Galileo did! But, it was kind of a trick question, but Galileo does deserve more credit than he gets. Galileo came up with the Relativity Principle, and Einstein centuries later extended it. In the 17th century, Galileo conducted experiments that showed the laws of physics are the same regardless of an object’s constant motion. This is the essence of the Relativity Principle. For example, if you’re on a smoothly sailing ship , as long as the motion is constant, you won’t be able to tell if you’re moving or not. This is a great trivia question because even textbooks frequently credit Einstein with it. To great shame, many books ignore this nuance.
Einstein built upon it and, in 1905, published his groundbreaking work on his special theory of relativity. It incorporated the concept of constant motion from Galileo’s work but also introduced revolutionary ideas about the speed of light and the relationship between space and time. This is when Einstein introduced the world to is his famous E=mc2 formula, which simply says:
“energy equals matter, and matter equals energy.”
In 1907, with Einstein’s famous happiest thought, he realized that a person in free fall feels no gravity. This idea is known as the equivalence principle. In a very real way, Einstein’s equivalence principle is just Galileo’s relativity principle in motion. From here, he will conclude that gravity and acceleration are locally equivalent—and that gravity might be a property of spacetime rather than an ordinary force. Within the next decade, he published his general theory of relativity which explains all this by adding gravity to his framework. From Galileo to Newton to Einstein, their work helped pave the way to understanding how the universe expands.