Q: Are shadows 2-dimensional? Are there any real examples of 2-dimensional things in the universe?
It’s impossible to get something that’s genuinely 2-D in our universe for several reasons, the simplest of which is that atoms themselves are 3-D. That said, there are innumerable examples of things that behave as though they were 2-D. For example, ripples on the surface of water are an example of 2-D waves that behave very differently from ordinary 3-D waves. Weather systems that are substantially bigger across than they are tall, like hurricanes (e.g., Sandy was about 800 miles across, and only about 5-10 miles high), roughly obey 2-D fluid dynamics. However, very weirdly, in a 2-D fluid you find that the opposite tends to be true: energy starts out in small eddies and moves to larger.