by godelski | 2025-07-09 00:15:49+00:00
> how weird large-dimensional space is

It's rather crazy that we humans can't really even intuit about a single extra dimension. Or even a single fewer! There's a lot of people who will say that they can visualize things in the 4th dimension but I've yet to find someone who can actually do this. This includes a large number of mathematicians (it's never the mathematicians that claim this...)

I really like the animation in this Math Overflow post[0], because it has a lot of hidden complexity that most people don't think about. The animation is actually an illusion, and you are "hallucinating". That top image projecting a cube down onto a plane? Well... that isn't a cube. We've already projected the cube into 2D! Technically this is 3D. But the 3rd dimension isn't a spacial dimension, it is a time dimension. Which itself is a helpful lesson in learning about the abstraction of dimensions! So we hallucinate a cube, rotating, and then see the projected image on a plane, which we hallucinate as a square that isn't skewed but instead has depth. This is all rather wild in of itself.

The truth is that we struggle to imagine 2D! And most people will claim to be able to visualize 2D and the claim will go uncontested.

If you haven't read Flatland[1], I'd encourage everyone to do so. A lot of people get it wrong. They read it as an analogy 1 dimension down. Where we 3 dimensional creatures are analogous to the 2D creatures and a 4D creature would be as baffling as a 3D creature is to the Flatlander. While that is true, there is a trick being played on you. You think understanding 2D is really easy. But I guarantee you what you're visualizing is inaccurate. Frankly, the book isn't perfectly accurate either.

But really put yourself in the Flatlander's shoes. In a real Flatlander's shoes, not the ones of the book. Be the Square Flatlander and imagine yourself looking at a Triangle. What do you see? I'm betting it is a line? But this is incorrect. You've given it thickness, you've given it a third dimension. Try this again and again, adding more depth and challenging yourself to imagine a real Flatland. You'll find you can't.

Instead, we can visualize and reason about a 2D space embedded within 3D. You might say I'm being nitpicky here, but if I weren't then it would be perfectly fine to say that this[2,3] is a 4-dimensional hypercube instead of a representation of a 4D hypercube.

I actually think understanding this goes a long way to help understanding very high dimensions. If you are forced to face the great difficulty of accurately visualizing one more or one fewer dimension, you are less likely to fool yourself when trying to reason about much higher dimensions.

And as Feynman once said:

The first principle is that you must not fool yourself and you are the easiest person to fool.

[0] https://math.stackexchange.com/a/2286226

[1] http://www.geom.uiuc.edu/~banchoff/Flatland/

[2] https://en.wikipedia.org/wiki/Tesseract#/media/File:8-cell-s...

[3] Good video of Carl Sagan where he holds a 3D projection of the hypercube. The shadow. But anything I show you has to be embedded in 2D... He picks it up at 6:20 https://www.youtube.com/watch?v=UnURElCzGc0