Physicists may soon be able to detect and verify the existence of reality's extra dimensions, which could extend over distances as large as a millimeter.
All the matter and forces we know of-with the sole exception of gravity-are stuck to a "wall" in the space of the extra dimensions. Electrons, protons, photons and all the other particles in the Standard Model cannot move in the extra dimensions; electric and magnetic field lines cannot spread into higher-dimensional space. The wall has only three dimensions, and so far as these particles are concerned, the universe might as well be three-dimensional. Only gravitational field lines can extend into the higher dimensional space, and only the particle that transmits gravity, the graviton, can travel freely into the extra dimensions. The presence of the extra dimensions can be felt only through gravity.
Particles such as electrons and photons are like tiny lengths of string that each have two end points that must be stuck to a D-brane, Gravitons, on the other hand, are tiny closed loops of string that can wander into all the dimensions because they have no end points anchoring them to a D-brane.
The membranes of other three-dimensional universes could lie parallel to our own, only a millimeter removed from us in the extra dimensions. Similarly, although all the particles in the Standard Model must stick to our own membrane universe, other particles beyond the Standard Model might propagate through the extra dimensions. Far from being empty, the extra dimensions could have a multitude of interesting structures.