Re: [Phys-l] Rest mass again?

[Moses Fayngold <moshfarlan@yahoo.com>]

 Derek McKenzie wrote on Wed, October 20, 2010 6:42:13 PM

> The term 'rest mass' seems to be quite misleading to me because although we may 
> choose to view a lump of matter as 'at rest' in a particular frame, it >is made 
> up of many moving constituents in that same frame...

 Yes, so what? A "particular frame" is unambiguously defined as a frame in which 
the net momentum of a system is zero. It is a rest frame of a system.
What is misleading about it? 
  Similarly, the rest (invariant) mass is defined as the norm of a 4-momentum of 
a system. Applying this to the rest frame of the system where the system's 
3-momentum, by above definition, is zero, you get the rest mass. And since it 
does not matter what system you use for calculating an invariant characteristic, 
the invariant mass and the rest mass are synonyms. What is so misleading about 
it? 
 
     >...and what one person calls 'rest mass' could equally well be called a 
sum of relativistic masses of its constituents. 

 Yes, so what? If we have one photon, we usually call it massless. I do 
it myself, unfortunately, just to pay tribute to what is "customary". But if one 
insists that there is no mass other than the rest mass, then the term "massless" 
for a photon becomes misleading, for a very simple reason. Add another photon 
with the opposite momentum of the same magnitude. Now you have a system of two 
photons which (the system) has a rest frame and a non-zero rest mass. In my 
view, it is more convenient to say that this rest mass comes from the 
relativistic masses of constituents than to say that it is rest energy of the 
system divided by c^2. The first possibility avoids any necessity to explain the 
students (without invoking energy, which is a different property of an 
object), why is each photon separately massless but the system of both is 
massive.
  The first possibility DOES NOT deny the second one. But those who discard the 
first one, should explain their students how come that energy/c^2 determines 
mass in one special reference frame, but does not determine anything meaningful 
in all other frames. This would single out that one frame as something really 
special, quite in the spirit of pre-Galilean physics, and totally against 
relativity. 

Moses Fayngold,
NJIT

                           
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