We don’t usually associate movement with size. Saying that “up” and “down” are big is nonsensical. A dimension is a dimension is a dimension. Right?
Wrong – at least, according to string theory. The world we can observe is three dimensional, and these dimensions are extended in space. They are big, unlike the hidden dimensions of string theory. Those ones are invisible even to the most powerful particle accelerator on Earth, the Large Hadron Collider.
Extra dimensions are everywhere in space, and they are indeed tiny, according to string theorist Brian Greene in his book The Elegant Universe. So if you believe string theory, it makes sense to say that the 4th, 5th, 6th, 7th, 8th and 9th dimensions are small, and that left, right, forward, back, and up and down are big.
Even though we can’t see the small dimensions, scientists have the math to describe them, and they can even imagine what they might look like to our 3-D-perceiving eyes. To visualize nine dimensions, string theorists think about six-dimensional entities such as Calabi-Yau shapes. Stack an infinite number of these infinitesimal 6-D figures in a 3-D grid everywhere in space and Voila! You have a 9-D world.
Of course, the ten dimensions of string theory – nine spatial and one temporal – is just an approximation, according to famed theoretical physicist Ed Witten. Alternative string theories predict as many as twenty-six!
To wrap your head around just ten, check out this cool animation.