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I Why does mass/energy have inertia?

  1. Nov 6, 2018 #1
    I have been googling this topic for some time, but I still don't know if this is still an unsolved mystery of physics (it's just so) or if there is a deeper underlying theory.

    I get the idea that mass/energy distorts spacetime, justified by thought experiments with moving objects and photons bouncing between mirrors. I see how the distortion in spacetime produces the effects we call gravity. But where does inertia come in ? All mass/energy has inertia, but I don't see that inertia (or resistance to acceleration) comes naturally out of the arguments about spacetime and gravity. Why is inertial mass equal to gravitational mass, such that all free falling objects accelerate at the same rate in a gravitational field ?

    I am really not qualified to come up with ideas at this level, but I thought about the problem and came up with this: An object comprising mass/energy creates a distortion in spacetime around it, its gravitational well, which extends to infinity. If the object is pushed so that it begins to move, the gravitational well around it will move too, so as to stay centered on the mass. However we know that the effect on the gravitational well cannot propagate faster than light, so some of it will remain as it was until a ripple in spacetime reaches it. Could inertia be the result of an object trying to climb out of its own 'lagged' gravitational well ? Climbing out of a graviational well requires input of energy, which requires work to be done on the object which could explain inertial forces...
  2. jcsd
  3. Nov 6, 2018 #2


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    Science Advisor
    Gold Member

    @elpidiovaldez5 you are jumping into General Relativity with both feet here. You would need to quote a reasonable reference about this as PF doesn't discuss personal theories.
    Are you sure that you have a solid knowledge of Newtonian Mechanics? I have to wonder, in the light of your question:-
    Objects with mass make no conscious effort to do things. Anthropomorphism really has no place in Physics.
  4. Nov 6, 2018 #3


    Staff: Mentor

    I'm not sure what kind of answer you are looking for.

    To me, Noether's Theorem (https://en.wikipedia.org/wiki/Noether_theorem), and the fact that Newton's Laws of Motion can be derived from that (plus the symmetries of nature) is a very satisfying answer. It is also much simpler than General Relativity.

    I can recommend Leonard Susskind's video course on Classical Mechanics. Spending the time to go through those 10 lectures is very entertaining and it will give you an excellent grasp of all those things including the use of Noether's Theorem.

  5. Nov 6, 2018 #4


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    Staff: Mentor

    Inertia isn't really related to gravity except by the totally unsurprising coincidence that both are proportional to the amount of matter you have. So are specific heat capacity and chemical binding energy and other properties.
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