Ask someone to name every dimension they know and they’ll likely list the following: length, width, and depth. They might also add time if they’re thinking outside the three-dimensional box. But asking a string theorist “How many dimensions are there?” would elicit a very different response.

According to this branch of theoretical physics, there are at least 10 dimensions of space, most of which are impossible for humans to perceive. Dimensions are the metrics that physicists use to describe reality. Sounds broad, right? Let’s start with the three dimensions most people learn in grade school.

The spatial dimensions—width, height, and depth—are the easiest to visualize. A horizontal line exists in one dimension because it has only length; a square is two-dimensional because it has length and width. Add depth and we get a cube, a three-dimensional shape.

These three coordinates are used to pinpoint an object’s location in space. But space isn’t the only plane we exist on; we also exist in time, which is where the fourth dimension comes in. Once we know a dot’s altitude, longitude, latitude, and position in time, we have the tools needed to plot its existence in the universe as we know it.

In a four-dimensional universe, this theory wouldn’t be possible, but once scientists tweaked the math to include 10 dimensions—11 including time—their equations worked.