if there are 3 dimensions in space, and 1 in time, what are the other 7 that are theorised?

Question

This might be a bit much for eli5, but I find any googling yields results far too complex for me to understand. Ive heard that there are up to 11 dimensions. If up/down, left/right, and forward/back are the 3 spatial dimensions, what are the others? And what of time being a dimension itself?

In: 2

Answer 1

We (i.e. pretty much any human) can’t visualise more that 3D space.

So don’t bother trying to understand “what it would look like”. It’s futile.

Instead, let’s try to understand “what it does”. A dimension is an INDEPENDENT axis/variable. If you have 3D space represented as three axis, all 90 degrees to eachother, you can draw a random line in that space. The problem is, this line is not independent, it will be some combination of the original three.

This is best understood by length of lines. In 1D, the length of a line is the line. In 2D, the length of a line is a combination of its perpendicular x and y components according to Pythagoras, c = sqrt ( x^2 + y^2 ). This holds in every higher dimension. In 3D, every line will have a length r = sqrt ( x^2 + y^2 + z^2 ). The direction in which it’s pointing can be expressed as angles from these axes.

So to get a new dimension, you need a new axis that is perpendicular to the other three, like z is to x and y. As in, r = sqrt ( x^2 + y^2 + z^2 + w^2 ). An independent axis. You can’t visualise it. Don’t bother. The point is that the math works the same for this new axis as it did for z when you went from 2 to 3. So basically, you say “this is a new independent axis because I’m giving it the properties of an independent axis”.

Now, to understand where the others of the 11 dimensions of string theory are hiding, the physicists declared that they are, in fact… Hiding. That their axes aren’t (apparently) straight lines like our three spatial dimensions. Those extra dimensions are all curled up on themselves really tightly, too small to interact with anything we perceive aside from gravity.

I can’t remember if they’re literally meant to be closed loops, so finite and repeating, or are they meant to be infinitely long but wound into coils. Maybe either, depending on the theory. It honestly doesn’t matter, because string theory is currently unprovable anyway, so it’s splitting hairs.