🚀 go-pugleaf

In article <1992Jul22.193837.18095@sfu.ca> palmer@sfu.ca (Leigh Palmer) writes:
>In article <9868@sun13.scri.fsu.edu> jac@ds8.scri.fsu.edu (Jim Carr) writes:
>
>>It should be added that I do not know of any physicist who works wiht 
>>relativistic kinematics -- either nuclear physicists, astrophysicists, 
>>or particle physicists -- who use this first definition.  There is the 
>>variable W = sqrt(s), where s is one of the Mandelstam variables, known 
>>as the total energy that can be used for such purposes. 
>
>Well, Jim, now you do. In the context of those disciplines the conventional
>definition of mass is *now* that which you advocate. I surely hope that you
>do not use that definition in teaching elementary physics, however, which 
>is how some of us astrophysicists make our livings. In order to illustrate
>my point, do you claim that a lump of condensed matter has a the same mass
>as the sum of the masses of its elementary constituents? Of course you don't.

Oh oh, of course not.  I even use the proton mass (gasp) when working out 
the kinematics for a proton despite the fact that its constituents have 
very little mass (in the sense of the current quarks one would use in an 
ab initio calculation of the proton mass).  

The risks of living a skizo life where sometimes one works at calculating 
the mass of something from the masses and binding energies of its 
constituents, and other times one treats the object as "elementary" 
for the purposes of calculating other things...  Thanks for the reminder!  

I guess I should say that no one uses the first definition when working 
out the kinematics of objects (composite or not), but that one has to 
be careful when speaking about other aspects of the prescriptions used 
to accomodate language and physics.  Of course the mass of a composite 
object should be the rest energy if you assume that a composite object is 
governed by the relativistic energy equation.  This is not the same as 
the first definition, where the mass depends on the reference frame, 
but there are some common elements in the two pictures.  Certainly 
one can compute correct answers either way, but the second method is 
simpler and more easily applied.

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)