Journey to the 1Oth Dimension!

Type of possible space #2: The universe as we know it is merely a three-dimensional brane suspended in a four-dimensional bulk.

Excellent question #2: What the hell is a brane?

Attempted answer #2: Here is the Powerpoint version of everything you need to know about branes:

You live on a brane.

A brane is like a membrane.

Imagine the skin that forms on your soup when it gets cold. A brane is like that.

A brane is some sort of lower-dimensional thing (the 2-D skin) sitting in a higher-dimensional space (your 3-D soup).

Brane theory says our 3-D world is really just a brane.

Our brane sits in a 4-D space called the bulk.

Like so much congealed fat, we are prevented from escaping the brane and going into the higher-dimensional soup.

Only gravity is allowed to do that.

This theory—generally referred to as brane theory—was devised in 1998, which is pretty recent by theoretical-physics standards (people have been noodling with various forms of string theory for about 30 years now). Three guys get the credit for coming up with it, one of whom—Nima Arkani-Hamed—was a 25-year-old fresh out of Berkeley with a Ph.D. He is now a professor in the physics department at Harvard. Smart guy. He and his cohorts Savas Dimopoulos and Gia Dvali had been working on a problem that had confounded big thinkers for, oh, a few decades, when they suddenly realized that all they had to do to get us out of it was to invent another dimension! Voil!


The problem that had been confounding all of these smart people for so long (and continues to confound them; did I mention that none of what I’m describing has yet been supported by a shred of experimental evidence?) was this: Gravity is weak.

Objection #1: That’s silly. Gravity is the strongest thing around—it’s what moves planets and clusters of thousands of galaxies, not to mention that it’s what keeps us pinned to the ground.

Rebuttal #1: When you compare it to the other forces—say, the electromagnetic force—gravity is incommensurably less powerful. Take for example a simple refrigerator magnet. Think about the forces acting on it as it pins a photo to the fridge. There’s the combined gravitational force of the entire Earth pulling the magnet down to the ground, and the magnetic attraction of a little strip of iron anchoring it to the fridge. Those few grams of magnetic material win; not even a planet-size helping of gravity is enough to overcome its intrinsic weakness.

Objection #2: OK, so gravity is weak. But that’s just the way it is; physicists can’t do anything to change gravity’s strength. All they can do—all they’re supposed to do—is describe it.

Rebuttal #2: Correct, sort of. There can be many ways to accurately describe something in nature. Yet there is only one way in which the thing in nature actually works, one physical process that determines how things happen, one Truth (big T) of the universe. Right now, particle physicists have a way to describe the workings of gravity. And while they think this description is useful—it accurately predicts the outcomes of experiments and the like—they do not think that it reflects the true physical processes that govern the universe.


The world according to the current theory of particle physics seems very ad hoc, in that it must treat the weakness of gravity with great care, it must introduce new assumptions and fine-tune all the parameters it can in order to replicate the weakness of gravity. Everything else works fine; gravity is the oddball of the particle family.