For Einstein, E=mc² was interesting—but not very relevant to the real world.

Decades after Einstein published his famous equation, other scientists realized that it did explain a number of physical phenomena. One important discovery answered a question that had puzzled scientists for centuries: Why does the Sun shine? Every star you can see in the night sky is powered by a process known as "fusion," in which atoms fuse together while some of their mass is converted to energy.

Perhaps most famously, E=mc² helps explain the energy released by atomic bombs and produced by nuclear power plants. Under the right conditions, certain atoms can split apart in a process called "fission." During fission, some of the mass of the original atoms is converted to energy. Scientists have learned how to exploit fission for weapons as well as for peaceful applications, such as nuclear power.

Why Does the Sun Shine?

The Sun is fueled by a process known as fusion: four hydrogen atoms undergo a series of collisions and eventually fuse together to form one helium atom. Such reactions—which occur in the Sun 100 million quadrillion quadrillion times each second—release a significant quantity of energy as predicted by E=mc². The mass of one helium atom is slightly less than the sum of the masses of four hydrogen atoms. During fusion, this missing mass is converted to energy. Our Sun has enough hydrogen to continue burning for another five billion years.

Atomic addition: fusion

H-atom = 1.008 units of mass
 plus
 H-atom = 1.008
 plus
 H-atom = 1.008
 plus
 H-atom = 1.008

adds up to a Helium atom = 4.003 units of mass
 LOSS: 0.029 units of mass

What happens to the missing 0.029 units? During the fusion reaction, particles known as positrons and neutrinos are produced, accounting for some of the missing mass. Half of the missing mass, however, is converted directly to energy in the form of radiation.

The energy of an atomic bomb or a nuclear power plant is the result of the splitting, or "fission," of an atom. Most nuclear power plants today draw their energy from the fission of uranium atoms.

Under certain conditions, a uranium atom will split apart into two smaller atoms, such as barium and krypton. The combined mass of the two smaller atoms is less than the mass of the original uranium atom. Why? Because some of the mass of the uranium atom has been converted to energy. In a nuclear power plant, this energy is used to superheat water. The resulting steam powers a turbine and a generator, thereby producing electricity.

Atomic division: fission

Uranium can split apart into barium and krypton:

U = 235 units of mass
 yields

Ba = 144 units of mass
 plus
 Kr = 90 units of mass

loss: 1 unit of mass

What happened to the missing unit of mass? During fission, particles known as neutrons are produced, accounting for some of the missing mass. The rest of the mass is converted to energy.