# What is Spacetime?

If you combine the three dimensions of space and one dimension of time into a single four-dimensional manifold, you call it spacetime. Spacetime diagrams can be used to show relativistic effects, like why different people see things in different places and at different times. Before the 20th century, it was thought that the three-dimensional geometry of the universe (the way it looked in terms of coordinates, distances, and directions) was not affected by one-dimensional time. This is not true anymore. When Albert Einstein came up with his theory of relativity, he came up with the idea of spacetime. Before his work, scientists had two different theories to help them understand how things worked in the world. Isaac Newton’s laws of physics explained how big things move, and James Clerk Maxwell’s electromagnetic models explained how light works. The laws of physics are the same in all inertial systems (frames of reference that don’t move) and the speed of light in a vacuum is the same for all observers, no matter how fast the source of the light moves. In 1905, Einstein wrote a paper called “Special Relativity.” It was based on two assumptions: The result of taking these two statements together is that the four dimensions of space and time, which had been thought of as separate, must be joined together. In addition to being independent of the speed of the light source, the speed of light is the same no matter what frame of reference is used to measure it. This is called the relativity of simultaneity. The linear additivity of velocities also doesn’t work anymore. This is called the relativity of simultaneity. Einstein thought about how things move when he came up with his theory (the study of moving bodies). After Lorentz and PoincarĂ© came up with their theories of electromagnetic phenomena in 1904 and 1879 respectively, his was a step up. Einstein came up with the Lorentz transformation, but these theories used equations that were very similar to those he came up with. They were made up to explain the results of experiments, like the Michelsonâ€“Morley interferometer experiment, that were hard to fit into existing theories. A math professor in ZĂĽrich named Hermann Minkowski came up with a geometric interpretation of special relativity that combined time and the three spatial dimensions of space into a single four-dimensional continuum called Minkowski space in 1908. Minkowski was once a young Einstein’s math teacher. A big part of this interpretation is how the spacetime interval is defined in a more formal way. Even though measurements of distance and time between events can be made in different reference frames, the spacetime interval is the same no matter which frame of reference they are made in. Mathematical interpretation: Minkowski’s geometric interpretation of relativity was important for Einstein when he came up with his general theory of relativity in 1915. He showed how mass and energy made spacetime curve into a “pseudoremannian manifold.”