Why don't we see the other dimensions ?

[Abidal Sala]

There might be 11 dimensions like some physicists say, but why can't we see them? Is it because we are too small or too big to see them? what if dimensions were something relative? the bigger/smaller you become in comparison with something else, the less/more dimensions you can experience from it?

 

[Nabeshin]

There might be 11 dimensions like some physicists say, but why can't we see them? Is it because we are too small or too big to see them? what if dimensions were something relative? the bigger/smaller you become in comparison with something else, the less/more dimensions you can experience from it?

This is precisely correct. If you imagine a sheet of paper, to us it looks 2-Dimensional, but to a small microbe, it is definitely 3-dimensional. This effect exists because the scale of the third dimension (the paper's width) is much smaller than that of the other two. So goes the story in string theory, except now you need to do something like this with 7 extra dimensions, and you need to worry a little more about continuity than in my paper example. What I mean by this is that the width dimension for the paper is just like the other two in that you travel a finite distance along the width dimension before it 'ends'. The trick is to curl this width dimension in on itself, much as you would tie two ends of a string together to produce a continuous circle, so that you never hit any edge. Again, the story in string theory is vastly more complicated, but this is the footnotes version.

 

[Chalnoth]

The other possibility is that while there may potentially be large extra dimensions, we are confined onto a surface in this higher-dimensional space. To take Nabeshin's sheet of paper example, the sheet of paper is sort of a two-dimensional object in three-dimensional space. But imagine that that sheet of paper is now made into a perfect two-dimensional object, with little two-dimensional beings in it. They can't interact with the larger three-dimensional world, and so can't ever observe it.

One thing that distinguishes this view from the one Nabeshin pointed out, of the extra dimensions simply being small, is that while it is easy to confine the forces like electricity and magnetism or the nuclear forces to such a surface, gravity cannot be contained to the surface. This has interesting implications in that it may potentially explain why gravity is so weak compared to the other forces: it is spread over a much large volume.

If the extra dimensions are small, then all forces are confined in the exact same way. Note that reality might be a combination of the two approaches: we could have, say, two large extra dimensions and a bunch of small extra dimensions.