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Inertia - Why?

  1. Sep 29, 2011 #1
    I was reading "Reinventing Gravity - A Physicist Goes Beyond Einstein" by Mr. John Moffat, btw it's a really good book for understanding introductary level of classical and theoretical physics, anyway a passage there caught my eye. I'm not finished with book yet but the author expressed/chose his words carefully as he said "the questions related to source of inertia have yet to be answered." eventhough physicists like Einstein or others tried to express the situation but couldn't come up with an idea which could be acknowledged by scientists.

    Might sound like stupid but I couldn't find an answer on Wikipedia either.

    Is there a scientific consensus, sort of update, on this subject? I always thought inertia and gravity goes hand to hand as far as Newton's laws are concerned.

    Answers are appreciated.

    EDIT: Oops I might have opened this on a wrong section when I was reading through the threads, sorry about that.
    Last edited: Sep 29, 2011
  2. jcsd
  3. Sep 29, 2011 #2


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    Warning - don't believe everything you read in this book. Moffat does not subscribe to standard physics, and expounds his own theories that are not widely accepted. If you must read this book, suggest you do it only after reading and understanding more standard treatments of physics.
  4. Sep 29, 2011 #3


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    There is more than one way of looking at this. The following is one, conventional view:

    Prior to general relativity, there were two laws:

    - the law of inertia: objects move at uniform speed in straight lines, unless acted on by some force. The amount of acceleration produced by a force is inversely proportional to the (inertial) mass.

    - the law of gravitation: gravity acts between objects with a force in proportion to the product of their gravitational masses and inversely by the square of their distance. It is an intriguing coincidence that gravitational mass is always equal to inertial mass (no other source of force, e.g. charge, has this property).

    General Relativity replaced this with (in very non-technical terms):

    - (new law of inertia): Objects move on the straightest possible path through spacetime (geodesic). Their resistance to deviating from such a path, under the influence of a force, is given by their mass.

    - Mass, energy, and pressure modify the geometry of spacetime, according to a Einstein's equations, thus providing the background to the new law of inertia, as given above. Note that there is a dynamic interaction between these laws - motion changes geometry, which influences motion.

    Thus, two laws of motion are replaced by one. However, in no sense is it explained why there is a phenomenon of inertia, or why there is mass. A further feature is that it actually turns out that you can dispense with the law of inertia (which is only approximate in general relativity anyway), because it is contained in the equations relating mass, energy, pressure and geometry.
  5. Sep 29, 2011 #4
    Although I am strictly a non-technical amateur, this is the first I've heard of pressure in connection with relativity. What is meant by pressure?
  6. Sep 29, 2011 #5


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    See, for example:


    the section:

    Identifying the components of the tensor
  7. Sep 30, 2011 #6
    I never like to use the term "inertia", though I consider it to be a valid one. Instead, I always refer to the Law of Conservation of Momentum, which can be converted into a mathematical format that defines the precise state of the elements of an experiment.
  8. Sep 30, 2011 #7
    I've often wondered about the source of inertia. Maybe it's intimately related in some way to the universal isotropic expansion. But I have no idea how that might work. Of course the expansion is another big mystery.

    You asked about a scientific consensus on the subject. As far as I know (which isn't very far), nobody has a clue. It's a fascinating mystery.
  9. Oct 2, 2011 #8
    GR was meant to give an answer to that, by taking ideas from Mach and by doing away of the need for using inertial reference systems - but in the end it didn't really. Einstein attempted to make acceleration as "relative" as uniform motion, but that idea and the related term "General Principle of Relativity" are little used nowadays.

    As far as I know GRT doesn't fully use Mach's ideas; consequently Mach's objections against Newton's conclusions* were not supported by a working theory. Perhaps someone else can give more details.

    *see: http://en.wikipedia.org/wiki/Bucket_argument
  10. Oct 2, 2011 #9
    "hand in hand" is not a term we would all agreed upon.

    If you mean that in general F = ma and where gravity is the acceleration then F = mg...ok.
    So it utrns out gravitational mass and inertial mass are equal; but I'm pretty sure there is
    no known reason why they have to be the same. And the mass of a body does determine how much it is affected by gravity: F = GmM/r2 is one view.

    However, mass is an observable, a constant in varying gravity: The mass of your computer, for example, is the same on the moon as on the earth; but it's weight is not.

    Also, it still takes a force to accelerate a mass even if NO gravity is present: so you have to
    expend energy,say in outer space, without any gravity to get your rocketship accelerating.

    Ultimately, "why" mass, energy, the forces, space and time exist at all is not well understood.
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