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 ... followups to sci.physics.relativity ...


meron@cars3.uchicago.edu wrote:
}...
} No, it doesn't.  Mass is the invariant length of the energy-momentum
} 4-vector.

Fred McGalliard <frederick.b.mcgalliard@boeing.com> writes:
>
>Mati, this is certainly a sound, but rather myoptic view. 

 Have you read the FAQ on this?  Given the broad cross-post, I will 
 assume you did not read the interesting remarks Einstein made on 
 this subject (they appeared only in sci.physics.relativity and 
 sci.physics).  Quoted in American Journal of Physics 55, 739 (1987):

  Whatever Einstein's precise early views were on the subject, his
  view in later life appears clear.  In a 1948 letter to Lincoln
  Barnett, he wrote
  "It is not good to introduce the concept of the mass
  M = m/(1-v^2/c^2)^{1/2} of a body for which no clear definition
  can be given.  It is better to introduce no other mass than
  'the rest mass' m.  Instead of introducing M, it is better to
  mention the expression for the momentum and energy of a body
  in motion."

 In any case, and regardless of this particular appeal to authority, 
 it is not short sighted to use the convention used by every person 
 I know who is doing anything remotely related to relativity.  Even 
 more so, it is far better to use a sense for the word that agrees 
 with what (noting your posting host) engineers designing aircraft 
 a Boeing use for "mass". 

>From
>fundamental phenomina, isn't mass the character that assignes momentum?

 No.  Light carries momentum.  Momentum tells you something that can 
 be related to the energy of motion, while mass tells you something 
 that can be related to the intrinsic energy (the energy unrelated 
 to motion, an invariant) of a body. 

>So more properly said, it is not invariant at all. It is just the total
>energy contained in the object, nes't ce pas?

 The total energy _of_ the object is E, which is _not_ invariant.  

 The total energy _contained_in_ the object (that is, E in the rest 
 frame of the object) is mc^2, which _is_ invariant. 

 They are related by E^2 = (mc^2)^2 + (pc)^2. 

-- 
 James A. Carr   <jac@scri.fsu.edu>     | Commercial e-mail is _NOT_ 
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