I can't put my finger on a really good link to the word "subdimension" in Golden Age SF and comics, maybe some readers can come up with it. It's basically a place-holder word, a liguistic MacGuffin, used to fill in for any type of weird science that happens to be needed. But I've always wanted to visit the subdimensions.
Experientially, I think of going into the subdimensions as being something like SCUBA diving…here's a photo of a guide, my big brother Embry and me diving near Yap Island in Micronesia.
In recent years I decided to retrofit the word "subdimensional" and use it to apply to a hypothetical cosmos that lies "inside the Planck length," in a sense that I'll explain at the end of this post.
I introduced this SFictional usage three yeares ago in a story with Paul DiFilippo, "Elves of the Subdimensions," which is still online in issue #1 of my webzine Flurb.
And I used it again in my novel Postsingular. You can either buy the paperback or download a free Creative Commons PDF release from my site for Postsingular. Here's a drawing from my online working notes for the novel (these notes are also online my Postsingular site).
The beings who live in the subdimensions are called "subbies," and generally speaking, you're better off not having any dealings with them!
It's always nice have some kind of scientific justification for what I write about, and, by way of justifying the reality of the subdimensions, I found the following passage in Michio Kaku, Parallel Worlds, where he discusses a 1984 theory of “string duality” ascribed to Keiji Kikkawa and Masami Yamasaki. String duality allows for interesting physics below the Planck length (which is roughly a quadrillionth of the diameter of a proton). The Planck length becomes something like an interface between two worlds. As Kaku puts it:
Let's say we take a string theory and wrap up one dimension into a circle of radius R. Then we take another string and wrap up one dimension into a circle of radius 1/R. By comparing these two quite different theories, we find that they are exactly the same. Now let R become extremely small, much smaller than the Planck length. This means that the physics within the Planck length is identical to the physics outside the Planck length. At the Planck length, spacetime may become lumpy and foamy, but the physics inside the Planck length and the physics at very large distances can be smooth and are in fact identical.