Gravity and All

[Ghetta Life]

http://www.nytimes.com/2005/11/01/science/01prof.html?8hpib

In the mid-90's, theorists discovered that [string] theory was even richer 
than its founders had thought, describing not just strings but so-called 
branes, as in membranes, of all dimensions. Our own universe could be such a 
brane, an island of three dimensions floating in a sea of higher dimension, 
like a bubble in the sea. But there could be membranes with five, six, seven 
or more dimensions coexisting and mingling like weird cosmic soap bubbles in 
what theorists sometimes call the multiverse.

[...]

Gravity should be 10 million billion times as strong as it is. You might 
find it hard to imagine gravity as a weak force, but consider, says Dr. 
Randall, that a small magnet can hold up a paper clip, even though the 
entire earth is pulling down on it.

[...]

They began by drawing pictures and making crude estimates over ice cream and 
coffee in that ice cream parlor, which is now a taqueria. What they drew 
pictures of was a kind of Oreo cookie multiverse, an architecture similar to 
one first discovered as a solution of the string equations by Edward Witten 
of the Institute for Advanced Study and Petr Horava, now at Berkeley. Dr. 
Randall and Dr. Sundrum's model consisted of a pair of universes, 
four-dimensional branes, thinly separated by a five-dimensional space 
poetically called the bulk.

When they solved the equations for this setup, they discovered that the 
space between the branes would be warped. Objects, for example, would appear 
to grow larger or smaller and get less massive or more massive as they moved 
back and forth between the branes.

Such a situation, they realized to their surprise, could provide a natural 
explanation for the hierarchy problem without invoking supersymmetry. 
Suppose, they said, that gravity is actually inherently as strong as the 
other forces, but because of the warping gravity is much much stronger on 
one of the branes than on the other one, where we happen to live. So we 
experience gravity as extremely weak.

"You can be only a modest distance away from the gravity brane," Dr. Randall 
said, "and gravity will be incredibly weak." A result was a natural 
explanation for why atomic forces outgun gravity by 10 million billion to 1. 
Could this miracle be true? Crazy as it sounded, they soon discovered an 
even more bizarre possibility. The fifth dimension could actually be 
infinite and we would not have noticed it.

In this case, there would be only one brane, ours, containing both gravity 
as we know it and the rest of nature. But it would warp space in the same 
way as in the first model, trapping gravity nearby so that we would 
experience space-time as four-dimensional. This new single brane model did 
not solve the weak gravity problem, Dr. Randall admitted, but it was a 
revelation, that an infinite ocean of space could be sitting next to us 
undetected.

"So when we wrote this paper, what we were concentrating on was this amazing 
fact that really had been overlooked for 100 years - well, years, whatever - 
that you can have this infinite extra dimension," she said. "I mean it was 
quite wild."

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