's principle in developing GR, but eventually rejected it because it seemed to require instantaneous action at a distance.
  4. Apr 6, 2006 #3


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    Try this thought experiment. Empty the universe of all mass except yourself. Say you have a method like rockets or something to make yourself spin. How would you know you are spinning? If you are not spinning against anything else, could you be said to be spinning? Mach concluded not only that you couldn't tell, but that with no way to define spin, there would be no centrifugal force. He concluded that centrifugal force is created by all the other mass in the universe.
  5. Apr 6, 2006 #4

    If there was nothing in the universe but a single particle, it couldn't spin. If there was more than one particle, then I would notice measurable non-inertial motion during spinning.
  6. Apr 8, 2006 #5
    I'd answer like this: a spinning object should radiate. If I'm massive enough, sooner or later, the light will bend towards me and I'll understand that I'm spinning. Anything wrong with that?
  7. Apr 8, 2006 #6
    It assumes electromagnetism? Inertia should be a more fundamental property than EM. That's my opinion at least.
  8. Apr 8, 2006 #7


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    Mach's principle is very confusing. I don't understand what he was saying either, although from what I have read it is somewhat similar to what krab said.
    Almost all websites are vague about it, claim that the description on other websites is not in the same spirit as Mach intended, but themselves fail to convey the essence of the principle in a clear way.

    The term was first coined by Einstein, but people argue that Einstein's view differed from Mach's.
    Last edited: Apr 8, 2006
  9. Apr 8, 2006 #8


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    The following might be interesting in that it shows a bit about what physicists think of Mach's principle on a general basis (the poll results).


    So, after the conference, a fair number of people changed "no" to undecided, and one changed no to yes, but the vast majority of the peole polled don't believe that GR is Machian.

    A lot of the debate is over just exactly what it means for the theory to "be Machian", of course.
  10. Apr 8, 2006 #9
    From what I've read .. just popular accounts of it .. Einstein was enamored of Mach's ideas, tried to encorporate them in GR, but couldn't fully do it. And those accounts say that if the Universe is finite and unbounded, GR could fully encorporate Mach's Principle.

    From what I get out of MP, it seems to try to say that not only are gravitational and inertial forces equivalent, but the exact same thing. But heh, I don't see how it can be considered as anything other than speculation, especially when it comes to GR. GR assumes there is mass (I'm not stating that as fact; only my very limited understanding of GR) and builds upon that .. a fundamental principle. So, proving MP should be like trying to prove an assumption.

    If the essense of MP can be reduced to just the question: "What does a spinning body spin with respect to", and you consider Mach's answer just speculation, then other speculation could be considered, like: Inertial (and gravitational) forces arise by spinning/moving with respect to a point in a higher spatial dimension.
  11. Apr 8, 2006 #10


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    Mach´s principle is something like the "absolute relativity principle" - even acceleration is relative. This may be appealing, but it doesn´t work unless you accept that spacetime is something rather than nothing. GR claims that spacetime has properties, eg it gives you the "rest frame" for everything concerning acceleration. If I recall correctly what Sexl said about it, even an empty spacetime (where you could place your test mass) would be enough to determine rotation.
  12. Apr 8, 2006 #11
    Well a single particle without extension just cannot rotate. And as I have said, if there were 2 particles, the non-inertial motion would be directly observable.
  13. Apr 8, 2006 #12
    But, that is something that can't be achieved, and leads to the question: If there was only 1 particle in the Universe, would it still have mass? If you believe in Mach's Principle, that gravitational and inertial forces are the same, how could you answer anything but "No"?
  14. Apr 8, 2006 #13
    It could have any mass you wanted, to be honest. But without human minds to define these quantities...
  15. Apr 8, 2006 #14
    Then how would you reconcile that mass and inertia are supposed to be the same? In your scenerio there would be no inertia.
  16. Apr 8, 2006 #15


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    To express myself more clearly:
    If you have a single particle in an empty space, you have a single particle and spacetime. The latter also exists. And it defines acceleration and rotation.
  17. Apr 14, 2006 #16
    in a single particle universe how can it be experimentally ascertained that spacetime exists?
  18. Apr 14, 2006 #17
    Mass and inertia are the same up to some constant anyway (which we have chosen to be 1). So in my scenario, I'd just redefine that constant.

    My point is, single particle universes are just silly. We could discuss them ad infinitum (in fact probably not, but certainly ad nauseam) and not get very far. The universe is NOT single particle. That's empirical fact.

    I don't really believe in Mach's principle.
  19. Apr 14, 2006 #18
    Don't tell Garth
  20. Apr 14, 2006 #19

    My mistake. I thought you were making a point for MP. I don't believe it either. But, sometimes I think I'm looking at it wrong. Some of the accounts say that the origin of inertia is the "combined mass of the Universe". It might make more sense to term it as "the distribution of mass in the Universe".
  21. Apr 15, 2006 #20


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    The questions are:

    In empty space -

    1. "What is the physical significance of any coordinate system?"

    2. "What determines whether that system is inertial or not? - What do you 'hang' an inertial coordinate system on?"

    Now add particles with inertial mass to the system -

    3. "What is the source of their inertia?"

    4. "What determines the ratio of their gravitational and inertial mass, i.e. the value of G?"

    If you are happy to say that there are no answers to these questions, 'they just are', then there is no need for Mach's Principle.

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