Editing Mass in special relativity (section)

{{More footnotes needed|date=August 2023}}

{{short description|Meanings of mass in special relativity}}

The word "[[mass]]" has two meanings in [[special relativity]]: ''[[invariant mass]]'' (also called rest mass) is an [[invariant quantity]] which is the same for all [[Observer (special relativity)|observers]] in all [[reference frames]], while the '''relativistic mass''' is dependent on the velocity of the observer. According to the concept of [[mass–energy equivalence]], invariant mass is equivalent to ''[[rest energy]]'', while relativistic mass is equivalent to ''[[relativistic energy]]'' (also called total energy).

The term "relativistic mass" tends not to be used in particle and nuclear physics and is often avoided by writers on special relativity, in favor of referring to the body's relativistic energy.<ref name=roche>{{cite journal |last=Roche |first=J |year=2005 |title=What is mass? |journal=[[European Journal of Physics]] |volume=26 |issue= 2|page=225 |bibcode= 2005EJPh...26..225R |doi=10.1088/0143-0807/26/2/002 |s2cid=122254861 |url=http://www.marco-learningsystems.com/pages/roche/what-is-mass.pdf}}</ref> In contrast, "invariant mass" is usually preferred over rest energy. The measurable inertia of a body in a given frame of reference is determined by its relativistic mass, not merely its invariant mass. For example, photons have zero rest mass but contribute to the inertia (and weight in a gravitational field) of any system containing them.

The concept is generalized in [[mass in general relativity]].