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Mass what is it?

  1. Jan 25, 2005 #1
    Here's a controversial topic... mass... i was wondering maybe all mass is just rotational inerta i mean think abt it, electrons move around the protons like a guy swinging his sling causing inertia at the center or at the protons and then electrons spin themselves so mayb their mass is proprtional to their momentum... what do u guyz think? i think i can work out the mathematics for it if u guyz say it could be right...

    Photons don't spin, they have no mass.....
  2. jcsd
  3. Jan 25, 2005 #2


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    Sorry, photons do have spin angular momentum, with magnitude [tex]\hbar[/tex].
  4. Jan 26, 2005 #3


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    Mass is one of those hard to talk about questions, b/c the fact is no one really knows. We do however have a consistent interpretation for the 'number' that pops out of our experiments.

    Various theories define mass in slightly different ways. In my opinion quantum mechanics treats it in a very classical sense, regarding it as akin to a 'lump' of matter, though you could probably debate that. Of course that had to change with E = mc^2 and special relativity. So in early relativistic quantum mechanics, its really the mass/energy of a field excitation. Then its further complicated in QFT, b/c of additional vacuum quantities polarizing the number. One has in mind a relative concept, where some 'thing' drags a long other 'things' and acquires a concept we can measure. This is the bare vs renormalized mass people talk about. Note that its hard to make sense of the concept to the 'field' itself. Does a field have mass or energy? Well kinda, but we don't really say 'oh an electron field weighs so and so'. Rather again only excitations thereof are interpreted that way.

    Not entirely satisfying. Add curved spacetime to the mix, and well you get additional difficulties. It then becomes frame dependant. One observer might not even measure a field excitation, whereas another does.
  5. Jan 26, 2005 #4
    odd consept, u would think that something that is used so easily everyday would be understood by now.... oh well... just goes to show how far we still have to advance as a species.... huh... thx...
  6. Jan 26, 2005 #5
    Is mass the same on every planet? Or does it directly relate to gravity as weight does?
  7. Jan 26, 2005 #6
    Mass does not depend on gravity. You have the same mass here on earth as you do on the moon. While your weight will vary due to the value of g.
  8. Jan 26, 2005 #7


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    I hope u're talking about the REST MASS,the fundamental intrinsic property of matter...

  9. Jan 26, 2005 #8
    Typically when I read mass, I really read rest mass (especially on these boards). And I assumed, that in this case, the question asked was pertaining to rest mass. But feel free to explain all about relativistic mass since you brought it up!
  10. Jan 26, 2005 #9


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    Relativistic mass [tex]m_r=\gamma m_0[/tex]
    So Matt, when your on a different planet your mass varies depending on the force of gravity pushing down on you, which is dependent on the planet's mass and density. Relativistic mass is related to velocity, for example, traveling near light speed, mass would be much heavier then it was at rest, in a vaccum. So as a mass is being dropped from the sky of a planet, the force of gravity accelerates the object, giving it its velocity, therefore making it heavier and heavier with every second. Relativistic mass. :biggrin:
  11. Jan 26, 2005 #10


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    Not accurately.U've given him the relativistic mass in the case GRAVITY IS NOT CONSIDERED.

    So your formula won't hold for a gravity field...It doesn't matter whether the particle's moving is not...It can be at rest,and yet,due to the gravity of the celestial body its mass would be different...

    Last edited: Jan 26, 2005
  12. Jan 26, 2005 #11


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    Yes, I see your point. And know I have a question. :tongue2:

    [tex]E_{g.field}=4\pi G \frac{m}{4\pi r^2}[/tex]
    to get the accelerated force on a planet, how would you come up with it's relativistic mass, and is relativistic mass on a surface of a planet = to the mass that would be shown on a scale on that planet?
  13. Jan 26, 2005 #12


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    No,the GTR (General Theory of Relativity) brings the necessary changes to the formula from the Special Relativity
    [tex] m_{SR}=\gamma m_{0} [/tex]

    ,not the theory of Newton...

  14. Jan 26, 2005 #13


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    Ok, so how would I find the accelerated gravitational force on a planet and then find the mass of an object on the planets surface, knowing the objects rest mass in a vacuum. According the GR.
  15. Jan 26, 2005 #14


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    U need to solve two sets of equations:the Einstein equations which would give the metric and then the equations of geodesic movement for the body...

  16. Jan 26, 2005 #15
    Maybe this is a good place to interject some thoughts: Why is there momentum? I figured this question was relevent to the topic of discussion.

    All of classical and modern physics are predicated on the concept of momentum, but the the property itself is not discoursed about in and of itself and IMHO is not very well understood. This one has been bugging me for a while and I would like to get some feedback.

    Specualtion: What if we were to construct the Lagrangian/Hamiltonian in a very superfulous way. Instead of

    [tex] p \Rightarrow F=0[/tex]

    i.e. there is no outside force acting upon the particle, say we assume an 'ominforce' on the particle by making the following statement:

    [tex] p \Rightarrow \Sigma F=0[/tex]

    Then construct our dynamics form there. I have not worked it out myself yet but I have been wondering if this would yeild the same results and perhaps yeild an insight into this property called momentum/mass.

    Thoughts, speculations, incriminations?

    Fire away!
    Last edited: Jan 26, 2005
  17. Jan 26, 2005 #16
    The definition of mass appears to not have any obvious definition.

    Is it possible to consider "mass" as anything that occupies space-time, be it at absolute rest or relative rest?
  18. Jan 27, 2005 #17
    Interesting point of view. This means that mass does not only constitute energy but intertwined with surrounding forces as well. In another words, mass is equal to the product of some portion of energy and some portion of surrounding forces?

  19. Jan 28, 2005 #18


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    The current account of mass in quantum physics is that it is an interaction of some kind. The Higgs mechanism gives mass to most elementary particles, but the major part of what we think of as mass in our world is the binding energy of the gluon sea inside the proton and neutron.
  20. Jan 28, 2005 #19
    Does this definition cover the BE of the gluon sea? "..."mass" as anything that occupies space-time, be it at absolute rest or relative rest."
  21. Jan 28, 2005 #20
    What sA means is that mass can also come from any kind of interaction going on because these interactions correspond to energy and via E=mc², energy is equivalent to mass. The mass of a quark is indeed the rest mass plus the mass coming from interactions of the quarks with gluons and pions. In QFT this is called dynamical effective mass, a bit analoguous (but not exactly the same) as the effective mass in solid state physics.

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