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What is the cause of inertia.

  1. Jun 16, 2010 #1

    My name is Edward Solomon and I have found a very particular concept in classical physics to be more mystifying and puzzling than even more advanced topics covered in modern physics. This is the theory of "inertia."

    Although there is much literature covering the effects of inertia, I have yet to come across any texts that state the "cause" of inertia. The best answer I've come across is that it is an "inherent property of mass."

    What is interesting is that inertia cannot be a force. This follows from the very definition of inertia. Here is a simple (and hopefully well-known) proof of this claim.

    F = ma

    We can interpret inertia to say that an object will not accelerate unless acted upon by some force. In other words inertia preserves scalar and angular momentum, thus the "force" of inertia is equal to zero.

    So now for my ultimate question.

    Photons carry electro-magnetic radiation, gluons carry the strong force, the predicted-but-yet-unobserved gravitons carry the gravitational force (recall that inertia and gravity are derived from the same quantity "mass"). Is there any hypothesized particle that may carry inertia?

    Another question: The centrifugal force, Coriolis Force and the Euler force are known as the three fictitious forces. Is the concept of inertia simply a "fictitious" entity itself to explain what we do not know.

    Last question:
    Even if inertia is real, its force is equal to zero, so how could we ever detect it? How can it even interact with our universe? For instance if in some strange universe, Newton's Gravitational Constant (G) was equal to 0, then we could hypothesize that gravity exists, but we could never detect it because as far as our universe is concerned, gravity does not exist. The same concept seems to apply to inertia in OUR universe, unless there is a different concept of force that must be invented/discovered in order to adequately explain how it does interact with our universe.
  2. jcsd
  3. Jun 16, 2010 #2
    Inertia is a property of mass (or perhaps, a definition of mass). Like most fundamental properties, nobody knows what 'causes' it.

    In my opinion this kind of question is not a question that we should expect science or physics to answer. Science models and describes our observations, the fundamental 'causes' and 'reality' are forever hidden from us. Perhaps someday we will explain it in light of a more fundamental property, but for now it is seen as fundamental and thus no further explanation is expected.
  4. Jun 16, 2010 #3


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    There are two parts to inertia. First, the tendency of things to stay at rest. You don't expect something at rest to suddenly start moving for no apparent reason. That'd be bad. You wouldn't be able to predict anything. Of course, once you establish that, going into a moving frame of reference gives you tendency to move in a straight line. Again, no mystery here. Finally, the the thing that makes it do something other than stay at rest we call a force. That's the definition of force, so asking why a force accelerates mass is pointless. A better question is why effect is linear, and that can also be explained by considering different coordinate system. If you want, I can go into a bit more detail here.

    Second part, and probably what you are really thinking of, is the tendency of objects to push back when you push against them. So it's really the Newton's Third Law that makes you think of inertia, not second.

    So why must there be a reaction force equal to the applied force? Fundamentally, this is going to be because of conservation of momentum, and that, again, has to do with symmetries in space. But for very simple explanation, think of what causes interaction forces? You normally come across gravitational and electrostatic forces. For both, the particle that's affected by the field, creates a proportional field of its own. So if particle A experiences force F in field from particle B, the particle B is in the field of A, and will experience force -F.

    Both of these work within the framework of existing forces, so we really do not need any new particles. If you apply a force via gravity, your reaction force, which you perceive as result of inertia, is also gravitational. No new carrier particles are needed.

    Fictitious forces aren't fictitious because they are made up or undetectable or anything of the sort. It's just a name. They are called fictitious because they depend on the choice of coordinate system.

