Has relativistic mass gone out of fashion?

[ChrisVer]

Well, you can also have imaginary mass without having superluminal propagation...http://en.wikipedia.org/wiki/Tachyon#Fields_with_imaginary_mass

 

[stevendaryl]

There was a discussion in Physics Forums that showed that the concept of "relativistic mass" was of some theoretical interest in understanding how the group theory of the Poincare group relates to the group theory of the Galilei group in the nonrelativistic limit.

You actually need an 11th degree of freedom to define the Galilei
limit. The 11th parameter (in Galilei) is the "central charge"
associated with mass. To make Poincare' suitable for taking the limit
to Galilei (that is: to implement the correspondence principle with
respect to non-relativistic physics!), one needs to split the energy
generator E into two parameters -- kinetic energy H and "relativistic
mass" M. E does not have a Galilean limit. The mass shell condition (E/
c)^2 - P^2 = (mc)^2 (where P is the momentum and m the rest mass)
needs to be generalized to P^2 - 2MH + (1/c)^2 H^2 = constant. An
additional invariant emerges: M - (1/c)^2 H = constant.

So it's not 100% correct that nobody cares about relativistic mass anymore. Maybe 99%.

https://www.physicsforums.com/showthread.php?t=174039