Re: What Hath Einstein Writ?

[Joel Rauber <Joel_Rauber@SDSTATE.EDU>]

Hmm? Well let's see. What is it we need mass for? We need a proportionality
constant between momentum and v/Sqrt(1-v^2)  {units of c=1}.  I rather guess
that rest mass will suffice quite well and we don't need a concept like
relativistic mass.



My point:  momentum is

m_o*v/Sqrt(1-v^2)

One has two options, you may associate the ugly looking Sqrt term with m_o
and invent the concept relativistic mass.  The main justification being that
you may then write formulas for momentum that "look" like the old Newtonian
formulas, i.e. m*v. (The price paid is losing a nice invariant, the rest
mass.)

The other choice is to simply realize that the "look" of the correct special
relativistic formula for momentum does not have to be the Newtonian "look"
and one may simply realize that the proper way to write momentum is

m*v/Sqrt(1-v^2)     where m is the invariant rest mass.

Neither choice is wrong! You can develope a fully consistant and correct
special relativistic kinematics either way; but one does see relativistic
mass less and less these days. . .

Joel Rauber


> Hmm?  Well let's see.  What is it we need mass for?   We need a
> proportionality constant between momentum and velocity, and we need
> something to be proportional to the total energy of a particle.
> In neither case will the rest mass suffice so we are sort of
> stuck with
> the relativistic mass.   That said, I must admit that the
> concept of the
> rest mass is useful in the theory in an way that is analogous to the
> usefulness of the proper time.                      WBN