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Why do bodies have inertia?

  1. Sep 26, 2009 #1
    Stupid question, but Im no Einstein.

    So why do all the bodies have it? Does enegry also have it? What's it caused of?

    Thanks in advance,
  2. jcsd
  3. Sep 26, 2009 #2
    I don't think that would be a stupid question.
  4. Sep 26, 2009 #3
    We don't know why things have inertia, we don't know why gravity attracts, etc... we can only describe it!
  5. Sep 26, 2009 #4


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    Mach thought that inertial effects arise through matter's interaction with all other matter (Universe as a reference-frame). Einstein refuted this view in the 1920-1924 time-frame (Leyden address, "On the Ether" essay) and said that such interaction would amount to "spooky action-at-a-distance". Instead, he claimed that inertial effects arose from matter's interaction with the space in which it is embedded. He also claimed that space was a player in gravitational interaction and had properties necessary to allow the propagation of light. All of this was pretty unpopular with his contemporaries, since his earlier work had already given physicists reason to abandon the idea that space was an "ether" with physical properties.

    The origin of inertia is a mystery, and asking about it is far from being a dumb question - it is one of the most fundamental unanswered questions.
  6. Sep 27, 2009 #5


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    I'm quoting the part of the message that I agree with.

    In our physics theories inertia is described, but we have no way of moving the description to a deeper level.

    As an aside:
    Very often Ernst Mach is mentioned when the origin of inertia is discussed, and usually a point of view is attributed to Mach that was not Mach's point of view.

    In one of his books Mach had discussed the following example (which had earlier been discussed by Newton in the Principia). A bucket is filled with water, and then you start a rapid spinning motion of the bucket (around a vertical axis). Initially the water is not co-rotating with the bucket, friction gradually spins up the water. Only when the water is spinning does the water's surface take the distinctive hollow shape.

    The bucket example illustrates that it's not motion relative to the walls of the bucket that counts, but motion relative to some other, larger structure.

    In his own discussion Mach mentions the suggestion: "What if the bucket would have walls that are miles and miles thick?" (Meaning the question: if the walls would be thick enough, would there be so much mass that the motion relative to those walls would be what counts.) Mach's answer to that boils down to: there's no need to think about that. Mach's philosophy of physics was that physics theories should be as economical and as sparingly as posible. Mach advocated a level of caution that was much higher than usual in science. For example, in his time Mach argued that the evidence for the existence of atoms was not strong enough. In Mach's time there was a lot of evidence for the existence of atoms (but only a fraction of what is known today), and it was all very indirect evidence. Mach argued that science should only consider things as proven when the evidence was very strong and direct.

    When Mach in writing mentioned the question: "What if the bucket would have walls that are miles and miles thick?", his attitude was to argue that such a question falls outside the scope of science. The job of physics, Mach argued, was to find the most economical way to formulate laws of physics. The laws of motion take their simplest form when they are formulated as motion relative to the fixed stars, so that is how laws of motion should be formulated. Speculations that "the fixed stars", or "distant matter" are the origin of inertia fall outside the scope of science, argued Mach.

    Many years later Einstein had a thought about the origin of inertia, and he attributed that thought to Ernst Mach. Einstein formulated it in the form of a principle, and he coined the name "Mach's principle" for it. Instead of claiming the thought for himself Einstein credited Mach, but in this case Einstein didn't do Mach any favor.

  7. Sep 27, 2009 #6
    I think that the inertia is one of the conservation law of the Nature.
    There is conservation laws:
    1. Energy
    2. Momentum
    3. Velocity (kinematic state or characteristic)
    and so on.
    Inertia is a conservation of the kinematic characteristics of free body.
  8. Sep 27, 2009 #7


    Staff: Mentor

    It is not a stupid question, but IMO it is unscientific. Science does great on "how" questions, but "why" questions are generally the purview of priests and philosophers. Think about it, what kind of experiment could you design to answer pretty much any "why" question?

  9. Sep 27, 2009 #8


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    I beg to differ.

    My favorite example is Kepler's laws of planetary motion. In Kepler's time it could not be excluded that those laws, such as the area law, were irreducible laws.

    However, Newton showed that all three of Kepler's laws are interconnected, and reducible to aspects of a deeper theory: Newton derived Kepler's laws from the laws of motion and the law of universal gravity.

    Modern examples are plentiful. Is the Pauli exclusion principle irreducible or not? We cannot exclude that at some point in time a theory will be developed in which the Pauli exclusion principle will be a theorem instead of an axiom.

    It rather seems that taking on "why"-questions is a matter of timing. If Kepler would have insisted on first solving the question "Why is the area law valid" then he would never have published. That would have been bad; in Kepler's time taking on that "why"-question was too early. But Newton was ready for it.

    Science can do very well on "why"-questions - but not if you get ahead of yourself.

  10. Sep 27, 2009 #9


    Staff: Mentor

    I'm sorry, but that isn't an answer to a "why" question at all, it is an answer to a "how" question ("how are Kepler's laws connected"). The answer to "why masses gravitate" is still untouched by science today.
  11. Sep 27, 2009 #10


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    If I understand you correctly you argue that as soon as a "why" has been answered it has become a "how".

    In Kepler's time there was no way of knowing in advance that Kepler's laws are connected, so it would have been wrong to ask "How are Kepler's laws connected?"

    The questions were "why" questions. Why are the planetary orbits not circular, but ellipses with the Sun at one focus? Is that a law in itself, not otherwise reducible? Or is there a description on a deeper level?

    Last edited: Sep 27, 2009
  12. Sep 27, 2009 #11
    Could you elaborate please.
    I do not see how energy has inertia . I actually do not see inertia as something to be had by anything.It is more of a word used to describe how object behave in relation to the frame of reference.
  13. Sep 27, 2009 #12
    Inertia is connected with the mass. The bigger the mass of a body the bigger its inertia.
    So, why a body has mass?

    According to standard model, please correct me if that's wrong, the mass, and therefore the inertia of a body, exist because of a field called Higgs field.

    Bodies interact with that field and acquire mass and therefore inertia.
  14. Sep 27, 2009 #13


    Staff: Mentor

    Sure. Say you have a container of gas and you measure its inertia. Now, say you take the same container of the same gas and add energy so that the gas becomes very hot. The inertia of the hot gas will be greater than the inertia of the cold gas by an amount E/c² where E is the amount of energy added. You can derive this effect using the ideal gas law.
  15. Sep 27, 2009 #14


    Staff: Mentor

    No, that is not my argument at all! My point is that the answer to a "why" question speaks to motive or choices. Science addresses mechanisms which are probed by "how" questions.

    You are certainly free to disagree with me, but it is my experience on these forums and others that when posters ask a question with a "why" they are very rarely satisfied with the mechanistic answers provided by science. Also, the scientific answer to any "why" question is simply yet another "why" question, which most people asking find dissatisfying. By the way, I note that your post #8 did not actually pose any "why" questions, which makes me suspect that you may understand my point at a visceral level even if you don't like it at a cognitive level.

    This is why (my personal motive and choice) I always respond to "why" questions in this manner.
    Last edited: Sep 27, 2009
  16. Sep 27, 2009 #15


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    I think this is a semantics issue. 'How' and 'why' are used interchangeably. 'Why is the sky blue?'

    I'm not disagreeing with you that there is often a problem with motive/intent. Biology and evolution are rife with this.

    But I think that, in general, and particularly here, the 'how' meaning can be assumed.
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