system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 the concept of extra dimensions

I don't understand how it's possible to have a 4th or 9th dimension, like those proposed in string theory. What type of shape would these have and how would we interact with them?
gpt
Think of a large SUV moving on a narrow street. Technically, the SUV is free to move in 2D dimensions: forwards/backwards and left/right. But if the street is very narrow compared to the SUV, it can't, in practical terms, move left/right. Essentially, its movement is constrained to 1D, despite it existing on a 2D surface. If the SUV were a creature, it would've evolved without the ability to sense the second dimension.

For contrast, a motorcycle is small enough to be able to manoeuvre on the narrow street, so it can feel the two dimensions. 

Essentially, string theory proposes 3 "large" space dimensions, one time dimension, and extra "small" space dimensions. *We* can't interact with the extra dimensions, but small particles can. If these dimensions exist, we would observe extra particles with different masses but otherwise identical the ones we already know; in truth, these aren't extra new particles but the same old ones moving differently across the small dimensions.

These extra particles have never been observed.