What is spacetime?

very simple the idea that you can't have 1 with out the other

they work together to make it all happen

Spacetime is the new model of both space and time that was discovered at the beginning of the 20th century, that explains how gravity makes energy and matter interact with space and time. Prior to this discovery, space was considered distinct from time, and it was thought they had nothing to do with each other. Also prior to this, it was thought that space and time had nothing to do with matter or energy. It was thought that space and time were just the nondescript background stage over which matter and energy interact, but that the stage itself was unaffected by these interactions.

After the discovery of the new model, we now know that not only are space & time related to each other, they also interact with matter and energy. Spacetime may still be a stage, but it's a very flexible interactive stage. An apt description is that matter-energy tells spacetime how to flex, and spacetime tells matter-energy how to move.

Because both space and time are flexible no, we need something else that's constant that binds the two of them together. That constant is the speed of light. The speed of light can be thought of as the speed at which the space and the time dimensions slide by each other. Time slows down, the faster you move through space, and time speeds up the slower you move through space. Alternately, distance shrinks when you're going closer to the speed of light, and distance expands, when you're much lower than the speed of light.

In physics, spacetime is any mathematical model that combines space and time into a single interwoven continuum. Since 300 BCE, the spacetime of our universe has historically been interpreted from a Euclidean space perspective, which regards space as consisting of three dimensions, and time as consisting of one dimension, the "fourth dimension". By combining space and time into a single manifold called Minkowski space in 1908, physicists have significantly simplified a large number of physical theories, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.

Light years