Albert Einstein's theory of relativity, developed in the early 20th century, fundamentally reshaped our understanding of space, time, and gravity. It consists of two parts:

Special Relativity (1905) deals with objects moving at constant speeds, showing that the speed of light is constant for all observers, and introducing concepts like time dilation (where time moves slower for objects in motion) and length contraction (where objects appear shorter in the direction of their motion). It also famously states that mass can be converted into energy according to the equation E=mc2E=mc^2E=mc2.

General Relativity (1915) extends these ideas to include gravity, proposing that massive objects like planets and stars bend the fabric of space and time around them. This curvature explains gravity as the result of objects following the shortest path in curved space-time. General Relativity also predicts phenomena such as gravitational time dilation (where time moves slower in stronger gravitational fields) and gravitational waves (ripples in space-time caused by accelerating masses).

Implications: Einstein's theories have practical applications in GPS satellites (which require corrections for both time dilation and gravitational effects to remain accurate) and in understanding cosmic phenomena like black holes and the bending of light around massive objects. Overall, relativity revolutionized physics by showing that space and time are intertwined, dynamic, and influenced by mass and energy in ways that classical physics could not explain.