The relativistic mass-energy equivalence is one of the most famous concepts in physics, captured by Einstein’s iconic idea that mass and energy are two forms of the same thing. This means that any object with mass has an equivalent amount of energy, even when it’s at rest.
In relativity, this principle tells us that a small amount of mass can be converted into a tremendous amount of energy, and vice versa. This is the fundamental idea behind nuclear reactions—like those in the Sun or in atomic bombs—where tiny amounts of mass are transformed into large amounts of energy.
This concept also means that as an object’s energy increases (for example, by moving faster), its total relativistic mass increases as well. However, modern physics prefers to speak in terms of rest mass (which doesn’t change) and treats the increase in energy through the lens of relativistic dynamics, rather than saying mass itself increases.
Mass-energy equivalence is a cornerstone of modern physics, linking matter and energy in a profound and measurable way.