Before Einstein, scientists believed that space and time were completely separate things. Space was viewed as a fixed stage where events happened, and time was like a universal clock that ticked at the same rate for everyone, everywhere. Einstein changed this view forever with his Special Theory of Relativity. He realized that because the speed of light is always the same, space and time must be connected. He showed that if you move very fast through space, time actually slows down for you compared to someone standing still. This meant that space and time are not independent; instead, they are woven together into a single, flexible fabric known as “spacetime.”

This new way of thinking also connected matter and energy. Previously, people thought mass (how much “stuff” is in an object) and energy (the ability to do work) were totally different and could not change into each other. Einstein discovered that mass is really just a super-concentrated form of energy. His famous equation, $E=mc^2$, explains this relationship. It shows that a very small amount of mass is equal to a huge amount of energy. This proved that matter and energy are essentially the same thing in different forms, similar to how ice and steam are both just different forms of water.