🚀 go-pugleaf

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.
That only begins the parade of difficulties attendant to using the currently
fashionable definition outside its proper area of use. "Mass" as you use it
is equal to neither inertial nor gravitational mass as those terms apply to
any system more complicated than a particle at rest! To further confuse the
beginning physics student you could explain that the mass of that lump of
matter is independent of its temperature. You could then define a "relativ-
istic mass" which just happens to be identical to its "inertial mass" and,
by the way, to its "gravitational mass". A more parsimonious observer might
argue that, if those three entities are always equivalent to one another,
perhaps it would be conceptually simpler to designate them as the "mass"
and define a related parameter intrinsic to a particle, its "rest mass",
as the derivative, less "fundamental", entity.

For the benefit of those too young to know I will point out that this con-
vention has changed over the years. It caught on about 25 years ago and has
been a problem ever since, especially conceptually. People actually believe
that there is *scientific* justification for their positions regarding this
convention. It is exactly as scientific as the observation that the bird we
North Americans call a "robin" is not a *true* robin! Actually the *only*
justification for the current convention is that one would have to use the 
term "rest mass" rather more frequently in particle contexts than the term 
"relativistic mass". In earlier times both terms were used to minimize con-
fusion.

Leigh