2 Relativistic mass depends on the frame of reference
The relativistic mass of an object corresponds to the total energy of an object (invariant proper mass plus kinetic energy).
Zero-particles have zero rest-mass [m.sub.0], zero relativistic mass m, and non-zero gravitational-rotational mass M, which is described in the [[??].sub.4] as
These particles are essentially thinner structures than light, because their relativistic masses m are zeroes.
Coming back to the main topic, in relativity we can either write our equations in terms of proper time or alternatively we can write them in terms of relativistic mass
Thus the observer's intrinsic frame (-[phi][??]*, -[phi]c[phi][??]*) containing negative intrinsic relativistic mass, -[phi]m* = -[gamma] [phi][m.sup.*.sub.0], in the intrinsic affine space coordinate -pXt*, will automatically appear in the negative universe, upon the particle's intrinsic frame ([phi][??]', [phi]c[phi][??]') containing positive intrinsic rest mass pm0 of the particle in the first quadrant making transition into the second quadrant.
There are, however, more formal and more complete derivations of these concepts along with the concepts of absolute intrinsic spacetime containing absolute intrinsic rest mass, which underlies absolute spacetime containing absolute rest mass and relativistic intrinsic spacetime containing relativistic intrinsic mass, which underlies relativistic spacetime containing relativistic mass, to be presented elsewhere with further development.
It is strange suggestion as Einstein has used relativistic mass
in his work including in the expression of relativistic kinetic energy  from which rest mass energy is derived.
over the surface of a light hypercone) bear zero rest-mass [m.sub.0] =0 but their relativistic mass (mass of motion) is nonzero m [not equal to] 0.
They are in the state of matter for which rest-mass is definitely zero [m.sub.0] = 0 but the relativistic mass is imaginary.
Regular mass-bearing particles possessing non-zero rest-masses and relativistic masses
Particles of non-zero rest-masses [m.sub.0] [not equal to] 0 (substance) can be moved: (1) along real world-trajectories cd[tau] > d[sigma], having real relativistic masses
m = [m.sub.0]/[square root of (1 - [v.sup.2]/[c.sup.2])]; (2) along imaginary world-trajectories cd[tau] < do, having imaginary relativistic masses
m = I[m.sub.0]/[square root of ([v.sup.2]/[c.sup.2] - 1)](tachyons).