relativistic mass, in the special theory of relativity, the mass that is assigned to a body in motion. ... The relativistic mass m becomes infinite as the velocity of the body approaches the speed of light, so, even if large momentum and energy are arbitrarily supplied to a body, its velocity always remains less than c.

Why is relativistic mass important?

It explains simply and quantitatively why a body subject to a constant acceleration cannot reach the speed of light, and why the mass of a system emitting a photon decreases. In relativistic quantum chemistry, relativistic mass is used to explain electron orbital contraction in heavy elements.

Is relativistic mass outdated?

Relativistic mass is equivalent to energy, which is why relativistic mass is not a commonly used term nowadays. ... The mass is then independent of velocity and is closer to the old Newtonian concept.