Many people give the tube visualization. If you've been playing video games for a long time, you have another option: Consider the Asteroids game field: https:/

Many people give the tube visualization. If you've been playing video games for a long time, you have another option: Consider the Asteroids game field: https://www.youtube.com/watch?v=_TKiRvGfw3Q https://www.youtube.com/watch?v=_TKiRvGfw3Q . This is well known to be topologically a torus. There are no boundaries on this field, yet it is finite in both dimensions. Note that while Asteroids itself has no equivalent to a "wave" in the game, you could imagine having a wave inside this space. Even though there is no boundary to this space, you can see how any stable wave would have to be related to the size of the space. Now take the Asteroids playing field, and collapse one of the dimensions to be very small. Now you have a very small spatial dimension, even though it has no boundary. The waves that wrap around that dimension are confined even more tightly than before to certain wavelengths. This still appeals to human intuitions; your "small" dimension is probably, say, 10 pixels. But you could keep shrinking it until it was microscopic, you just wouldn't be able to fit the game entities in it anymore. Alternatively, you could expand the playing field into three dimensions, then shrink the third one down, though that requires a bit more geometrical intuition. Either way you can see that the nature of that coordinate starts to change fairly significantly, and anything moving through it does not move the way you are used to because it wraps around so quickly.