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Mass in relativity

  1. Jan 29, 2009 #1
    Dalespam posted in another thread (as have others in other threads) responding to a comment:

    .

    I still find that very confusing unless this is simply a modern convention.

    In SIX EASY PIECES for example (copyright 1997) pages 87 to 91 Richard Feynman seems to say repeatedly mass DOES increase with speed. For example:

    and separately:
    Explanations appreciated.
     
  2. jcsd
  3. Jan 29, 2009 #2
    Rest mass + energy needed for relativistic speed = relativistic mass.
    Relativistic mass is only observed from outside the inertial frame.
    All non accelerating frames are at rest so maybe relativistic mass is an illusion.
    Next - Someone that knows what they are talking about.
     
  4. Jan 29, 2009 #3

    DrGreg

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    I refer the honourable gentleman to the answer I gave a few threads ago:

    The problem is that not everybody agrees which convention to use. Feynman was referring to relativistic mass, but many users in this forum insist on following the modern convention of referring to invariant mass a.k.a. rest mass. It's the one I prefer myself, but both conventions are valid, as long as you know which one the author is using. The confusion arises when two thread-posters are using different conventions and maybe not realising it.
     
  5. Jan 29, 2009 #4
    I don't like that explanation (much) because KE and relativistic mass vary by the same Lorentz transformation amount ....so for anyone to explain relativistic mass increase is less real than KE energy increase due to inertial observation frame makes no sense to me.
     
  6. Jan 29, 2009 #5
    DrGreg...thanks for your input...I DID see that earlier post and forgot about it.....if that's the convention generally used, that's just peachy by me...(If you just tell me the same thing a few more times I might even remember it!!)

    Until I was just rereading Feynmann's book this morning I had also forgotten how blatently he describes KE and relativistic mass being "one"....when he says because the resulting object of a collision is heavier due to energy content, "it will be a different object" lights went off....

    His writing is just great!!!
     
  7. Jan 29, 2009 #6
    So let's say we have two isolated particles and an arbitrary reference frame (I don't see how in this case you could define an Inertial Reference Frame). Can you write equations of motion?
     
  8. Jan 29, 2009 #7

    jtbell

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    Staff: Mentor

    In a similar vein to Dr. Greg, I offer this post and this one, which I've written previously on this subject.
     
  9. Jan 29, 2009 #8
    I said "Relativistic mass is only observed from outside the inertial frame.
    All non accelerating frames are at rest so maybe relativistic mass is an illusion."

    DrGreg said "...but many users in this forum insist on following the modern convention of referring to invariant mass a.k.a. rest mass. It's the one I prefer myself,..."

    I am in no way in DrGreq's league but it sounds like we are saying something similar. It seems that maybe KE should be ignored (until impact) unless there is an ether to compare speed to.
     
  10. Jan 29, 2009 #9

    Dale

    Staff: Mentor

    It is simply the modern convention. Unfortunately there is a lot of historical work out there that uses "relativistic mass" either explicitly or implicitly.

    The reason the modern convention is used is because of the four-momentum. Relativistic mass is the timelike component of the four-momentum, which is also (and more commonly) known as energy. Invariant mass is the norm of the four momentum, which does not have another common name. So just from a communication standpoint it makes more sense to use the word "mass" to refer to the norm and "energy" to refer to the timelike component.

    Secondly, even though laypeople and pop-science books tend to use relativistic mass it is a source of confusion. Mass is generally thought to be a property of an object rather than an observer, so the idea that motion increases mass leads to a feeling that motion is still absolute. Also, since mass is usually thought to be a property of an object it does not make sense that it could have multiple different values at the same time as relativistic mass does.

    IMO (and in the opinion of the modern physics community) there is no reason to use "relativistic mass" instead of energy. E.g. "An object cannot accelerate to the speed of light because it would have infinite energy" works just as well as "An object cannot accelerate to the speed of light because it would have infinite (relativistic) mass"

    However, in the end it is just a convention, as is the use of any word.
     
    Last edited: Jan 29, 2009
  11. Jan 29, 2009 #10
    Wouldn't their motion relative to each other define the inertial frame?
     
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