In physics, mass–energy equivalence states that
anything having mass has an equivalent amount of energy and vice versa, with these fundamental quantities
directly relating to one another by Albert Einstein's famous formula: "E=MC2"
This formula states that the
equivalent energy (E) can be calculated as the mass (m) multiplied by the speed of light (c = ~3×108 m/s) squared. Similarly, anything having energy
exhibits a corresponding mass m given
by its energy E divided by the speed of
light squared c2. Because the speed of light is a
very large number in everyday units, the formula implies that even an everyday
object at rest with a modest amount of mass has a very large amount of energy
intrinsically. Chemical reactions, nuclear reactions, and other energy transformations may
cause a system to lose some
of its energy content (and thus some corresponding mass), releasing it as
the radiant energy of light or
as thermal energy for
example.
Mass–energy equivalence arose originally from special relativity as
a paradox described by Henri Poincaré.[2] Einstein
proposed it on 21 November 1905, in the paper Does the inertia of a
body depend upon its energy-content?, one of his Annus Mirabilis (Miraculous Year) papers.[3] Einstein
was the first to propose that the equivalence of mass and energy is a general
principle and a consequence of the symmetries of space and time.
A consequence of the mass–energy equivalence is that if a body is
stationary, it still has some internal or intrinsic energy, called its rest energy, corresponding
to its rest mass. When the body is in motion, its total energy is
greater than its rest energy, and equivalently its total mass (also
called relativistic mass in
this context) is greater than its rest mass. This rest mass is also called the
intrinsic or invariant mass because
it remains the same regardless of this motion, even for the extreme speeds or
gravity considered in special and general relativity.
The mass–energy formula also serves to convert units of mass to units of energy (and
vice versa), no matter what system of measurement
units is used.
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