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How relative is inertia ?

  1. Sep 17, 2007 #1

    I understand (I think) the basic concept of 'relativity' of physical properties.
    That is, physical properties (laws) are always the same in every 'inertial reference frame'.
    When I'm standing on earth observing a bouncing ball I can measure it's behaviour by measuring it against my reference frame; I may have drawn out an x-axis and y-axis on the floor, put a stick (a meter) in the ground for the z-axis and I have a clock in my hand. My own personal inertial reference frame !

    If I was to observe the same bouncing ball on a moving train, at constant speed, I would find the same physical properties, providing I put my 3 axis rulers and my clock on the train's floor. My moving train reference frame !

    With this classical moving train example, the concept of these frames is very intuitively presented.

    Even the 'changing of frames' is intuitive : if the train suddenly accelerates, the bouncing ball will start to act 'weird'; we changed reference frames by accelerating.

    But is inertia also relative to these reference frames ?

    A new example based on the infamous Paradox Twins :

    Assume an empty space and our twins are just floating around a bit in their space suits.
    There is nothing around to see; as I said : it's an empty space.
    They both have a lifeline attached to them, almost infinitely long : they can't see the origin.
    Suddenly the distance between the two twins increases.
    This is their conversation :

    Twin A : "Hey, what's happened ? We moved away from each other ? Did you move away from me or did I move away from you or did we just both move away relative to eachother ?"
    Twin B : "Well, I felt something pulling me. Probably my lifeline was pulled."
    Twin A : "I didn't feel a thing. Now, we don't have a visual clue like a star to know who moved away from whom, but given that you felt a pull and I felt nothing, I would conclude that YOU MOVED away from me and I REMAINDED AT REST."
    Twin B : "Well, you didn't remain at rest from my point of view. You say you remained at rest...but relative to what ?"

    If this conversation can occur, doesn't that prove that there is a 'general' reference frame for inertia ?

    Hence my question.
    Physical properties are relative, but how relative is inertia ?

    I have read that Einstein gradually grew more sympathetic to the 'aether' idea.
    Am I talking about the same thing here ?

    (p.s. I may have used the terminology inaccurately (properties, laws, forces...), but please focus on the Twin's example in responses : I'm just wondering if the situation I describe is correct and if it implies a general reference frame)
    Last edited: Sep 17, 2007
  2. jcsd
  3. Sep 17, 2007 #2
    Consider this: is it even possible to move them relative to each other whilst they're both inertial frame, or would some force (so acceleration) be necessary?
  4. Sep 17, 2007 #3
    The example wasn't clear enough, so I just added the line "Probably my lifeline was pulled." to explain the move.
    Last edited: Sep 17, 2007
  5. Sep 17, 2007 #4
    The guy who was pulled is no longer in an inertial frame.
  6. Sep 17, 2007 #5
    Hi, I am new on this forum, so hello for everyone. Moreover, my mothertongue is not English. So please forgive me, if I misunderstand questions, and use non-correct terminology.
    I think, Eribons question goes around the fact, whether can it be absolutely decided, if a ref. frame is interial or not.
    The answer I would give: it can be desided, whether the frame where I am sitting is inertial, by checking Newtons 1st law. If Newtons 1st law applies, so it is inertial. If not, not.
    On the other hand, there are unlimited number of inertial frames, and there is no experiment, that could differentiate any of them, on the condition, that the same experiment is carried out the same way.
    In his example one of the twins sensed something, that was against the 1st law. Therefore he was not in an inertial frame. Better to say: his motion was not inertial.
  7. Sep 17, 2007 #6
    Mutant: very good -- it's much clearer than I managed. Welcome to the forum!
  8. Sep 17, 2007 #7
    "Hence my question.
    Physical properties are relative, but how relative is inertia ?

    I have read that Einstein gradually grew more sympathetic to the 'aether' idea.
    Am I talking about the same thing here ?"

