WHY do we live in a space with only three dimensions? Because, at some time in the past, all the universes with four or more dimensions collided and destroyed each other, while our 3D space survived by slipping between collisions. Or at least, that is what a new theory claims.
It sounds like a tall tale. Can鈥檛 we accept that our universe has three space dimensions and that鈥檚 that? Not if you believe in string theory 鈥� physicists鈥� best bet for a fundamental description of all particles and forces 鈥� which needs nine spatial dimensions.
In one interpretation of string theory, called braneworld, those extra dimensions are large, perhaps even infinite, and our universe is just a 3D membrane drifting in a higher-dimensional space. However, that does not explain why our 鈥渂rane鈥� has three rather than, say, four or seven dimensions.
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Now a team led by Ruth Durrer of the University of Geneva in Switzerland has an explanation. The idea is that the cosmos once included branes with up to eight dimensions, floating about at random in a nine-dimensional space. In their model, this 9D space has the form of a torus, or doughnut, with each dimension circling back on itself ().
In such a model, the higher-dimensional branes are far more likely to intersect and collide with each other than low-dimensional ones. For example, in a 3D box, sheets of 2D paper are more likely to touch each other than 1D strings. When branes collide, they fuse and evaporate in a burst of gravitational particles, or gravitons. Durrer and her team worked out that only branes with three or fewer space dimensions are skinny enough to avoid this fate. Perhaps one of them became our universe.
鈥淚t is an interesting premise, but I鈥檓 not so sure about the details,鈥� says braneworld expert Lisa Randall of Harvard University. Durrer admits the idea still has weaknesses: for instance, the model does not destroy 1D and 2D branes, raising the question of why we don鈥檛 find ourselves living in flatland. Durrer has an anthropic explanation. A 1D or 2D universe would be too simple for life to evolve. 鈥淚n one and two dimensions it is difficult to make something complex,鈥� she says.
Also, there is no way yet to test the model. 鈥淲e have to go into more detail and make a prediction,鈥� says Durrer. She speculates that all the energy from those evaporating branes might have driven the big bang on our brane. If so, detailed calculations might yield predictions about the precise texture of the cosmic microwave background. Then astronomers could look for the signs, and find out whether our universe really is a nomadic survivor drifting around a nine-dimensional doughnut.
