# Relativistic mass

I've always seen older books talking about relativistic mass all over their special relativity introduction $$m=\gamma m_0$$. But I can't stand it. It makes no sense to define this quantity at all.

To explain myself, mass can't be measured for a moving object, so why even bother bringing this concept up in the first place. Also, it is not needed in any way to derive any other formula from special relativity.

I just think it's pointless to even point out that mass could change for a moving particle when no one could ever measure it.

Maybe I'm wrong in all my explanation, if so, is there any meaningful use of this relativistic mass?

• "Mass can't be measured for a moving object..." can you say more about this? – M. Enns Mar 4 at 22:57
• how else to measure mass? I thought you should move it.... – Žarko Tomičić Mar 4 at 22:58
• Einstein never derived an equation for "relativistic mass", and in later years he expressed his dislike of the idea:[23] "It is not good to introduce the concept of the mass $\displaystyle M=m/{\sqrt {1-v^{2}/c^{2}}}$ $M = m/\sqrt{1 - v^2/c^2}$ of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the ’rest mass’ m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion." en.wikipedia.org/wiki/Mass_in_special_relativity – Gert Mar 5 at 0:13
• From the same link: "The concept of "relativistic mass" is subject to misunderstanding. That's why we don't use it. First, it applies the name mass – belonging to the magnitude of a 4-vector – to a very different concept, the time component of a 4-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of spacetime itself."[7] – Gert Mar 5 at 0:25
• "mass can't be measured for a moving object" Yes it can. Including the invariant kind, the so-called "transverse mass" $\gamma m$, and the "longitudinal mass" $\gamma^3 m$. I invite you to consider—for instance—the operation of a mass spectrometer. Questions for the student: which quantity (or quantities) can be found from the results? Why do you say that? – dmckee Mar 5 at 3:45