    In view of all of the above, I think you'll agree that your last question is a non-sequitur.
  5. Jun 17, 2010 #4
    I don't think it's a silly question, but it's more a philosophical question than a scientific one. There are certain qualities in the universe which have no apparent cause. Not just inertia, but time and space, gravity, magnetism, etc. These just have to be called "phenomena". They all come into existence with the Big Bang, and we can't investigate that, because at that point in the history of the universe the tools science needs don't even exist yet. The Big Bang is apparently a causeless cause, so ultimately it isn't a rational universe. We just have to accept the phenomena that are presented to us and deduce or infer laws that govern how it all works. That's all science can do. Inertia is just a phenomenon whose cause is unknown.
  6. Jun 17, 2010 #5
    If I give kinetic energy to a body, they will accelerate and this new velocity in the same frame of reference will be an increase in inertial mass. So, energy can be converted in inertial mass. I can increase and decrease inertia given or taken energy to a body.
  7. Jun 17, 2010 #6
    Why should the cause be unknown? In my mind just using the little twist we call big bang as the cause explains the motion I still feel today because our continuum was created when the original hot dense state broke into our atoms 300,000 light years after the singularity. Outward motion is the motion of energy, which is the motion we count as time, mass is nothing more than the dilating momentum of matter from the inside out imparted at this break up. A dilating area is all we need from the singularity.
  8. Jun 17, 2010 #7
  9. Jun 17, 2010 #8
    There is a book by Amitabh Ghosh (Manufacturing Engineering emeritus professor!) named 'The Origin Of Inertia'. He takes a different and uncommon (but not new) view of inertia and Newtonian Laws.
    As of fictitious forces, if we call them fictitious because of their dependence on reference frame (of course non inertial) then what is real (non-fictitious) for every force is dependent on choice of frame- take a frame with acceleration 'a' and force,F[=ma] is gone! No gravity in freely falling lift!!
  10. Jun 18, 2010 #9
    Sounds like a situation of some gravity to me.
  11. Jun 18, 2010 #10
    I don't find inertia so odd. Most things in physics are constant, in-and-of-themselves, and change with external conditions, inertia included.
  12. Jun 18, 2010 #11
    The cause of inertia IS a question we should expect science to answer. The objective of science is to explain what happens (which we are getting pretty good at doing) and why it happens (we are not so strong here.).

    Inertia is not necessarily any more strange that electric charge or energy or time. We don't know where any of them originate, but we are lucky they did or we would not be having this discussion.
  13. Jun 18, 2010 #12
    A small spherical shell of charge has electromagnetic mass ... an external agent must exert a force on the sphere in order to accelerate it. When the charge is accelerated, its own fields INDUCE an electric field component right at the charge itself. This component is such as to always point opposite to the acceleration. It is helpful to think of the charge as a "conduit" or point of contact between the driving agent and the electromagnetic field. The agent exerts a force on the field (by pushing/pulling on the field source) and the field pushes back, ala Newton 3, via an electric force experienced by the charge in its own field. Using the field solutions for point charges, this can easily be modeled on a computer, and it can be demonstrated that the accelerating charge does indeed experience a self-induced electric field right at its own position. "The Feynman Lectures ...", V2, Sect. 28-4 discuss "The force of an electron on itself."
  14. Jun 18, 2010 #13


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  15. Jun 18, 2010 #14


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    Regarding inertia as an "inherent property of mass", that doesn't work.
    If you throw, say, a marble towards a chunk of clay the marble will embed itself into the clay. Penetrating the clay takes considerable force. The fact that the fast-moving marble can penetrate the clay cannot be seen as just an inherent property of the marble; it cannot be the case that the marble is pushing itself into the clay.

    For comparison: there is a Baron von Munchhausen story in which he pulls himself out of a swamp by grabbing his own hair and pulling himself up with all this strength! Clearly that is unphysical, and the idea of a marble pushing itself into the clay is just as unphysical.

    I agree with that, but it's not a very deep statement. The thing is, the very concept of force is defined as: 'that which causes acceleration'. We have that inertia opposes acceleration, so in order to be consistent with the definition of force inertia cannot be categorized as a force. As I said, this isn't so much physics considerations, it's about using the chosen definition of force consistently.

    Electromagnetism and the Strong Nuclear force are interactions between pairs of particles. The interaction is thought of as being mediated by a field. Each particle is thought of as the source of a field.

    Inertia is quite different in nature; it's not interaction between pairs of particles. While you can get away from the source of an electric field, you can't get away from inertia. At present the properties of inertia must be assumed in order to formulate a theory at all. Possibly at some point in the future we'll be able to account for inertia in terms of a deeper theory, but as yet there are no clues as to what such a theory should look like.

