## Parallel Universes: The Brane Multiverse

When Albert Einstein’s theory of general relativity began to come to public attention in the 1920s, many people speculated about the “fourth dimension” that Einstein had allegedly invoked. What might be in there? A hidden universe, maybe?

This was nonsense. Einstein was not proposing a new dimension. What he was saying was that time is a dimension, similar to the three dimensions of space. All four are woven into a single fabric called space-time, which matter distorts to produce gravity. Even so, other physicists were already starting to speculate about genuinely new dimensions in space.

The first intimation of hidden dimensions began with the work of the theoretical physicist Theodor Kaluza. In a 1921 paper Kaluza showed that, by adding an extra dimension to the equations of Einstein’s theory of general relativity, he could obtain an extra equation that seemed to predict the existence of light.

That looked promising. But where, then, was this extra dimension? The Swedish physicist Oskar Klein offered an answer in 1926. Perhaps the fifth dimension was curled up into an unimaginably small distance: about a billion-trillion-trillionth of a centimetre.

The idea of a dimension being curled may seem strange, but it is actually a familiar phenomenon. A garden hose is a three-dimensional object, but from far enough away it looks like a one-dimensional line, because the other two dimensions are so small. Similarly, it takes so little time to cross Klein’s extra dimension that we do not notice it.

Physicists have since taken Kaluza and Klein’s ideas much further in string theory. This seeks to explain fundamental particles as the vibrations of even smaller entities called strings.

When string theory was developed in the 1980s, it turned out that it could only work if there were extra dimensions. In the modern version of string theory, known as M-theory, there are up to seven hidden dimensions.

What’s more, these dimensions need not be compact after all. They can be extended regions called branes (short for “membranes”), which may be multi-dimensional.

A brane might be a perfectly adequate hiding place for an entire universe. M-theory postulates a multiverse of branes of various dimensions, coexisting rather like a stack of papers.

If this is true, there should be a new class of particles called Kaluza-Klein particles. In theory we could make them, perhaps in a particle accelerator like the Large Hadron Collider. They would have distinctive signatures, because some of their momentum is carried in the hidden dimensions.

These brane worlds should remain quite distinct and separate from each other, because forces like gravity do not pass between them. But if branes collide, the results could be monumental. Conceivably, such a collision could have triggered our own Big Bang.

It has also been proposed that gravity, uniquely among the fundamental forces, might “leak” between branes. This leakage could explain why gravity is so weak compared to the other fundamental forces.

As Lisa Randall of Harvard University puts it: “if gravity is spread out over large extra dimensions, its force would be diluted.”

In 1999, Randall and her colleague Raman Sundrum suggested that the branes do not just carry gravity, they produce it by curving space. In effect this means that a brane “concentrates” gravity, so that it looks weak in a second brane nearby.

This could also explain why we could live on a brane with infinite extra dimensions without noticing them. If their idea is true, there is an awful lot of space out there for other universes.

**Next Page: The Quantum Multiverse**

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