    Newton referenced inertial reaction to an absolute space. After SR, Einstein thought about the problem of acceleration in connection with his development of the General Theory. He considered what properties the universe must possess in order for acceleration to be relative - he concluded that it was not possible for an observer to determine whether an acceleration took place relative to universe, or whether the object remained at rest and the universe were accelerated. Perhaps this is where you would find Einstien leaning toward a more holistic view of space and time - which one might call an aether to the extent it functioned as he believed it should, in order for acceleration to be relative on the cosmic scale
  9. Sep 17, 2007 #8


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    The fact that B felt a pull and A didn't just shows that B accelerated while A did not, and any inertial observer watching them would agree that B's velocity changed while A's remain constant. But they wouldn't all agree A remained "at rest", all observers moving at constant velocity feel the same thing regardless of whether they are at rest in a given frame or if they are moving at 0.99c.
  10. Sep 18, 2007 #9
    It appears that most people here (appart from yogi who indeed made the distinction between Newton's absolute space and Einstein's relative one) are assuming that it is quite clear which person moved and which one doesn't. Making my question look rather silly.

    But if you're sure which person moved (ok, accelerated) and which one didn't, you're implying an absolute space (thus the Newton point of view, not the relativistic one).

    Correct me if I'm wrong but from a relativistic point of view, the two twins only move away RELATIVE from each other.
    Or as yogi specified :
    But this is exactly my point : if an observer in the universe accelerated instead of the universe, wouldn't the observer FEEL this acceleration ? Vice versa, if the universe accelerated (including planets, stars...) and the observer didn't, wouldn't the people on the planets FEEL the acceleration, whereas the observer didn't feel a thing ?

    We can't test this of course, but the Twin example I described is more or less doable, or at least imaginable.

    Again my question, in a few steps.

    1. Is inertia completely relative in the standard model ?
    2. If so, doesn't that contradict with what the twins experience?

    Probably this is a non-issue in current physics, but I'm just wondering if everything is considered RELATIVE, including inertia.

    Or to rephrase the question again :
    Is acceleration relative or absolute ?
    And if relative, how come that only one twin feels the acceleration ?
    (And don't reply again that only one feels it because only one moves : if acceleration is relative, they both move away from eachother. Also don't say that only twin B moved relative to twin A. If it's relative, you can turn it around : twin A moved relative to twin B.)

    To try to answer my own question :
    Since one one twin feels the acceleration, acceleration is NOT relative but is absolute and thus an ABSOLUTE reference frame exist, causing only the accelerating person (in relation to this absolute frame) to feel inertia.

    This boils down my question to an even more simple one : is my answer Right or Wrong ?

    (sorry that I'm mixing 'moving' and 'accelerating' to describe the same thing, the latter obviously, but I'm not used to 'physical talk'.)
    Last edited: Sep 18, 2007
  11. Sep 18, 2007 #10
    When reading my last post here, I noticed that I'm actually asking two different questions :
    1. is inertia relative or absolute ?
    2. is acceleration relative or absolute ?

    I assume both questions should come up with the same answer.
  12. Sep 18, 2007 #11
    Yes, acceleration (the fact, not the measure of it) is absolut, in the meaning, that there is an experimental way to show, whether the body moves inertial or is accelerating wrt an inertial frame. Similarly: the fact of a motion to be inertial is also absolut.
    This however does not mean, that there exist an absolut frame of reference for measuring distances or motion.
    To be said on an other way: if a motion is accelerating in one inertial frame, it is accelerating in all of them. If not, not. That is why I say it is "absolute".
  13. Sep 18, 2007 #12


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    I believe these two questions are really equivalent. Inertia is, of course, the resistance of mass to an applied force or, equally, the resistance of mass to acceleration. Therefore I think you are really asking

    'can the two twins both legitimately claim to be the one who accelerated'.

    The answer to this is no. If both twins were carrying accelerometers only one of them would have registered an acceleration and only that one could legitimately claim to have accelerated.

    However, I believe either twin could choose a frame of reference in which the other one accelerated and they would be mathematically equivalent. Still the fact remains that one of them can demonstrate, unequivocally, an acceleration.
  14. Sep 18, 2007 #13


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    Acceleration is definitely absolute. But how do you define "inertia"? It's usually defined in terms of resistance to acceleration, in which case the answer to whether inertia is relative or not depends on whether you're talking about proper acceleration or coordinate acceleration in a single inertial frame, and also on how you define "resistance" (force or energy needed to accelerate the object a given amount, and is it the force or energy in the object's instantaneous rest frame or in some other frame?)
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