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Mass Dialation question

  1. May 7, 2008 #1
    I just have a question relating to Mass Dilation;

    Mass is defined (by the dictionary) as

    Now, mass is a collection of particles - protons, neutrons and electrons. Mass dilation states that when approaching the speed of light, from an outside observer, the mass of the object will be recorded as being greater than that recorded by the object itself.

    [The way Michio Kaku says it] How could this be?

    This would mean that particles appear from no where from one point of view but not from another. There will be two different counts of particles from the same thing!!!

    Could someone please explain?
  2. jcsd
  3. May 7, 2008 #2


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    Gold Member

    The definition of mass you give is rubbish.

    Inertial mass is the tendency to resist acceleration, and gravitational mass is the source of gravity. It has nothing to do with 'collections of incoherent particles'.

    Get a better dictionary.

    No, it does not. The apparent increase in 'ponderability' ( an old term for mass) would apply to individual entities.
  4. May 7, 2008 #3
    One must do work to make a mass at rest accelerate (one must overcome its inertia, which is directly related to the mass' rest energy content, as mentioned by Mentz114).

    By performing this work, one expends their own energy, which is then transferred to the mass in the form of kinetic energy. Since the total amount of energy within the mass has now increased (rest + kinetic = total), its inertia has also increased, making the mass even harder to further accelerate. By the time the mass has reached nearly the speed of light, its kinetic energy becomes incredibly great, and so it would take an incredibly great amount of extra energy to accelerate it further even by the slightest amount.

    In other words, it's a no-win situation for anyone wanting to accelerate a mass to the speed of light. One must add energy to the body to accelerate it, but by doing so, one makes the body harder to accelerate further.

    This is why it's impossible to accelerate a mass to the speed of light -- it would take an infinite amount of energy to do so, and there is not an infinite amount of energy at one's disposal to do so.
    Last edited: May 7, 2008
  5. May 8, 2008 #4


    Staff: Mentor

    That definition of mass sounded like a medical definition, as in "your MRI shows a mass in your liver, we need to do a biopsy". What you need is a physical definition, not a dictionary definition. Among other ways to define it mass is the norm (aka magnitude) of the momentum divided by the norm of the velocity.
    You have stepped into the middle of a running argument here on PF (though the usual participants seem to have missed this thread). There are basically 2 distinct meanings for the term "mass" in physics. They are "relativistic mass" and "rest mass". Generally physicists say "mass" without any qualifiers they mean "rest mass". Rest mass does not dilate and it is the quantity that is a fundamental property of the particle. Relativistic mass is as much a property of the observer as the particle, so it is not necessary that there be more particles when it increases.
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