Higher Dimensions in Superstring and M-Brane Theory
On "The Speed-of-Light Limit Argument" page the possibility was discussed of circumventing the speed-of-light barrier by somehow making use of possible freedom to travel through the additional dimensions that are part of modern superstring and M-brane theory, ideas at the forefront of modern physics. Within the past few years physicists have begun to theorize that our universe might actually be a three-dimensional "membrane" (four-dimensional if you add time following the dictates of special relativity) that lies within a much higher-dimensional super-space (or super-spacetime). Until recently, these extra dimensions that are an essential part of superstring theory, were thought to be incredibly small: on the order of the Planck scale, 10-33 meters or so in size. But recently the potential size scales of the extra dimensions have been revised up dramatically, to perhaps as large as a millimeter or so. This has occurred not just because of overfertile imaginations on the part of physicists (well, perhaps a little), but because what can be called macroscopic extra dimensions might solve some long-standing puzzles in both elementary particle physics and cosmology.
All particles and forces are confined to the "membrane" that constitutes our four-dimensional spacetime universe... except for gravity. Modern theory predicts that the gravitational force would spread out into the extra dimensions. In fact, this is how theorists set upper limits to the size scales of the extra dimensions. The inverse square nature of the gravitational force has only been measured down to a millimeter or so (owing to its extreme weakness) and that is how the millimeter size scale cited above came about. If forthcoming experiments find the gravitational force to follow an inverse 2+n power at very small distances, this will be a good indication of the reality of n extra dimensions.
For over two decades astronomers have been searching for the apparent "dark matter" that is thought to be responsible for gravitational effects in the rotations of galaxies, the virial motions in clusters of galaxies, and other astrophysical anomalies. It has been thought that 90 percent of the matter in the Universe must consist of dark matter. But the extradimensional ideas of modern superstring and M-brane theory have begun to open yet another possibility: that there is no dark matter within our Universe after all; rather that the gravitational force of matter in adjacent membrane-universes is spreading out and spilling over into our universe. In other words, entire other universes might exist a tiny fraction of an inch away from our Universe in one or more extra dimensions. We cannot see these universes because their photons of light are stuck to their membrane-universe just like our photons are stuck to our membrane-universe, but gravitational forces can reach from one membrane-universe to another.
As wild as these ideas sound, they are respectable concepts in modern physics. They may or may not pan out, but no physicist would be hauled away from her lectern for discussing these ideas in front of other physicists. Moreover, there is no reason to think that if other parallel universes were to exist, they could not harbor habitable planets. Why then should we not consider, with a skeptical but open mind, the possibility that if extraterrestrial visitors are indeed nosing around, they might not necessarily be coming from distant star systems light years away in our own Universe, but rather from planets in other membrane-universes that may be right under our noses, so to speak? It does put a different spin on the ease and frequency of possible visitation.