**Albert Einstein** (1879 – 1955) was a German-born theoretical physicist, widely acknowledged as a once-in-a-generation genius. He is known for developing the theory of relativity, and contributed to quantum mechanics. His mass–energy equivalence formula *E* = *mc*^{2} has been dubbed "the world's most famous equation". He received the 1921 Nobel Prize in Physics for photoelectric effect, as his theory of relativity had only been accepted around 1925, 20 years after its inception, even though scientists like Max Planck had supported it. Relativity was so ground-breaking that it deserved the Nobel prize, but an individual can only win it once per field.

Some of his contributions to science:

- Light and the photoelectric effect
- Brownian motion
- Special relativity
- Mass-energy equivalence (
*E*=*mc*^{2}) - General relativity with field equations of motion, and his theory of gravitation which predicted gravitational waves.
- Cosmology; collaborated with Nathan Rosen to produce a model of a wormhole, often called Einstein–Rosen bridges.
- Einstein rings formed by gravitational lensing.
- Bose–Einstein condensate (BEC) is the fifth state of matter, predicted by Einstein crediting a pioneering paper by Satyendra Nath Bose on the field of quantum statistics.
- Wave–particle duality
- Zero-point energy
- Stimulated emission which makes possible the maser and the laser.
- Co-invented the absorption refrigerator

He spent the latter years of his life trying to find a unified field theory by generalizing his theory of gravitation to include electromagnetism, but never succeeded. His quest has motivated a modern search for a theory of everything, in particular string theory.