    About the Higgs field.
    What is interesting about the Higgs field is that it is thought of as a field that is uniformly present everywhere. The Higgs field is not attributed to a source, the Higgs field is just there.

    It's my understanding that for certain particles interaction with the Higgs field imposes an energy cost. It is the energy associated with that energy cost that gives rise to the mass of those particles. As described by special relativity, Energy has inertial mass. When a quantity of energy is confined to a finite volume of space then that quantity of energy has a corresponding inertial mass. The formula m=E/c^2 expresses the magnitude of the inertial mass in that case.

    In other words, the Higgs mechanism does not account for inertia itself; the Higgs mechanisms does not explain why matter and energy are subject to inertia.
  16. Jun 18, 2010 #15


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    But if the marble had zero (or negligible) mass, it could not push itself into the clay.

    I think inertia is closely related to momentum, which is comprised of its mass and its kinetic energy. You have imbued it with kinetic nergy, but its mass is intrinsic.
  17. Jun 18, 2010 #16
    Related yes, but different.

    IMHO The best non mathematical explanation of inertia is the idea of

    "resistance to change"

    This can be applied to physical systems other than mechanical which also exhibit such a resistance.

    In mechanical systems you need to also consider rotational inertia, which is independant of linear momentum.
  18. Jun 18, 2010 #17


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    I like to think of inertia as opposition to change. Inertia opposes change of velocity, but does not prevent it.

    There is an intriguing analogy with with the physics of electric current in a circuit. Electric resistence is analogous to friction. Electric resistence limits current strength. Given a particular electromotive force and a particular resistence there is a corresponding current strength.

    Then there is inductance.
    Take a current circuit that includes a coil with self-induction. When there is current the coil gives rise to a steady magnetic field. This setup does not resist current. However, when there is change of current strength the magnetic field changes, the change in magnetic field induces an electric field that opposes the change in current strength. In a circuit with high inductance it is hard to get a current flowing, and it is hard to decrease current strength. Starting with zero current: if you apply a voltage the current will steadily climb. The higher the inductance, the slower the increase in current strength.

    (Inductance can be quite dangerous. If you have current flowing in a circuit with high inductance then when you break the circuit the current will continue nonetheless, and it will arc between the separated contact points.)

    Inductance is a responsive process. The system as a whole responds to any change, with a reaction that opposes the change. Inductance does not prevent change in current strength because it arises in response. For the opposing force to arise there must be change to begin with.

    Summerizing the analogy:
    - Electric resistence relates to current strength, mechanical friction relates to velocity.
    - Inductance relates to change of current strength, inertia relates to change of velocity.
    Last edited: Jun 18, 2010
  19. Jun 19, 2010 #18
    Or we can view matter as field with wave properties. In such a view where we talk waves, a propagating disturbance tends to continue in its propagation. Normally we see this as inertia if the propagation velocity is greater than c (with a group velocity less than c). For particles that propagate at c, and so would always propagate at c, the association with inertia is not made, yet the difference between the two disturbances is somewhat trivial.

    In this view, the concept of inertia loses value as a meaningful physical phenomenon. One varies under a Lorentz boost, the other is constant. With this difference in mind, we can ask why one sort of propagation is massive, where the wave velocity must exceed c, but where the natural propagation velocity of Minkowski space is exactly c.
  20. Jun 19, 2010 #19
    Inertia as resistance to velocity change is a partial story. We must not forget the equivalence principle- Gravitation too has an equally prominent role.
    Mass is also defined by gravitation.
    Charge is analogous to mass in terms of electrostatic field and gravitational field. And mass has two equivalent forms. But charge still does not have more than one equivalent form. I see a broken symmetry here - quite interesting!!!!!!!
  21. Jun 19, 2010 #20


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    What? No it isn't.

    Weight is defined by gravitation. Mass is defined only by matter.

    A 1kg massive object floating in a void between superclusters a billion light years from the next nearest object will have a weight that is incalculably small, but its mass will still be 1kg. If you push on it, it will still react as a 1kg mass. a=F/m.
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