TSR formulas diverge when speed between inertial references approaches c speed of light in vacuum Particles moving with the speed of light in a vacuum have zero #MassAtRest
In 1967, #physicist Gerald Feinberg proposed #tachyon concept which requires extending definitions of mass and energy.
If #tachyons are accepted,
key problem is presented by time because Heisenberg uncertainty principle links energy of a quantum state with a lifetime
IMHO if u are not #Oviedo trolls,do not waste time in this

Right now there's a discussion running on about #RelativisticMass and its relation to limit of the speed of light in vacuum in the TSR
As u would know m0 #MassAtRest is a constant and the m that appears in the expression E=mc^2 is the so-called #RelativisticMass
When four-vectors are built in #Minkowski space,magnitude of energy-momentum is scalar and from there expression E^2 = m0^2c^4 + p^2c^2 is deduced
Concept is useful if it is known how to apply, if not fallacies that populate TSR pop up