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1. Relativistic Mass

   Linked via "Albert Einstein's theory of Special Relativity (1905)"

   Relativistic mass is a concept originating from Albert Einstein's theory of Special Relativity (1905)/), which describes how physical properties of an object change when it moves at speeds approaching the speed of light ($c$) relative to an observer [frame of reference]. It is defined historically as the ratio of the total relativistic energy ($E$) of an object to the square of the speed of light ($c$), such that $E = m_{\text{rel}} c^2$.
   The formal relationship connecting re…

2. Relativistic Mass

   Linked via "Special Relativity"

   Conceptual History and Context
   The notion of "relativistic mass" gained prominence in the early 20th century as physicists reconciled classical Newtonian mechanics with the new postulates of Special Relativity/), particularly the constancy of the speed of light in all inertial frames. Early presentations of relativity often utilized this concept to maintain the Newtonian form of [momentum](/entries/momen…