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I'd like to clear up a misconception: there is no fourth dimension.
As a matter of fact, there is no third dimension, either.
Now, by that I don't mean that there aren't three dimensions… there are, but just we
can't tell these dimensions apart: there's no special direction you can call "the 1st
dimension," [north?
east?
up?] so it doesn't make sense to say "this is the 2nd one," or "what's the fourth one?"
It's like if you have a jug with three cups of water in it... it's easy to tell that there
ARE three cups of water, but which is the first cup? the second? or the third?
Those questions don't even make sense!
That said, there may be different KINDS of dimensions that you CAN tell apart - like
if you poured three cups of water and one cup of oil into our dimensions jug.
Then you could say "I have three dimensions that behave in the same way, and one that
behaves a different way."
And that's what we mean when we say that we live in three spatial dimensions and one time
dimension.
And there's nothing crazy about adding another cup of water so that you'd have four watery
cups and one oily cup; likewise we can contemplate what the universe would be like if there were
FOUR spatial dimensions and one time dimension… or in the case of string theory, nine spatial
dimensions and one time dimension.
And while we're at it, it's not even too hard to conceive that there might be two dimensions
of time… but I don't have even have enough of one time dimension to explain that now.
So if you'd like to try your brain at visualizing a world with four spatial dimensions, check
out the book Flatland or try solving a 4D rubiks cube.