Time: yes, it’s a dimension, but no, it’s not like space | by Ethan Siegel | Starts With A Bang! | Oct, 2024

The fabric of spacetime is four-dimensional, with three for space and only one for time. But wow, time sure is different from space!

When it comes to dimensions, most of us intuitively understand the notion that space possesses those properties. You can move forwards or backwards by any amount you desire in any of the three dimensions — up-down, left-right, or forwards-backwards — in which motion is possible. It’s why, if you ask yourself to draw a path that describes the shortest distance between any two points, most of us will give the same answer that Archimedes gave more than 2,000 years ago: a straight line. If you take a flat sheet of paper and put two points down on it absolutely anywhere, you can connect those two points with any line, curve, or geometrical path you can imagine. So long as the paper remains flat, uncurved, and unbent in any way, the straight line connecting those two points will ultimately represent the shortest path possible.

This intuitive answer correctly describes how the three dimensions of space work in our Universe: in flat space, the shortest distance between any two points is a straight line. This is true regardless of how you rotate, orient, or otherwise position those two points. However, our Universe…