Exploring the Concept of Relative Mass

[Mr_Quantum]

Is the mass relative?

 

[PeterDonis]

Is the mass relative?

The term "mass" in relativity usually refers to invariant mass, which is, as its name implies, invariant, i.e., not relative.

There is an older concept of "relativistic mass", but that's really just another name for energy. It is relative.

 

[DrGreg]

See our FAQ: What is relativistic mass and why it is not used much?

 

[Ibix]

"Invariant mass" or "rest mass" are two names for the same thing. In modern professional texts, this is what is meant by mass unless otherwise noted. It is not relative.

"Relativistic mass" is a different thing, and is the total energy of the body divided by $c^2$. Older serious textbooks, and popular science sources that are more concerned with cool than helpful do sometimes refer to this as mass. It is relative.

You may also come across "longitudinal mass" and "transverse mass", which are part of the reason for giving up on relativistic mass. These are also relative.

So the answer is that it depends. Modern serious sources regard "mass" as meaning invariant mass, which is invariant (as the name suggests). This is the best convention to adopt. When reading pop sci or old texts, keep your wits about you - "mass" could be either relative or not.

 

[sweet springs]

Mass of body is energy measured in special system where its momentum = 0. By this definition mass cannot be relative. All the system use the same value that was measured in the appointed system.

 

[jtbell]

Mass of body is energy measured in special system where its momentum = 0. By this definition mass cannot be relative.

You can also define it via the equation $mc^2 = \sqrt {E^2 - (pc)^2}$ which gives the same result regardless of which inertial reference frame you measure $E$ and $p$ in.